Acid : Preparation, Reaction

Acid

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.

Nomenclature

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