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-:
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. The simplest stable acyl chloride is ethanoyl chloride or acetyl chloride; methanoyl chloride is not stable at room temperature, although it can be prepared at –60 °C or below. Acyl chloride is not soluble in water. Instead, it decomposes in water.
Synthesis
Industrial routes
The industrial route to acetyl chloride involves the reaction of acetic anhydride with hydrogen chloride: Propionyl chloride is produced by chlorination of propionic acid with phosgene: Benzoyl chloride is produced by the partial hydrolysis of benzotrichloride:
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, phosphorus trichloride, or phosphorus pentachloride : The reaction with thionyl chloride may be catalyzed by dimethylformamide. In this reaction, the sulfur dioxide and hydrogen chloride generated are both gases that can leave the reaction vessel, driving the reaction forward. Excess thionyl chloride is easily evaporated as well. The reaction mechanisms involving thionyl chloride and phosphorus pentachloride are similar; the mechanism with thionyl chloride is illustrative: Another method involves the use of oxalyl chloride: The reaction is catalysed by dimethylformamide, which reacts with oxalyl chloride in the first step to give the iminium intermediate. The iminium intermediate reacts with the carboxylic acid, abstracting an oxide, and regenerating the DMF catalyst. Finally, methods that do not form HCl are also known, such as the Appel reaction: and the use of cyanuric chloride :
Reactions
Nucleophilic reactions
Acyl chlorides react with water yielding the carboxylic acid: Acyl chlorides are used to prepare 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, or excess amine is desirable to remove the hydrogen chloride byproduct, and to catalyze the reaction. While it is often possible to obtain esters or amides from the carboxylic acid with alcohols or amines, the reactions are reversible, often leading to low yields. In contrast, both reactions involved in preparing esters and amides via acyl chlorides are fast and irreversible. This makes the two-step route often preferable to the single step reaction with the carboxylic acid. With carbon nucleophiles such as Grignard reagents, acyl chlorides generally react first to give the ketone and then with a second equivalent to the tertiary alcohol. A notable exception is the reaction of acyl halides with certain organocadmium reagents which stops at the ketone stage. The nucleophilic reaction with Gilman reagents also afford ketones, due to their lesser reactivity. Acid chlorides of aromatic acids are generally less reactive those of alkyl acids and thus somewhat more rigorous conditions are required for reaction. Acyl chlorides are reduced by lithium aluminium hydride and diisobutylaluminium hydride to give primary alcohols. Lithium tri-tert-butoxyaluminium hydride, a bulky hydride donor, reduces acyl chlorides to aldehydes, as does the Rosenmund reduction using hydrogen gas over a poisoned palladium catalyst.
