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Concept
Acyl chloride
Summary
In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group . Their formula is usually written , where R is a side chain. They are reactive derivatives of carboxylic acids (). A specific example of an acyl chloride is acetyl chloride, . Acyl chlorides are the most important subset of acyl halides.
Where the acyl chloride moiety takes priority, acyl chlorides are named by taking the name of the parent carboxylic acid, and substituting -yl chloride for -ic acid. Thus:
When other functional groups take priority, acyl chlorides are considered prefixes — chlorocarbonyl-:
Lacking the ability to form hydrogen bonds, acyl 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.
The simplest stable acyl chloride is acetyl chloride; formyl chloride is not stable at room temperature, although it can be prepared at –60 °C or below.
Acyl chlorides hydrolyze (react with water) to form the corresponding carboxylic acid and hydrochloric acid:
RCOCl + H2O -> RCOOH + HCl
The industrial route to acetyl chloride involves the reaction of acetic anhydride with hydrogen chloride:
(CH3CO)2O + HCl -> CH3COCl + CH3CO2H
Propionyl chloride is produced by chlorination of propionic acid with phosgene:
CH3CH2CO2H + COCl2 -> CH3CH2COCl + HCl + CO2
Benzoyl chloride is produced by the partial hydrolysis of benzotrichloride:
C6H5CCl3 + H2O -> C6H5C(O)Cl + 2 HCl
Similarly, benzotrichlorides react with carboxylic acids to the acid chloride. This conversion is practiced for the reaction of 1,4-bis(trichloromethyl)benzene to give terephthaloyl chloride:
C6H4(CCl3)2 + C6H4(CO2H)2 -> 2 C6H4(COCl)2 + 2 HCl
In the laboratory, acyl chlorides are generally prepared by treating carboxylic acids with thionyl chloride (). The reaction is catalyzed by dimethylformamide and other additives.
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