Acyl halide
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In chemistry, the term acyl halide (also known as acid halide) refers to a compound derived from an acid by replacing a hydroxyl group with a halide group.
If the acid is a carboxylic acid, the compound contains a -COX functional group, which consists of a carbonyl group singly bonded to a halogen atom such as chlorine. The general formula for such an acyl halide can be written RCOX, where R may be, for example, an alkyl group, CO is the carbonyl group, and X represents the halogen atom. See simple diagram at right.
The RCO- group in an acyl halide can be thought of as an acyl group and the -X radical as the halide part of the molecule; for example, -Cl is chloride. Acyl chlorides are the most commonly used acyl halides. Acyl halides are commonly made by halogenating a carboxylic acid, hence the name acid halide.
An example of an acyl halide is acetyl chloride CH3COCl. See simple diagram at right.
Another example is replacing hydroxyl by chloride in a sulfonic acid to create a sulfonyl chloride.
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[edit] Acyl halide synthesis
A common method for the synthesis of acyl halides in the laboratory is by reaction of carboxylic acids with reagents such as thionyl chloride and phosphorus pentachloride for acyl halides, phosphorus tribromide for acyl bromides and cyanuric fluoride for acyl fluorides
[edit] Acyl halide reactions
Acyl halides are artificial (meaning not found in nature), rather reactive compounds often synthesized to be used as intermediates in the synthesis of other organic compounds. For example, an acyl halide can chemically react with:
- water to form a carboxylic acid
- an aromatic compound, using a catalyst such as AlCl3, to form an aromatic ketone. See Friedel-Crafts acylation.
In the above reactions, HX (hydrogen halide or hydrohalic acid) is also formed. For example, if the acyl halide is an acyl chloride, HCl (hydrogen chloride or hydrochloric acid) is also formed.
Because acyl halides are rather reactive compounds, they have toxicity and special precautions should be taken handling them. They are lachrymatory chemicals because they can react with water at the surface of the eye producing hydrohalic and organic acids irritating to the eye. Similar problems can result if one inhales acyl halide vapors.
[edit] Multiple functional groups
A molecule can have more than one acyl halide functional group. For example, "adipoyl dichloride", usually simply called adipoyl chloride, has two acyl chloride functional groups; see the structure at right. It is the dichloride (i.e., double chloride) of the 6-carbon dicarboxylic acid adipic acid. An important use of adipoyl chloride is polymerization with an organic di-amino compound to form a polyamide called nylon or polymerization with certain other organic compounds to form polyesters.
Phosgene (carbonyl dichloride, Cl-CO-Cl) is a very toxic gas which may be thought of as the dichloride of carbonic acid (HO-CO-OH). Both chloride radicals in phosgene can undergo reactions analogous to the preceding reactions of acyl halides. Phosgene is used a reactant in the production of polycarbonate polymers, among other industrial applications.
