Part 2

Chapter 10:

coordination compounds

Addition Compounds :

They are formed by the combination of two or more stable compounds in stoichiometric ratio.

Double salts :

Those addition compounds which lose their identity in solution are called double salts.

K₂SO₄+ Al₂(SO₄)₃ + 24H₂O ®  K₂SO₄ . Al₂(SO₄)₃ . 24H₂O  2K⁺ (aq.) + 2Al⁺³ (aq.) + 4SO₄^2– (aq.)

Other examples are carnallite (KCl. MgCl₂ . 6H₂O), Mohr’s salt  [FeSO₄ . (NH₄)₂SO₄ . 6H₂O],

Potash alum [KAl(SO₄)₂.12H₂O] etc.

Coordination Compounds :

Those addition compounds which retain their identity (i.e. doesn’t lose their identity) in solution are called coordination compounds.

Fe(CN)₂ + 4KCN —® Fe(CN)₂ . 4KCN or K₄ [Fe(CN)₆] (aq.)  4K⁺ (aq.) + [Fe(CN)₆] ^4-(aq.)

Other examples are,  

[Cu(NH₃)₄]SO₄ (aq.)  [Cu(NH₃)₄]²⁺ (aq.) + SO₄^2– (aq.)

K₂[Zn(CN)₄] (aq.)  2K⁺ (aq.) + [Zn(CN)₄]^2– (aq.)

Coordination compound is defined as a species in which metal atom or ion is attached to group of neutral molecules / ions by coordinate covalent bonds.

Coordination Entity/Coordination Sphere :

A coordination entity constitutes a central atom/ion, usually of a metal, to which are attached a fixed number of other atoms or groups each of which is called a ligand. It may be neutral or charged. Examples being : [Co(NH₃)₆]³⁺, [PtCl₄]^2–, [Fe(CN)₆]^3–, [NiCl₂(OH₂)₄] and (NH₃), (Cl^–), (CN^–), (H₂O) are the ligands.

The central atom/ion and the ligands attached to it are enclosed in square bracket and is collectively called as coordination sphere.

Note : The remaining ions apart from complex ions i.e. outside the coordination sphere are called counter ions, free ions or ionisable ions.  For example, in K₄[Fe(CN)₆], the potassium (K⁺) ion is counter ion of coordination entity [Fe(CN)₆]^4–.

Central Atom/Ion :

In a coordination entity–the atom/ion to which are bound a fixed number of ligands in a definite geometrical arrangement around it, is called the central atom or ion. For example, the central atom/ion in the coordination entities : [NiCl₂(OH₂)₄], [CoCl(NH₃)₅]²⁺ and [Fe(CN)₆]^3– are Ni²⁺, Co³⁺ and Fe³⁺, respectively. These central atoms / ions are also referred to as Lewis acids.

Ligands :

The neutral molecules, anions or cations which are directly linked with central metal atom or ion in the coordination entity are called ligands.

These may be simple ions such as Br–, small molecules such as H₂O or NH₃, larger molecules such as H₂NCH₂CH₂NH₂ or N(CH₂CH₂NH₂)₃ or even macromolecules such as proteins.

When a ligand is attached to a metal atom ion through a single donor atom, as with Cl^–, H₂O or NH₃, the ligand is said to be unidentate. Similarly when a ligand is bound through two donor atoms, as in H₂NCH₂CH₂NH₂ (ethane-1, 2-diamine) or C₂O₄^2– (oxalate), the ligand is said to be bidentate and when several donor atoms are present in a single ligand as in N (CH₂CH₂NH₂)₃ or ethylenediaminetetraacetic acid (EDTA), the ligand is said to be polydentate.

Ambidentate Ligand :

Ligands which can ligate through two different atoms present in it are called ambidentate ligands. Examples of such ligands are the CN^–, NO₂^– and SCN^¯ ions. NO₂^– ion can coordinate through either the nitrogen or the oxygen atoms to a central metal atom/ion. Similarly, SCN¯ ion can coordinate through the sulphur or nitrogen atom. Such possibilities give rise to linkage isomerism in coordination compounds. For example,

                   nitrito-N

M ¬ O —N=O         nitrito-O

M¬ SCN                 thiocyanato or thiocyanato-S

M¬ NCS                isothiocyanato or thiocyanato-N

Chelate ligand :

Chelate ligand is a di or polydentate ligand which uses its two or more donor atoms to bind a single metal ion producing a ring. The complex formed is referred to as a chelate complex and the process of chelate formation is called chelation. The number of such ligating groups is called the denticity of  the ligand.

Chapter 11: Reaction Mechanism

Introduction :

Alkyl halides :

There are three major classes of organohalogen compounds ; the alkyl halides, the vinyl halides, and the aryl halides.

An alkyl halide simply has a halogen atoms bonded to one of the sp³ hybrid carbon atoms of an alkyl group. (A vinyl halide or Aryl halide has a halogen atom bonded to one of the sp² hybrid carbon atoms of an aromatic ring. They are different from alkyl halides because their bonding and hybridization are different.)

Classification of halides :

(a) Alkyl halides or haloalkanes (R—X) Compounds Containing sp³ C–X Bond :

They are classified as primary, secondary or tertiary according to the nature of carbon to which halogen is attached.

 (b)  Allylic halides :

These are the compounds in which the halogen atom is bonded to an sp³–hybridised carbon atom next to carbon-carbon double bond (C=C) i.e. to an allylic carbon.

(c)  Benzylic halides :   

These are the compounds in which the halogen atom is bonded to an sp³–hybridised carbon atom next to an aromatic ring.

(d)  Compounds Containing sp² C–X Bond :  Vinylic halides  Aryl halides

Structure of alkyl halide :

The carbon-halogen bond in an alkyl halide is polar because halogen atoms are more electronegative than carbon atoms. Most reactions of alkyl halides result from breaking this polarized bond. The carbon atom has a partial positive charge, making it some what electrophilic.

Chapter 12: Aldehydes, Ketones and Carboxylic Acids

Aldehydes and Ketones

Introduction

Aldehydes & ketones have general formula C_nH_2nO and contains >C = O group.  Thus aldehydes (R–CHO) and ketones (R–CO–R) are collectively called as carbonyl compounds. Aldehyde is always at terminal position while ketone is never at terminal position.

Structure and bonding in aldehydes and ketones

The carbonyl carbon atom is sp² hybridized. The un hybridized p-orbital overlaps with a p-orbital of oxygen to form a pi bond. The double bond between carbon and oxygen is shorter, stronger, and polarized.

