ORGANIC REACTION MECHANISM-FUNDAMENTAL CONCEPTS

In an organic reaction, the organic molecule (called substrate) reacts with an attacking reagent to form one or more intermediates and finally the products.

Substrate + attacking reagent → Intermediate → Products

A sequential account of different steps in which the reactants are converted to products is called reaction mechanism.

Fission of a covalent bond

A covalent bond can be broken either by homolysis or by heterolysis.

1. Homolysis:

In homolysis or homolytic cleavage, each of the bonded atoms gets one of the electrons of the shared pair. Here the movement of a single electron takes place. The single electron movement is shown by half – headed arrow or fish hook arrow ().

The species formed as a result of homolysis is called free radical. These are species which contain an odd electron or an unpaired electron. There are three types of free radicals – primary (10), secondary (20) and tertiary (30). Their stability increases in the order 10 < 20 < 30.

Organic reactions, which take place by homolytic fission are called free radical or homopolar or nonpolar reactions.

2. Heterolysis:

In heterolysis or heterolytic cleavage, the bond breaks in such a manner that the shared pair of electrons remains with one of the parts.

After heterolysis, one atom has a sextet of electron and a positive charge and the other atom has an octet of electron with atleast one lone pair and a negative charge.

For example the bond cleavage in methyl bromide takes place in the following manner.

A species having a carbon atom possessing sextet of electrons and a positive charge is called a

carbocation (carbonium ion). They are of three types – primary, secondary and tertiary.

Carbocations are highly unstable and reactive species. Their stability increases in the order 10 < 20 < 30. The high stability of tertiary carbocations is due to inductive effect and hyper conjugation. In carbocations, carbon atom is in sp2 hybridisation and hence they have trigonal planar (planar triangular) shape.

If the group attached to the carbon atom is less electronegative than C, due to heterolytic cleavage, a species with C atom containing a shared pair of electrons and negative charge is formed.

Such a species carrying a negative charge on carbon atom is called carbanion. They are also unstable and reactive. Their stability increases in the order : 30 < 20 < 10.

The organic reactions which proceed through heterolytic bond cleavage are called ionic or heteropolar or polar reactions.

Nucleophiles and Electrophiles

A reagent that brings an electron pair is called a nucleophile (:Nu) and the reaction is called nucleophilic reaction. Or, nucleophiles are electron rich species attack at electron deficient centre. (The word

nucleophile means nucleus seeking).

Example for nucleophiles are OH, CN, NO , Cl, Br, I, H O, NH , R-NH etc.

2 2 3 2

A reagent that takes away an electron pair is called an electrophile (E+) and the reaction is called electrophilic reaction. Or, electrophiles are electron deficient species attack at electron rich centre. (The word electrophile means electron seeking).

Example for electrophiles are carbocations (R+), -CHO, >CO etc.

Electron displacement effects in covalent bonds

In an organic molecule, the electron displacement may take place either under the influence of an atom or in the presence of an attacking reagent. The important types of electron displacement effects are inductive effect, electromeric effect, resonance effect and hyper conjugation.

1. Inductive effect (I effect):

It is a permanent effect arising due to the shifting of sigma electrons through a carbon chain in presence of an atom or group of atom (having different electronegativity) attached to a carbon chain. This effect propagates only through C – C σ bonds. This effect decreases rapidly as the number of C atoms increases.

E.g. 1-chlorobutane CH3 – CH2 – CH2 – CH2 –Cl

Here Cl is more electronegative than C. So the electron pair in the C – Cl bond is shifted towards Cl and it gets a slight –ve charge (δ) and C gets a slight +ve charge (δ+). This carbon attracts the electron density from the second carbon and so the 2nd carbon gets a relatively smaller positive charge (δδ+).

Here Cl atom attracts electron towards it. So we can say that Cl atom has electron withdrawing effect or – I effect (negative inductive effect). So groups which have the ability to attract electron pairs towards it are called – I effect. Example for such groups are –X (F, Cl, Br, I), nitro (-NO2), Cyano (CN), Carboxy (-COOH), ester (-COOR), aryloxy (-OAr) etc.

Groups which donate electron pairs towards the carbon chain are said to have +I effect or electron donating (releasing) groups. Example for such groups are alkyl groups like methyl (-CH3), ethyl (-CH2-CH3) etc.

2. Electromeric effect (E effect):

 It is defined as the complete transfer of a shared pair of π-electrons to one of the atoms joined by a multiple bond in presence of an attacking reagent. It is a temporary effect. It is possible only in compounds containing multiple bonds( alkene or alkyne). This effect cancels when the attacking reagent is removed from the reaction site. The shifting of the electrons is shown by a curved arrow . There are two types of E effects:

a) Positive Electromeric effect (+E effect): Here the pi electrons are transferred to that atom to which the

The presence of alternate single and double bonds in an open chain or cyclic system is termed as a conjugated system.

E.g. for +R effect groups: – halogen, –OH, –OR, –OCOR, –NH2, –NHR, –NR2, –NHCOR etc.

E.g. for – R effect groups: – COOH, –CHO, >C=O, – CN, –NO2 etc.

3. Hyper conjugation:

It is a permanent effect. In this effect the σ electrons of C—H bond of the alkyl group enter into partial conjugation with the unsaturated system or with the unshared p orbital. i.e. the σ electrons of C –H bonds get delocalised.

e.g. ethyl cation (CH3-CH +)

Hyper conjugation stabilises the carbocation because electron density from the adjacent σ bond helps in dispersing the positive charge. In general, the greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyper conjugation interaction and stabilisation of the cation.

Thus the relative stability of carbocations is in the order: (CH3)3C+ > (CH3)2CH+ > CH3-CH2+ . CH3+.

Here tertiary carbocation has 9, isopropyl has 6, ethyl carbocation has 3 and methyl carbocation has zero hyper conjugative structures.

Hyper conjugation is also called no-bond resonance and it is also possible in alkenes and alkyl arenes.

Types of Organic reactions

Organic reactions can be classified into the following categories:

    1. Substitution reactions
    2. Addition reactions
    3. Elimination reactions
    4. Rearrangement reactions