Acyl chloride

In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group -CO-Cl. Their formula is usually written RCOCl, where R is a side chain. They are usually considered to be reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride, CH₃COCl. Acyl chlorides are the most important subset of acyl halides, e.g. acetyl bromide.

Nomenclature

Where the acyl chloride moiety takes priority, acyl chlorides are named by taking the name of the parent carboxylic acid, and substituting -ic acid for -yl chloride. Thus: :acetyl chloride CH₃COCl :benzoyl chloride C₆H₅COCl When other functional groups take priority, acyl chlorides are considered prefixes — chlorocarbonyl-: :(chlorocarbonyl)acetic acid ClOCCH₂COOH

Properties

Lacking the ability to form hydrogen bonds, acid chlorides have lower boiling and melting points than similar carboxylic acids. For example, acetic acid boils at 118 °C, whereas acetyl chloride boils at 51 °C. Like most carbonyl compounds, infrared spectroscopy reveals a band near 1750 cm⁻¹.

Synthesis

Industrial routes

The industrial route to acetyl chloride involves the reaction of acetic anhydride with hydrogen chloride. For benzoyl chloride, the partial hydrolysis of benzotrichloride is useful: :C₆H₅CCl₃ + H₂O → C₆H₅C(O)Cl + 2HCl

Laboratory methods

In the laboratory, acyl chlorides are generally prepared in the same manner as alkyl chlorides, by replacing the corresponding hydroxy substituents with chlorides. Thus, carboxylic acids are treated with thionyl chloride (SOCl₂), phosphorus trichloride (PCl₃), or phosphorus pentachloride (PCl₅): :RCOOH + SOCl₂ → RCOCl + SO₂ + HCl :3 RCOOH + PCl₃ → 3 RCOCl + H₃PO₃ :RCOOH + PCl₅ → RCOCl + POCl₃ + HCl

The reaction with thionyl chloride may be catalyzed by dimethylformamide. In this reaction, the sulfur dioxide (SO₂) and hydrogen chloride (HCl) generated are both gases that can leave the reaction vessel, driving the reaction forward. Excess thionyl chloride (b.p. 79 °C) is easily evaporated as well.

:RCOOH + Ph₃P + CCl₄ → RCOCl + Ph₃PO + HCCl₃ and the use of cyanuric chloride (C₃N₃Cl₃): :

Reactions

Nucleophilic reactions

Acyl chlorides are very reactive. Consider the comparison to its ROOH acid analogue: the chloride ion is an excellent leaving group while the hydroxide is not under normal conditions; i.e. even weak nucleophiles attack the carbonyl. A common reaction which is usually a nuisance is in fact with water yielding the carboxylic acid:

:ROCl + H₂O → RO₂H + HCl

Acyl chlorides can be used to prepare carboxylic acid derivatives, including acid anhydrides, esters, and amides by reacting acid chlorides with: a salt of a carboxylic acid, an alcohol, or an amine respectively. The use of a base, e.g. aqueous sodium hydroxide or pyridine,

Electrophilic reactions

With Lewis acid catalysts like ferric chloride or aluminium chloride, acyl chlorides participate in Friedel-Crafts acylations, to give aryl ketones:

The first step is the Lewis acid-induced dissociation of the chloride: :

This step is followed by nucleophilic attack of the arene toward the acyl group: :

Finally, a chloride atom combines with the released proton to form HCl, and the AlCl₃ catalyst is regenerated:

:

Because of the harsh conditions and the reactivity of the intermediates, this otherwise quite useful reaction tends to be messy, as well as toxic to the health and environment.

Hazards

Because acyl chlorides are such reactive compounds, precautions should be taken while handling them. They are lachrymatory because they can react with water at the surface of the eye producing hydrochloric and organic acids irritating to the eye. Similar problems can result if one inhales acyl chloride vapors.

References

* Category:Functional groups