Orbital diagram for the formation of carbonyl group is as follows:

This polarity confirms that there is nucleophilic addition reaction takes place in carbonyl compound.

The double bond of the carbonyl group has a large dipole moment because oxygen is more electronegative than carbon.

Carbonyl carbon act as an electrophile (Lewis acid)

Carbonyl oxygen act as a nucleophile (Lewis base)

Chapter 13: Organic Compounds contaning Nitrogen

Amines

Amines are derivatives of ammonia in which one or more hydrogen atoms are replaced by alkyl group(s).

Amines are classified as primary, secondary and tertiary depending on the number of alkyl groups attached to nitrogen atom.

General methods of preparation

(1) Ammonolysis of alkyl halides and alcohols :

(a) From Ammonolysis of alkyl halides (Hofmann's ammonolysis) : When an aqueous solution of ammonia is heated with alkyl halide all the three types of amines and quaternary ammonium salt are formed.

                                                                                                                                                                                      (Quaternary ammonium salt)

If ammonia is taken in excess, 1° amine is the main product.

(b) Ammonolysis of alcohols : When ROH and NH₃ are passed over At₂O₃ or ThO₂ at 350°  

Call the three types of amines are formed.                   

• Quaternary ammonium hydroxide is not formed.
• If excess of ammonia is used, then main product will be primary amine.

(2) By reduction :

(a) With RCONH₂ : RCONH₂   RCH₂NH₂

(b) With RCN : RCN + 4[H]   RCH₂NH₂

This reaction (b) is called mendius reaction.

The reduction of alkyl isocynides gives secondary amines.

R–NC + 4[H]RNHCH₃

(c) With Oximes : R­–CH=N–OH + 4[H]RCH₂–NH₂ + H₂O

(d) With RNO₂ : RNO₂ + 6[H]  RNH₂ + 2H₂O

Sn/HCl is used in laboratory preparation

(3)     By hydrolysis of :

(a) R–NC : Alkyl isocyanide undergoes hydrolysis with mineral acid and forms alkyl amine.

R­–NC + 2H₂ORNH₂ + HCOOH

(b) RNCO : Alkyl isocyarlate undergoes hydrolysis on heating with KOH and form alkyl amine.

(4) By Hofmann's bromamide reaction (Hofmann's Hypobromite reaction) : This is a general method for the conversion of alkanamides into primary amines having one less carbon.

(5)  From Grignard reagent : Alkyl magnesium iodide reacts with chloramine to yield alkyl amine.

(6) Gabriel phthalimide synthesis : Phthalimide is first treated with KOH to obtain potassium phthalimide which is then treated with alkyl iodide. Then alkyl phthalimide  on hydrolysis yields alkylamine. This method is used in the formation of pure aliphatic primary amines.

Phthalic acid

• Aniline is not formed by this reaction.

(7) Curtius reaction :

(8) Schmidt reaction : In presence of conc. H₂SO₄ alkanoic acid reacts with hydrazoic acid (N₃H) followed by hydrolysis to yield alkylamine.

Chapter 14: biomolecules & polymers

Carbohydrates (Saccharides) : (Latin word : saccharum = sugar)

Concept

Generally, carbohydrates are substances with the general formula Cx(H₂O)y , and were therefore called carboydrates (hydrates of carbon) because they contained hydrogen and oxygen in the same  proportion as in water. How ever, a number of compounds have been discovered which are carbohydrates by chemical behaviour but do not conform to the formula Cx(H₂O)y e.g ,2-deoxyribose, C₅H₁₀O₄.

Hence, carbohydrates are biopolymers of polyhydroxy aldehyde or polyhydroxy ketones. “There monomeric polyhydroxy aldehydes or ketones can also exist in hemiocetal and acetal forms in cyclic structures.”

# Note:

Almost all of these compounds are chiral & optically active. “An exception of this is  1,3-dihydroxypropanone, CH₂OHCOCH₂OH”.

2 Classification of carbohydrates :

(A) Classification on the basis of number of hydrolysed products     

(B) On the Basic of functional group

(C) On basis of carbon atoms.

Configuration of the D-Aldoses

3. Structure :

(i) All form of Aldose or ketose may exist in open chain form as well as in cyclic pyramose/furanose form.         

Cyclie structure of monosaccharide

Disaccharide

Polysaccharide

INTER CONVERSION OF a and b

Haworth projection

Chair conformation structures :

Anomers :

Anomers are diastereomers that differ in the configuration at the acetal or hemiacetal C atom of a sugar in its cyclic form or Anomers are epimers whose conformations differ only about C-1.

For example, a D(+) and b -D(+) glucose are anomers. a-D(–) and b-D(–) fructose are anomers.

Epimers :

Diastereomers with more than one stereocentre that differ in the configuration about only one stereocentre are called epimers.

i. D-Glyceraldehyde and L-glyceraldehyde (this pair is an enantiomer as well as an epimer).

ii. D-Erythrose and L-threose are epimers.

iii. D-glucose and D-galactose are C-4 epimers and

iv. D-idose and D-talose are C-3 epimers.

v.  D-glucose and D-mannose are C-2 epimers. 

vi. Epimerisation of glucose at C-2 gives mannose.

vii. Epimerisation of glucose at C-3 gives allose.

viii.  Epimerisation of glucose at C-4 gives galactose. 

Action of alkalies :

(a) With conc. NaOH glucose first turns yellow resinify. Fructose is not effected by conc. NaOH

(b) With dil. NaOH/KOH both Glucose & Fructose undergo a rearrangement to give equilibrium mixture of three sugars.

This rearrangement is known as Lobry De-bryn. van Ekenstein rearrangement.

4. Properties of Anomers : Mutarotation

When one of the pure glucose anomers dissolve in water, an intersting change in the specific rotation is observed. When the a-anomer dissolves, its specific rotation gradually decreases from an initial value of +112º to +52.7º. When the pure b- anomer dissolves, its specific rotation gradually increases from +19º to the same value of +52.7º. This change (mutation) in the specific rotation is called mutarotation. What is happening is that each solution, initially containing only one anomeric form, undergoes equilibrium to the same mixture of a-and b-forms. The open chain forms is in intermediate in the process.

For mutarotation atleast one hemiacetal group must be present in the sugar therefore all reducing sugars will mutarotate.

