2. Types of Reagents

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.