Acid : Preparation, Reaction


Acid or acyl chlorides may be prepared by the replacement of the hydroxyl in the carboxyl group by chlorine, are also known as acyl chlorides because they contain the acyl group.


According to the I.U.P.A.C. system the class suffix of the acyl chlorides is –oyl; the common names are formed by changing the suffix-ic of the trivial name of the acid into –yl, e.g.,

CH3COCl                                            (CH3)2CHCOCl
Ethanol chloride                             2-methylpropanoyl chloride
Acetylchloride                                 isobutyryl chloride

If the carboxyl group is considered as a substituent, then according to the I.U.P.A.C. system the nomenclature of all substances containing acyl groups is in all cases based on the name ‘carbonyl’ for the CO group. e.g.,

CH3CH2CH2COCl                              propane-1-carbonyl chloride

When naming amides, the ‘yl’ is elided before the suffix amide, e.g.,

CH3CH2CH2CONH2              propane-1-carnpmamide

General methods of preparation

1.  The acid is heated with phosphorus trichloride or pentachloride, e.g.,

3RCOOH   +   PCl3        3RCOCl   +   H3PO3
RCOOH   +   PCl5        RCOCl   +   HCl   +   COCl3

The reaction with phosphorus trichloride is accompanied by the formation of small amounts of volatile phosphorus compounds, e.g.,

RCO2H   +   PCl3        RCO2PCl2   +   HCl

2. By distilling the salt of the acid with either phosphorus trichloride, phosphoryl chloride or sulphuryl chloride, e.g.,

3CH3CO2Na   +   PCl3        3CH3COCl   +   Na3PO3
2CH3CO2Na   +   POCl3        2CH3COCl   +   NaCl   +   NaPO3
(CH3CO2)2Ca   +   SO2Cl2        2CH3COCl    +    CaSO4

This method is used industrially since the salts are cheaper than the acid.

General properties of acid

The lower acids are colorless liquids with irritating odours; the higher members are colorless solids. The chlorine atom is very reactive and so the acid chlorides are important regents. The infra-red absorption region of the carbonyl group in acyl chlorides is 1815-1785 cm-1 (s).

Reactions of acid

1.  The acid are readily hydrolysed by water, wthe lower members reacting vigorously :

RCOCl   +   H2O        RCO2H   +   HCl

Acid usually react rapidly with compounds containing ‘active’ hydrogen atoms, i.e. hydrogen attached to oxygen, nitrogen or sulphur; e.g., esters are formed with alcohols:

R1COCl   +   R2OH       R1COOR2   +   HCl

2.  Acid may be reduced catalytically to aldehydes or to alcohols:

RCOCl    (H2+Pd)     RCHO    (H2)     RCH2OH

3.  acid react with salts of the acids to form acid anhydrides

R1COCl   +   R2COONa   ⟶    R1CO.O.OCR2   +    NaCl

4. Acid may be used in the Friedel-Crafts reaction to produce an aromatic ketone; e.g., acetyl chloride reacts with benzene in the presence of anhydrous aluminium chloride to form acetophenone:

C6H6   +   CH3COCl    (AlCl3)     C6H5COCH3   +   HCl

5.  Acid react with Grignard reagents to produce ketones or tertiary alcohols, according to the conditions, e.g., acetyl chloride forms butanone with ethylmagnesium iodide:

CH3COCl   +   C2H5MgI       CH3COC2H5   +   MgClI