(C)    Classification on the basis of Reducing and non Reducing properties :

Monosaccharides :

(A) Glucose  (C₆H₁₂O₆) :

Glucose is the most common monosaccharide. It is known as Dextrose because it occurs in nature principally as the optically active dextrorotatory isomers. It is act as a reducing agent (reduces both Fehling’s solution and ammonical silver nitrate solution). When heated with sodium hydroxide, an aqueous solution of glucose turns brown. It is known as dextrose and found as grapes, honey, cane sugar, starch and cellulose.

Preparation :

(1) By acid hydrolysis of cane sugar (a disaccharide) :

a-Glucose being much less soluble in alcohol than fructose separate out by crystallization on cooling.

(2) By enzymatic action over starch :

Physical properties :

(i) It is white crystalline solids having melting point 146ºC. It is readily soluble in water.

(ii) Glucose is sweet in taste and also optically active (dextro rotatory).

(iii) Glucose shows mutarotation.

Chemical properties :

(1) Reactions due to OH group :

(2) Reactions due to –CHO group :

Glycosides-formation :

(B) Fructose :

• Also said to be fruit sugar
• It occur both in combined as well as free state.
• Fructose is named as fruit sugar because it is present in honey and most sweet fruit in free state.
• It is sweetest monosaccharide and present in cane sugar and insulin in combined state.
• It is also known as a-Laevulose i.e. natural occurring compound is laevorotatory

Preparation :

By acid hydrolysis of cane sugar :

By enzymatic action of sucrose

Note: Glucose and fructose obtained by acid hydrolysis of sucrose can be separated by treating with Ca(OH)₂ which forms calcium glucosate & calcium fructosate. Calcium fructosate being water insoluble. It is seperated out easily

Physical properties :

(1) It is colourless crystalline solid.

(2) It is soluble in water but insoluble in ether ketone and benzene.

(3) It is pentahydroxy ketone and shows mutarotation like glucose.

(4) It is reducing sugar.

Chemical Properties :

Due to OH group:

(1) It froms fructose penta acetate with acetyl chloride :

(2) Reaction due to keto group :

It also form osazone with excess of phenyl hydrazine thus we can say that osazone formation is characteristic of a-hydroxy carbonyl compounds.

Note :  1. Unlike other ketones, fructose can reduced Fehling sol. and Tollen’s reagents it is probably due to formation of an equilibrium between glucose, mannose and fructose.

Chain lengthing :

Chain shortening :

Disaccharides :-

(A) Sucrose : (Sucrose, Invert-sugar C₁₂H₂₂O₁₁)      

(a)  Sucrose (Cane sugar)  a–glucose + b-fructose

Formation of sucrose (C₁₂H₂₂O₁₁)

a–D-glucose + b-D-fructose

C1                     C2  

Pyranose form      Furanose form     

Two monosaccharides are joined together by an oxide linkage formed by loss of water molecule. Such linkage through oxygen atom is called glycosidic linkage.

In sucrose linkage in between C1 of a-glucose and C2 of  b-fructose. Since the reducing group of glucose & fructose are involved in glycosidic bond formation, sucrose is non reducing sugar.

(i) On hydrolysis with dilute acids sucrose yields an equimolecular mixture of D(+)-glucose and
D(–)-fructose :

Since D(–)-fructose has a greater specific rotation than D(+)-glucose, the resulting mixture is laevorotatory. Because of this, hydrolysis of cane-sugar is known as the inversion of cane-sugar or Inversion of sucrose (this is not to be confused with the Walden inversion), and the mixture of sugars are known as invert sugar Ex. D - Glucose & D-Fructose. The inversion (i.e., hydrolysis) of cane-sugar may also be effected by the enzyme invertase which is found in yeast.

(ii) Sucrose is not a reducing sugar, e.g., it will not reduce Fehling’s solution or Tollen’s reagnet. It does not form an oxime or an osazone, and does not undergo mutarotation. This indicates that hemiacetal group is not present in the rings.

Polysaccharides :-

(A) Starch (C₆H₁₀O₅)_n :

(i) Starch is the main contributor of carbohydrates in our diet. It exists exclusively in plants, stored in the seeds, roots, and fibres as food reserve. Example rice, potato.

(ii) Both amylose and amylopectin are composed of D-glucose units.

(iii) The amylose molecule is made up of D-glucose unit joined by a-glycosidic linkages between C-1 of one glucose unit and C-4 of the next glucose unit. The number of D-glucose units in amylose range from 60-300.

(iv)  Amylopectin has a branched-chain structure. It is composed of chains of 25 to 30 D-glucose units joined by a-glycosidic linkages between C-1 to one glucose unit and C-4 of the next glucose unit. These chains are in turn connected to each other by 1, 6-linkages.

The solution of a-amylose gives a blue colour with iodine.

The solution of Amylopectin gives a violet colour with iodine :

(B) Cellulose, (C₆H₁₀O₅)n  :

(i) Cellulose is the main structural material of trees and other plants. Wood is 50% cellulose, while cotton and wool are almost pure cellulose. Other sources of cellulose are straw, corncobs, bagasse, and similar agricultural wastes.

(ii)  Artificial silk, rayon, is used collectively to cover all synthetic or manufactured fibres from cellulose.

(iii)  The nitrates are prepared by the reaction of cellulose with a mixture of nitric and sulphuric acids, and the degree of ‘nitration’ depends on the concentrations of the acids and the time of the reaction. Cellulose trinitrate (12.2 – 13.2%N) is known as gun-cotton and is used in the manufacture of blasting explosives and smokeless powders.

(iv) Non-reducing sugar.

Chapter 15: Chemistry in Everyday Life

1.   Introduction to Drugs and Chemotherapy :

In general the drug may be defined as the substances used in the prevention, diagnosis, treatment or cure of disease in man or animals.

When administered to the ailing individual, its action should be localised at the site where it is desired to act. (In actual practice, there is no drug which behaves in this manner).

Chemotherapy :

“The use of chemicals to destroy infectious micro organisms without causing any injury to the host is called as chemotherapy” .

Drugs and Classification of drugs

(a) On the basis of Pharmacological effect

(b) On the basis of drug action

(c) On the basis of chemical structure

(d) On the basis of molecular targets

Drug Target Interaction :

Macromolecules of biological origin perform various functions in the body for example proteins which perform the role of biological catalysts in the body are called enzymes, and those which are crucial to communication system in the body are called receptors.

