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Acyl halides|Sureden:Your Education Partner

Acyl halides

Nomenclature, preparation and properties of i. Acid halides ii. Acid amides iii. Acid anhydrides and iv. Esters

Introduction of carboxylic acid derivatives

Carbon compounds containing a carboxyl functional group -COOH are called carboxylic acids. A carboxyl group is constituted of two groups - a carbonyl group

and a hydroxyl group -OH. Carboxylic acids may be aliphatic (R-COOH) or aromatic (Ar-COOH) depending on whether -COOH group is attached to aliphatic alkyl chain or aryl groups respectively.

Aliphatic monocarboxylic acids (containing one carboxyl group) are known as fatty acids because some of their higher members

(C12 - C18) like palmitic acid and stearic acid are found in natural fats as esters of glycerol.

Functional derivatives of acid refer to series of closely related organic compounds which can be structurally obtained by the replacement of -OH part of carboxyl group of the acids by -X, -NH2, -OR' or -OCOR respectively.

Acid chlorides

These are derivatives of carboxylic acids in which hydroxyl (-OH) part of carboxyl group in which hydroxyl (-OH) part of carboxyl group is replaced by halo group. The most reactive compounds among acyl halides are chloro compounds.

Acyl halides (sometimes also called acid halides) are named by identifying the acyl group (RCO) and then the halide. The name of the acyl group is derived from the name of the corresponding -ic acid by replacing the ending -ic acid by -yl or carboxylic acid by -carbonyl.

The most important acyl halides are acyl chlorides because they are more easily prepared, more stable and less expensive.

Preparation

The hydroxyl group of carboxylic acid like that of alcohols are easily replaced by chlorine atom on heating with PCl5, PCl3 or SOCl2.

RCOOH PCl5 ⟶ RCOCl + POCl3 + HCl

3RCOOH + PCl5 ⟶ 3RCOCl + H3PO3

RCOOH + SOCl2 ⟶ RCOCl + SO2 + HCl

Of the three, thionyl chloride is preferred because the other products are gaseous and excess from the reaction mixture, which makes the purification of the acyl chloride easier. Aqueous halogen acids are not used with carboxylic acids because the acyl chloride is easily hydrolyzed by water.

Reaction of acyl halides

Acyl halides undergo nucleophilic acyl substitution with several nucleophiles.

i) They react with water to give carboxylic acids. This is called hydrolysis.

ii) They react with ammonia and amines to give amides. The reaction involving cleavage of a bond with ammonia is called ammonolysis.

Tertiary amines do not react with acyl chlorides.

iii) Acyl chlorides react with alcohols and phenols to give esters. The cleavage of a bond with alcohol is called alcoholysis.

iv) Acyl chlorides react with salts of carboxylic to form acid anhydrides.

In these reactions, an acyl group is transferred to the nucleophiles. These reactions are known as acylation reactions. Acylation is generally done in the presence of a base in order to neutralise the HX formed.

Aliphatic acyl chloride are very reactive acylating agents. Aromatic acyl chlorides like benzoyl chloride are less reactive.

Schotten - Baumann reaction is the acylation of alcohols, phenol and amines by shaking with aromatic acyl chloride in the presence of a base usually aqueous NaOH. For eg.,

ROH – C6H5 COCl RCOC6H5 + HCl

These are also called benzoylation reaction.

v) Acid chlorides acylate aromatic hydrocarbons in the presence of anhydrous in the presence of anhydrous aluminium chloride to yield aromatic ketones. This is called Friedel - Craft's acylation reaction.

This is a good method for preparation of aromatic ketones. This reaction however fails when electron withdrawing group is present on the aromatic ring.

Reduction of acyl halides

Acyl halides are reduced to primary alcohols with LiAlH4 and NaBH4.

RCOCl + LiAlH4 (or NaBH4) ⟶ RCH2OH

Partial reduction of acyl chloride to aldehydes can also be done by catalytic hydrogenation over palladium catalyst supported on barium sulphate. Small accounts of quinoline and sulphur are added to the reaction which does not allow further reduction to alcohol. This reaction is known as Rosen Mund reduction.

 

Related Keywords
12    PMT    Chemistry    Aldehydes     Ketones and Carboxylic Acids    Acyl halides