In organic chemistry, an acyl chloride (or acid chloride) is an organic compound with the functional group -COCl. Their formula is usually written RCOCl, where R is a side chain. They are 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.
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-:
- (chlorocarbonyl)acetic acid ClOCCH2COOH
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 (formyl 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.
- (CH3CO)2O + HCl → CH3COCl + CH3CO2H
- CH3CH2CO2H + COCl2 → CH3CH2COCl + HCl + CO2
- C6H5CCl3 + H2O → C6H5C(O)Cl + 2 HCl
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):
- RCO2H + SOCl2 → RCOCl + SO2 + HCl
- 3 RCO2H + PCl3 → 3 RCOCl + H3PO3
- RCO2H + 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. 74.6 °C) is easily evaporated as well. The reaction mechanisms involving thionyl chloride and phosphorus pentachloride are similar.
Another method involves the use of oxalyl chloride:
- RCO2H + ClCOCOCl → RCOCl + CO + CO2 + HCl
The reaction is catalysed by dimethylformamide (DMF), which reacts with oxalyl chloride in the first step to give an iminium intermediate, which reacts with the carboxylic acid, abstracting an oxide, and regenerating the DMF catalyst.
Acid chlorides can be used as a chloride source.
- RCO2H + Ph3P + CCl4 → RCOCl + Ph3PO + HCCl3
- RCO2H + C3N3Cl3 → RCOCl + C3N3Cl2OH
Acyl chlorides react with water yielding the carboxylic acid:
- RCOCl + H2O → RCO2H + HCl
This hydrolysis is usually a nuiscance rather than intentional. 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 (when preparing amides) 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 (acyl chloride formation from carboxylic acid, followed by coupling with the alcohol or amine) 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 give the ketone, which is susceptible to the attack by second equivalent to yield the tertiary alcohol. The reaction of acyl halides with certain organocadmium reagents stops at the ketone stage, although cadmium compounds are highly toxic and carcinogenic. The nucleophilic reaction with Gilman reagents also afford ketones, reflecting the low reactivity to these lithium diorganocopper compounds. 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.
Because of the harsh conditions and the reactivity of the intermediates, this otherwise quite useful reaction tends to be messy, as well as environmentally unfriendly.
Low molecular weight acyl chlorides are often lachrymators, and they react violently with water, alcohols, and amines. One way to render inert old acyl chlorides is to slowly add them, dropwise, to some stirred and chilled ethanol or isopropanol, hydrolyzing them to the corresponding esters. The reaction proceeds with great release of heat and HCl fumes, so it should be carried out with proper safety measures. Adding the alcohol (or the water) to the acyl chloride will cause the mixture to heat to boiling, and should be avoided.
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