Enzymes as Drug Targets :

(a) Catalystic action of enzymes : 

In catalytic activity, enzymes perform two major functions as follows

(i) To hold the substrate for chemical reaction :

(ii) The second function of the enzyme is to provide functional group which will attacks the substrate to carry out chemical reaction.

Receptors as Drug Targets :

Receptors are proteins that are crucial to body’s communication process.

Denticity and Chelation :

Table : 1

Common Monodentate Ligands

Table : 2

Common Chelating Amines

Coordination Number :

The coordination number of the central atom/ion is determined by the number of sigma bonds between the ligands and the central atom/ions i.e. the number of ligand donor atoms to which the metal is directly attached. Pi-bonds, if any, between the ligating atom and the central atom/ion are not considered for the determination of coordination number. The sigma bonding electrons may be indicated by a pair of dots, preceding the donor atom in the ligand formula as in :

[Co(NH₃)₆]³⁺, [Fe(CN)₆]^3–, [Ni(CO)₄], [Co(Cl₄)]^2–.

Some common co-ordination number of important metals are as given below.

Coordination Polyhedron :

The spatial arrangement of the ligand atoms which are directly attached to the central atom/ion defines a coordination polyhedron about the central atom. Figure below shows the shapes of tetrahedral, square planar, octahedral, square pyramidal and trigonal bipyramidal coordination polyhedra. [Co(NH₃)₆]³⁺ has an octahedral geometry, while [PtCl₄]^2– and Ni(CO)₄, are square planar and tetrahedral, respectively.

Oxidation number of Central Atom :

The oxidation number of the central atom is defined as the charge it would carry if all the ligands are removed along with the electron pairs that are shared with the central atom. Metal oxidation number is represented by a Roman numeral in parentheses following the name of the coordination entity. For example oxidation number of iron in [Fe(CN)₆]^3– is +3 and it is written as Fe(III).

Homoleptic and heteroleptic complexes

Complexes in which a metal is bound to only one type of donor groups, e.g., [Cr(NH₃)₆]³⁺, are known as homoleptic. Complexes in which a metal is bound to more than one type of donor groups. e.g., [Co(NH₃)₄Br₂]⁺, are known as heteroleptic.

Nomenclature of Coordination Compounds

Writing the formulas of Mononuclear Coordination Entities :

The following rules are followed while writing the formulas :

(i) The central atom is placed first.

(ii) The ligands are then placed in alphabetical order. The placement of a ligand in the list does not depend on its charge.

(iii) Polydentate ligands are also placed alphabetically. In case of abbreviated ligand, the first letter of the abbreviation is used to determine the position of the ligand in the alphabetical order.

(iv) The formula for the entire coordination entity, whether charged or not, is enclosed in square brackets. When ligands are polyatomic, their formulas are enclosed in parentheses. Ligands abbreviations are also enclosed in parentheses.

(v) There should be no space between the ligands and the metal within a coordination sphere.

(vi) When the formula of a charged coordination entity is to be written without that of the counter ion, the charge is indicated outside the square brackets as a right superscript with the number before the sign. For example, [Co(H₂O)₆]³⁺, [Fe(CN)₆]^3– etc.

(vii) The charge of the cation(s) is balanced by the charge of the anion(s).

Writing the name of Mononuclear Coordination Compounds :

The following rules are followed when naming coordination compounds :

(i) Like simple salts the cation is named first in both positively and negatively charged coordination entities.

Examples :

[Ag(NH₃)₂]Cl,diamminesilver(I)chloride.K₃[Fe(CN)₆],potassium hexacyanidoferrate(III).

(ii)  The ligands are named in an alphabetical order (according to the name of ligand, not the prefix) before the name of the central atom/ion.

Examples :

[Pt(NH₃)BrCl(CH₃NH₂)], amminebromidochloridomethylamineplatinum(II).

[Co(H₂O)₂(ox)₂]–, diaquabis(oxalato)cobaltate(III).

(iii)  Names of the anionic ligands end in –o and those of neutral ligands are the same except aqua for H₂O, ammine for NH₃, carbonyl for CO, thiocarbonyl for CS and nitrosyl for NO. But names of cationic ligands end in–ium. The neutral an cationic are placed within enclosing marks ( ) .

Some more important examples of neutral and cationic ligands are :

tetraphosphorus   —    P₄

dioxygen              —    O₂

octasulphur          —    S₈

(iv) Prefixes mono, di, tri, etc., are used to indicate the number of the one kind of ligands in the coordination entity. When the names of the ligands include a numerical prefix are complicated or whenever the use of normal prefixes creates some confusion, it is set off in parentheses and the second set of prefixes is used.

Example ;

[CoCl₂(NH₂CH₂CH₂NH₂)₂]⁺, dichloridobis(ethane-1,2-diamine)cobalt(III).

[NiCl₂(PPh₃)₂], dichloridobis(triphenylphosphine)nickel(II).

(v) Oxidation state of the metal in cation, anion or neutral coordination entity is indicated by Roman numeral in the parentheses after the name of metal.

(vi) If the complex ion is a cation, the metal is named same as the element. For example, Co in a complex cation is called cobalt and Pt is called platinum. If the complex ion is an anion, the name of the metal ends with the suffix - ate. For example, Co in a complex anion, [Co(SCN)₄]^2– is called cobaltate. For some metals, the Latin names are used in the complex anions.

Examples ;     

[Co(NH₃)₄Cl₂]⁺, pentaamminechloridocobalt(III).

(NH₄)₂ [Co(SCN)₄], ammonium tetrathiocyanato-S-cobaltate(II).

(vii) The neutral complex molecule is named similar to that of the complex cation.

Example ;      

[CrCl₃(py)₃], trichloridotris(pyridine)chromium(III).

(viii) The prefixes cis- and trans- designate adjacent and opposite geometric locations. For examples,

[Pt(NH₃)2Cl₂], cis- and trans-diamminedichloridoplatinum(II),

[CoCl₂(NH₃)₄]⁺, cis- and trans-tetraamminedichloridocobalt(III).

Effective Atomic Number Rule given by Sidgwick :

Effective Atomic Number (EAN) = No. of electron present on the metal atom/ion + No. of electrons  donated by ligands to it.

OR

Effective Atomic Number (EAN) = Atomic no. of central metal – Oxidation state of central metal + No. of electrons donated by ligands.

The complexes in which the EAN of the central atom equals the atomic number of the next noble gas, are found to be extra stable.

