CBSE Class 12 Chemistry Notes: Carboxylic Acids – Conversion into functional derivatives

1.  Conversion into functional derivatives

(a)   Conversion into acid chlorides

Examples:

Illustration 1. Benzoyl chloride is prepared from benzoic acid by

                        (A)  Cl₂,                                         (B)  SO₂Cl₂

                        (C)  SOCl₂                                             (D)  Cl₂, H₂O

Solution:          (C)

(a)   Conversion into esters

Conversion into Esters (Esterification):

Carboxylic acid on reacting with alcohols in presence of dehydrating agent (H₂SO₄ or dry HCl gas) gives esters. The reaction is known as esterification.

This reaction is reversible and the same catalyst, hydrogen ion, that catalyzes the forward reaction, esterification, necessarily catalyzes the reverse reaction hydrolysis.

The equilibrium is particularly unfavourable when phenols (ArOH) are used instead of alcohol; yet if water is removed during the reaction, phenolic esters [RCOOAr] are obtained in high yield.

The presence of bulky group near the site of reaction, whether in alcohol or in the acid, slows down esterification (as well as its reverse, hydrolysis).

Reactivity                CH₃OH > 1° > 2° > 3°

In esterification  HCOOH>CH₃COOH>RCH₂COOH > R₂CHCOOH > R₃CCOOH

Conversion into amides

Example:

Illustration 1.    Discuss the reason for that a characteristic reaction of aldehydes and Ketones is one of nucleophilic addition while Acyl compounds yield Nucleophilic substitution product.

Solution:

The initial step in both reactions involves nucleophilic addition at the carbonyl carbon atom. It is after the initial nucleophilic attack has taken place that the two reactions differ. The tetrahedral intermediate formed from an aldehyde or ketone usually accepts a proton to form a stable addition product. By contrast, the intermediate formed from an acyl compound usually eliminates a leaving group: this elimination leads to regeneration of the carbon oxygen double bond and to a substitution product. The overall process in the case of acyl substitution occurs, therefore, by a nucleophilic addition–elimination mechanism. Acyl compounds react as they do so because they all have good leaving groups attached to the carbonyl carbon atom.

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