Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution (Page 3 of 4) in pdf

Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Page 3

Esters are polar and have higher boiling points than alkanes of comparable size and shape. Esters don’t form

Section 20.7

hydrogen bonds to other ester molecules so have lower boiling points than analogous alcohols. They can form

hydrogen bonds to water and so are comparable to alcohols in their solubility in water.

Esters react with Grignard reagents and are reduced by lithium aluminum hydride (Table 20.4).

Section 20.8

Ester hydrolysis can be catalyzed by acids and its mechanism (Mechanism 20.3) is the reverse of the

Section 20.9

mechanism for Fischer esterification. The reaction proceeds via a tetrahedral intermediate.

Ester hydrolysis in basic solution is called saponification and proceeds through the same tetrahedral

Section 20.10

intermediate (Mechanism 20.4) as in acid-catalyzed hydrolysis. Unlike acid-catalyzed hydrolysis,

saponification is irreversible because the carboxylic acid is deprotonated under the reaction conditions.

Esters react with amines to give amides.

Section 20.11

Thioesters have a less-stabilized carbonyl group than esters, and their reactions are characterized by more

Section 20.12

negative values of DG8.

Although the rates of hydrolysis of thioesters and esters are similar, thioesters are more reactive toward

nucleophilic acyl substitution by amines. Acetyl coenzyme A is a thioester involved in many biological

nucleophilic acyl substitutions.

Amides having at least one NOH unit can form intermolecular hydrogen bonds with other amide molecules.

Section 20.13

Compounds of this type have higher melting and boiling points than comparable compounds in which NOH

bonds are absent. Hydrogen bonding in amides influences the conformations of proteins.

Amides are normally prepared by the reaction of amines with acyl chlorides, anhydrides, or esters.

Carboxylic Acid Derivatives: Nucleophilic Acyl Substitution Page 3