Note : The EAN rule is generally found to be not valid in case of most of the complexes but in case of metal carbonyls this rule is found to be valid in all cases except one or two exceptions.

Werner's Theory :

Werner in 1983 presented a theory known as Werner's coordination theory. More important postulates of this theory are :

Most element exhibit two types of valencies :

(a) Primary valency and

(b) Secondary valency.

(a) Primary valency :  

This corresponds to oxidation state of the metal ion. This is also called principal, ionisable or ionic valency. It is satisfied by negative ions and its attachment with the central metal ion is shown by dotted lines.

(b) Secondary or auxiliary valency :

It is also termed as coordination number (usually abbreviated as CN) of the central metal ion. It is non-ionic or non-ionisable (i.e. coordinate covalent bond type). This is satisfied by either negative ions or neutral molecules having lone pair of electrons (e.g., H₂O, NH₃ etc.) or even sometimes by some positive groups. The ligands which satisfy the coordination number are directly attached to the metal atom or ion and shown by thick lines.

Every element tends to satisfy both its primary and secondary valencies. In order to meet this requirement a negative ion may often show a dual behaviour, i.e. it may satisfy both primary and secondary valencies since in every case the fulfillment of coordination number of the central metal ion appears essential. This dual behaviour is represented by both thick and dotted lines. For example, [CoCl(H₂O)₅]Cl₂ is represented as

The ions/groups bound by the secondary valencies have characteristic spatial arrangements corresponding to different coordination number. In the modern terminology, such spatial arrangements are called coordination polyhedra and various possibilities are

C.N. = 2      linear                                         

C.N. = 3      Triangular

C.N. = 4      tetrahedral or square planar       

C.N. = 6      octahedral.

To distinguish between the two types of valencies, Werner introduced the square brackets [ ] to enclose those atoms making up the coordination complex and which are, therefore, not ionized.

On the basis of the above postulates Werner formulated the coordination compounds, CoCl₃ . 6NH₃,
CoCl₃ . 5NH₃ and CoCl₃ . 4NH₃ as : [Co(NH₃)₆]Cl₃, [Co(NH₃)₅Cl]Cl₂ and [Co(NH₃)₄Cl₂]Cl respectively; the species within the square brackets being the coordination entitles (complexes) and the ions outside the square brackets the counter ions. He further postulated that octahedral, square, planar and tetrahedral geometrical shapes are more common in coordination compounds of transition metals. Thus, [Co(NH₃)₆]³⁺, [CoCl(NH₃)₅]²⁺, [CoCl₂(NH₃)₄]⁺ are octahedral entities, while [Ni(CO)₄] and [PtCl₄]^2– are tetrahedral and square-planar, respectively.

Bonding in coordination compounds

Valence bond theory :

The valence bond theory, VBT, was extended to coordination compounds by Linus Pauling in 1931. The formation of a complex involves reaction between a lewis base (ligand) and a lewis acid (metal or metal ion) with the formation of a coordinate-covalent (or dative) bonds between them. The model utilizes hybridisation of  (n-1) d, ns, np or ns, np, nd orbitals of metal atom or ion to yield a set of equivalent orbitals of definite geometry to account for the observed structures such as octahedral, square planar and tetrahedral, and magnetic properties of complexes. The number of unpaired electrons, measured by the magnetic moment of the compounds determines which d-orbitals are used.

These hybrid orbitals are allowed to overlap with ligand orbitals that can donate electron pairs for bonding.

Following table provides the types of hybridisation with different coordination number.

It is to be noted that the type of hybridisation of metal and shape of complex involved can be predicted conveniently, if some characteristic of the complex like magnetic nature, geometry or whether exhibits isomerism or not, etc., be known.

Coordination Number Six.

In the diamagnetic octahedral complex, [Co(NH₃)₆]³⁺, the cobalt ion is in +3 oxidation state and has the electronic configuration represented as shown below.

Co³⁺,[Ar]3d⁶     

[Co(NH₃)₆]³⁺    

Thus, the complex has octahedral geometry and is diamagnetic because of the absence of unpaired electron. Since in the formation of complex the inner d-orbital (3d) is used in hybridisation, the complex is called an inner orbital or low spin or spin paired complex.

The complex [FeF₆]^4– is paramagnetic and uses outer orbital (4d) in hybridisation (sp³d²) ; it is thus called as outer orbital or high spin or spin free complex. So :

Coordination Number Four :

In the paramagnetic  and tetrahedral complex [NiCl₄]^2–, the nickel is in +2 oxidation state and the ion has the electronic configuration 3d⁸. The hybridisation scheme is as shown in figure.

The compound is paramagnetic since it contains two unpaired electrons.

Similarly complex [Ni(CO)₄] has tetrahedral geometry and is diamagnetic as it contains no unpaired electrons. The hybridisation scheme is as shown in figure.

Complexes of  Pd(II) and Pt (II) are usually four-coordinate, square planar, and diamagnetic

Similarly the hybridisation scheme for [Ni(CN)₄]^2– is as shown in figure.

While the valence bond theory, to a large extent, explains the formation, structures and magnetic behaviour of coordination compounds, it suffers from the following shortcomings :

1. A number of assumptions are involved.

2. There is no quantitative interpretation of magnetic data.

3. It has nothing to say about the spectral (colour) properties of coordination compounds.

4. It does not give a quantitative interpretation of the thermodynamic or kinetic stabilities of coordination compounds.

5. It does not make exact predictions regarding the tetrahedral and square-planar structures of 4-coordinate complexes.

6. It does not distinguish between strong and weak ligands.

Magnetic Properties of Coordination Compounds :

Additional information for understanding the nature of coordination entities is provided by magnetic
susceptibility measurements. We have noted that coordination compounds generally have partially filled d orbitals and as such they are expected to show characteristic magnetic properties depending upon the oxidation state, electron configuration, coordination number of the central metal and the nature of the ligand field. It is experimentally possible to determine the magnetic moments of coordination compounds which can be utilized for understanding the structures of these compounds.

The number of unpaired electrons in any complex can be easily calculated from the configuration of the metal ion, its coordination number and the nature of the ligands involved( strong or weak from the spectrochemical series) and after that the magnetic moment of the complexes can be easily calculated using

Magnetic Moment =  Bohr Magneton                 

For metal ions with upto three electrons in the d-orbitals like Ti³⁺, (d1) ; V³⁺ (d²) ; Cr³⁺ (d³) ; two vacant d-orbitals are easily available for octahedral hybridisation. The magnetic behaviour of these free ions and their coordination entities is similar. When more than three 3d electrons are present, like in Cr²⁺ and Mn³⁺ (d⁴) ; Mn²⁺ and Fe³⁺ (d⁵) ; Fe²⁺ and Co³⁺ (d⁶) ; the required two vacant orbitals for hybridisation is not directly available (as a consequence of Hund’s rules). Thus, for d⁴, d⁵ and d⁶ cases, two vacant d-orbitals are  only available for hybridisation as a result of pairing of 3d electrons which leaves two, one and zero unpaired electrons respectively.

The magnetic data agree with maximum spin pairing in many cases, especially with coordination compounds containing d6 ion. However, there are complications with the coordination compounds / species having d⁴ and d⁵ ions.

[Mn(CN)₆]^3– has a magnetic moment equal to two unpaired electrons while [MnCl₆]^3– has a magnetic moment equal to four unpaired electrons.

Similarly [Fe(CN)₆]^3– has magnetic moment of a single unpaired electron while [FeF₆]^3– has a magnetic moment of  five unpaired electrons. [CoF₆]^3– is paramagnetic with four unpaired electrons while [Co(C₂O₄)]^3– is diamagnetic.

This anomalous behaviour is explained by valence bond theory in terms of formation of inner orbitals and outer orbitals complexes.

[Mn(CN)₆]^3– , [Fe(CN)₆]^3– and [Co(C₂O₄)₂]^3– are inner orbital complexes involving d²sp³ hybridisation, the former two are paramagnetic and the latter diamagnetic. [MnCl₆]^3– , [FeF₆]^3– and [CoF₆]^3– are outer orbital complexes involving sp³d² hybridisation and are paramagnetic having four, five and four electrons respectively.

Crystal Field Theory :

The drawbacks of VBT of coordination compounds are, to a considerable extent, removed by the Crystal Field Theory.

The crystal field theory (CFT) is an electrostatic model which considers the metal-ligand bond to be  ionic arising purely from electrostatic interaction between the metal ion and the ligand. Ligands are treated as point charges in case of anions or dipoles in case of neutral molecules. The five d orbitals is an isolated gaseous metal atom/ion have same energy, i.e., they are degenerate. This degeneracy is maintained if a spherically symmetrical field of negative charges surrounds the metal atom/ion. However, when this negative field is due to ligands (either anions or the negative ends of dipolar molecules like NH₃ and H₂O) in a complex, it becomes asymmetrical and the degeneracy of the d orbitals is lost. It results in splitting of the d orbitals. The pattern of spitting depends upon the nature of the crystals field.

(a) Crystal field splitting in octahedral coordination entities :

In an octahedral coordination entity with six ligands surrounding the metal atom/ion, there will be repulsion between the electrons in metal d orbitals and the electrons (or negative charges) of the ligands. Such a repulsion is more when the metal d orbitals is directed towards the ligand than when it is away from the ligand. Thus, the  and  orbitals (axial orbitals) which point towards the axis along the direction of the ligand will experience more repulsion and will be raised in energy ; and the d_xy , d_yz and d_zx orbitals (non-axial) orbitals which are directed between the axis will be lowered in energy relative to the average energy in the spherical crystal field. Thus, the degeneracy of the d orbitals has been removed due to ligand electron-metal electron repulsions in the octahedral complex to yield three orbitals of lower energy, t_2g set and two orbitals of higher energy, e_g set. This splitting of the degenerate levels due to the presence of ligands in a definite geometry is termed as crystal field splitting and the energy separation is denoted by D0 (the subscript o is for octahedral). Thus, the energy of the two e_g orbitals will increase by (3/5)D0 and that of the three t_2g will decrease by (2/5) D0.

The crystal field splitting, D0, depends upon the fields produced by the ligand and charge on the metal ion. Some ligands are able to produces strong fields in which case, the splitting will be large whereas others produce weak fields and consequently result in small splitting of d orbitals. In general, ligands can be arranged in a series in the orders of increasing field strength as given below :

     <

Such a series is termed as spectrochemical series. It is an experimentally determined series based on the absorption of light by complexes with different ligands. For d⁴ configuration, the fourth electron will singly occupy eg orbital (according to Hund’s rule) or will undergo pairing in t_2g orbital, which of these possibilities occurs, depends on the relative magnitude of the crystal field splitting, D0 and the pairing energy, P (P represents the energy required for electron pairing in a single orbital). The two possibilites are :

(i) If D0 < P, the fourth electron enters one of the eg orbitals giving the configuration t³_2ge_g¹. Ligands for which D0 < P are known as weak field ligands and form high spin complexes.

(ii) If D0 > P, it becomes more energetically favourable for the fourth electron to occupy a t_2g orbital with configuration t_2g⁴ e_g⁰. Ligands which produce this effect are known as strong field ligands and form low spin complexes.

(b) Crystal field splitting in tetrahedral coordination entities :

In tetrahedral coordination entity formation, the d orbital splitting is inverted and is smaller as compared to the octahedral field splitting. For the same metal, the same ligands and metal-ligand distances, it can be shown that Dt = (4/9)D0. This may attributes to the following two reasons.

(i) there are only four ligands instead of six, so the ligand field is only two thirds the size ; as the ligand field spliting is also the two thirds the size and

(ii) the direction of the orbitals does not concide with the direction of  the ligands. This reduces the crystal field spliting by roughly further two third. So Dt =  ×  = Do.

Consequently, the orbital splitting energies are not sufficiently large for forcing pairing and, therefore, low spin configurations are rarely observed.

Since Dt < Do, crystal field spliting favours the formation of octahedral complexes.

Colour in Coordination Compounds :

According to the crystal field theory the colour is due to the d-d transition of electron under the influence of ligands. The mechanism of light absorption in coordination compounds is that photons of appropriate energy can excite the coordination entity from its ground state to an excited state. Consider the Ti(III) ion in solution, that is [Ti(H₂O)₆]³⁺. This is a violet colour octahedral complex, where in the ground state of the complex a single electron is present in t2g level. The next higher state available for the transition is the empty eg level. If the light corresponding to the energy of yellow-green is absorbed by the complex, it would excite the electron from t_2g level to e_g level.  Consequently the complex appears violet in colour.

Table below gives the relationship of the wavelength absorbed and the colour observed.

Relationship between the wavelength of light absorbed and the colour observed In some coordination entitles

Note : 

(a) In absence of ligand, crystal field splitting does not occur and as a consequence the substance appears colourless. For example (i) removal of water from violet coloured complex [Ti(H₂O)₆]Cl₃ on heating makes it colourless. (ii) Similarly anhydrous copper sulphate (CuSO₄) is white, but hydrated copper sulphate (CuSO₄.5H₂O) is blue coloured.

(b) The nature of the ligand and the molar ratio of metal : ligand also influence the colour of the complex for example in the pale green complex of [Ni(H₂O)₆], the colour change is absorbed when ethylenediamine is progressively added to it.

Note : Ruby is Al₂O₃ in which 0.5–1% Cr³⁺ ions (d3 electron system) are randomly distributed in the positions normally occupied by Al³⁺. We may consider Cr(III) species as octahedral Cr(III) complexes incorporated into the alumina lattice ; d-d transition of electron at these centres give rise to the colour (red).

Emerland is the mineral beryl (Be₃Al₂Si₆O₁₈) in which Cr³⁺ ions occupy octahedral sites, but in this case low energy corresponding to yellow red and blue is absorbed and light corresponding to green region is transmitted.

Limitations of crystal field theory

(1) It considers only the metal ion d-orbitals and gives no consideration at all to other metal orbitals (such as s, p_x, p_y and p_z orbitals).

(2) It is unable to account satisfactorily for the relative strengths of ligands. For example it gives no explanation as to why H₂O is a stronger ligand than OH^– in the spectrochemical series.

(3) Account to this theory, the bond between the metal and ligands are purely ionic. It gives no account on the partly covalent nature of the metal ligand bonds.

(4) The CFT cannot account for the p-bonding in complexes.

Types of Reagents :

Reagents are of two types :

(i) Electrophiles             

(ii) Nucleophiles

Electrophiles :

Electrophiles are electron deficient species.

Ex.     (positively charged species),

          (species with vacant orbital at central atom).

Nucleophiles and their nucleophilicity :

Nucleophile is a species having negative charge or lone pair of electrons.

They are electron rich species.

Ex.     (l.p on O-atom),   (negaively charged species)

Note :  :CCl₂ is not a nucleophile because it is electron deficient species and act as electrophile.

• Negative ions have more nucleophilic than their neutral species

• Down the group nucleophilicity increases because the more polarizable donar atom is better nucleophyle

         Polarizability size of donar atom

• Across the period nucleophilicity decreases

• Bulky base has less nucleophilic character.

• Effect of solvent : In case of polar aprotic solvents nucleophilicity order of halides is just reversed.

           

Bases and their basicity :

Bases are the species which accept the proton or which donates l.p. of electron to proton.

• Basicity decreases down the group while nucleophilicity increases

         F– > Cl– > Br– > I– 

• Nucleophilicity and basicity order will be same across the period.
• For the same donor atom nucleophilicity and basicity order will be same

Leaving group ability :

• Weaker base is better leaving group.
• More resonance stabilised ion will be better leaving group.
• Weaker the carbon-leaving group bond (C–X) better will be the leaving group.
• If activation energy of a reaction is low then reaction will be fast and leaving group will be better.

Ex.  (a) I– > Br– > Cl– > F–   

 (b) CF₃SO₃– > RCOO– > C₆H₅O– > OH– > 

 (c)  

Note :  More stable anions are weak bases & hence better leaving group.

Types of solvents 

Note : If H atom is attached to oxgyen, nitrogen or sulphur then it is said to be protic solvent.

Nucleophilic substitution reactions of alkyl halides

Alkyl halide undergoes nucleophilic substitution reaction.

Nucleophilic substitution reactions are of two types :

Unimolecular nucleophilic substitution reaction (S_N1) :

• It is a two step process.
• First step is the heterolytic cleavage of carbon halogen bond () to give carbocation intermediate.

         (Ist step is r.d.s.)

• In the second step nucleophile attacks from either side of carbocation to generate product (racemic mixture).

         (IInd is fast step)

Mechanism :

• Carbocation intermediate is formed so rearrangement is possible in S_N1 reaction.

Kinetics :

• Rateµ [Alkyl halide]
• It is unimolecular and first order reaction.
• Rate of S_N1 reaction is independent of the concentration and reactivity of nucleophile.

Stereochemistry :

• Catalyst used is Ag⁺.
• More polar protic solvent is more favourable for S_N1.

         H₂O > ROH > NH3 (order of polar protic solvent).

• In S_N1 reaction carbocation is formed along with anion and to solvate these ions, polar protic solvent is used.
• Decreasing order of reactivity of alkyl halides.   ;   

         R–I > R–Br > R–Cl > R–F

Ex.  

Bimolecular nucleophilic substitution reaction (S_N2) :

It is a single step reaction as the rate of reaction depends upon concentration of substrate as well as nucleophile.

Mechanism :

• In this reaction nucleophile attack from back side on the carbon atom bearing leaving group. it is a concerted reaction where bond breaking and bond formation takes  place simultaneously to achieve a transition state (trigonal bipyramidal shape) where half bond has been formed and half bond has been broken.

Kinetics :  

• rateµ [alkyl halide] [nucleophile]
• It is a bimolecular, one step concerted process.
• It is a second order reaction because in the r.d.s. both species are involved.

Steriochemistry :

Only one product is formed where inversion of configuration takes place.

• Polar aprotic solvent is favourable not polar protic solvent. Becuase in case of polar protic solvent nucleophilicity of anion is decreased due to solvation and such solvation is not possible in case of polar aprotic solvent.
• Electron withdrawing group increases the rate of sN2 reaction.

         O=CH–CH₂–Br > CH₃–O–CH₂–Br > H–CH₂–Br > CH₃–CH₂–Br

Comparision between S_N1  / S_N2  reaction :       

Examples of S_N2 reaction of alkyl halides :

(i)  Williamson’s synthesis of ethers :

• It is used for the preparation of symmetrical as well as unsymmetrical ether.

           R—Br + NaOEt ¾¾®R—OEt + NaBr

• Williamson’s synthesis involve attack of an alkoxide on alkyl halide to give ethers. In place of alkoxide, phenoxide can also be used.
• This is an S_N2 reaction because alkoxide is a strong nucleophile.
• On using 2º & 3º alkylhalide we get alkene not ether as a product.

Ex.   

Note : Vinyl or Aryl halide should not be used because they don’t give SN2 reaction.

Ex.            

(ii) Reaction with AgCN and NaCN :

• NaCN is a more ionic hence ionized to give free  (an ambident nucleophile) where carbon side is more active than nitrogen side. That is why with NaCN, RCN is formed.

• On the other hand AgCN is more covalent so it is not ionized therefore only nitrogen side is free to act as a nucleophile and give isocyanide (R—NC)

(iii)  Reaction with NH₃ :

(iv)   Finkelstein reaction :

R—OH + HI¾¾®R—I  R—H + I₂ (reduced) [Final product is Alkane (R-H) not R-I]

This is a problems that is why iodides are best prepared through halogen exchange reaction. It is also known as Finkelstein reaction. In this reaction R—Cl and R—Br is treated with sodium iodide in acetone.

• NaI is soluble in acetone. In this reaction acetone is used because sodium iodide is soluble in acetone but NaBr and NaCl are insoluble so precipitated out. This eliminates any possibility of reverse reaction.
• It is an SN2 reaction therefore only 1ºRX and 2ºRX is used.

  

Aryl Halides

Preparation of Aryl Halides

1. By Halogenation :

X2 = Cl₂, Br₂  ;  Lewis acid = FeCl₃, AlCl_3, ZnCl₂, etc.

2. By Decarboxylation :

Chemical reaction of Aryl halide (Bimolecular nucleophilic substitution S_N2 Ar)

• An electron withdrawing group at ortho or para positions with respect to a good leaving groups are necessary conditions for S_N2 Ar.

Mechanism :

Intermediate ion is stabilized by resonance and are stable salts called Meisenheimer salts.

• A group that withdraws electrons tends to neutralize the negative charge of the ring and this dispersal of the charge stabilizes the carbanion.

G withdraws electrons : stabilizes carbanion, activates the Ar-S_N2 reaction.

(–(CH₃)₃, –NO₂, –CN, –SO₃H, –COOH, –CHO, –COR, –X)

• A group that releases electrons tends to intensify the negative charge, destabilizes the carbanion, and thus slows down reaction.

G (–NH₂, –OH, –OR, –R) releases electrons : destabilizes carbanion, deactivates  the Ar-S_N2 reaction.

Element effect :- The fact that fluoro is the best leaving group among the halogens in most ArS_N2 but in S_N1 & S_N2 mechanism where fluoro is the poorest leaving group among halogens.

Elimination reactions of alkyl halides

In an elimination reaction two atoms or groups (YZ) are removed from the substrate and generally resulting into formation of p bond.

Unimolecular elimination reaction (E1) :

Proton and leaving group depart in two different step.

Mechanism :

Step 1 : Formation of the carbocation (r.d.s.)

Step 2 : Base ( ) abstracts a proton (fast step)

• Reaction intermediate is carbocation, so rearrangment is possible

Kinetics:

• Rateµ[Alkylhalide]
• It is a unimolecular and first order reaction.
• Reactivity order is similar to S_N1 becuase Carbocation Intermediate is formed in the rds step.

S_N1 v/s E1 :

• In case of alkyl halides S_N1 product is generally more than E1 product

Ex.

Bimolecular elimination reaction (E2) :

• It is a second order reaction because rate of reaction depends upon conc. of substrate as well as base.

Mechanism :

• Orientation of eliminated proton and leaving group are antiparallel or antiperiplanar to each other because in anti conformation the transition state is more stable due to minimum electronic repulsion.

  

• E2 reaction is stereospecific.
• E2 reaction also depends upon size of base which decides major or minor product.

Kinetics : 

• Rate µ [R – X] [Base]       
• This is a single step, bimolecular reaction
• No carbocation intermediate is formed hence there is no rearrangement but a transition state is achieved because it is a single step reaction.
• More favourable substrate is tertiary alkyl halide because it will give more stable alkene according to saytzeff rule.  

Ex.  

Some important terms :

(a) Regioselective reaction : Those reaction in which more than one structural isomeric products are possible, said to be regioselective reaction.

(b) Regiospecific reaction : Those reaction in which only one structural isomer product is formed out of the possible products, said to be regiospecific reaction.

(c) Stereoselective : Those reactions in which mixture of two stereoisomeric products are formed with one major product.

Examples : S_N1, S_N2 and E2

(d) Stereospecific reaction : Those reactions in which two stereoisomeric reactants give two different stereoisomeric products, are called as stereospecific  reactions.

Examples : S_N2 and E2

Note : All stereospecific reactions are stereo selective.

Comparison between E1 and E2 reactions :

E1cB Reaction :

• It is two step reaction.
• In first step base takes proton from adjacent carbon atom of halogen bearing carbon and generate carbanion intermediate.                            

• 

• In second step there is loss of leaving group by carbanion to get alkene :

Ex.   

• b-hydrogen atom should be more acidic which is possible only if carbon atom having b-hydrogen atom should be link to with electron withdrawing group (-m, -I group).
• Leaving group should be more electronegative because it also increases acidity of b-hydrogen atom.
• Experimentally it is found that Ist step i.e. formation of carbanion intermediate is fast step and second step i.e. removal of leaving group is slow step thus r.d.s. in E1cB is second step.

Reactions of chloroform

• Purity of chloroform (presence of phosgene) can be tested before use as anaesthetic by treating with aqueous solution of AgNO₃ because the presence of COCl₂ may cause cardiac failure.
• Chloroform is stored in dark colour bottle containing small amount of ethyl alcohol. (It converts phosgene into diethylcarbonate).

Preparation methods of Aldehydes and Ketones 

By oxidation of alcohols :

Primary alcohols  Aldehydes

Secondary alcohols  Ketones

By dehydrogenation of alcohols :

Dehydrogenation means removal of hydrogen and reagent used is heated copper.

Ozonolysis of alkene :

It is used to get carbonyl compounds from alkene. The reaction is

Note :

(i) During the cleavage of ozonide Zn is used to check further oxidation of aldehyde into acid.

(ii) By this method we can locate double bond in olefin or exact structrue of hydrocarbon can be determined by knowing ozonolysis product i.e. by placing double bond at the place of two carbonyl oxygen atoms of two carbonyl compounds.

(iii) Among the three molecules of carbonyl compounds.

    (a) If one molecule contains two carbonyl groups, then hydrocarbon will be alkadiene.

    (b) If all the three molecules contain two carbonyl group then hydrocarbon will be cycloalkatriene.