Acid Chlorides

FIELDS OF STUDY: Organic Chemistry; Inorganic Chemistry

ABSTRACT

The characteristic properties and reactions of acid chlorides are discussed. Acid chlorides are useful compounds in both organic and inorganic synthesis reactions for the relatively easy formation of complex molecular structures from simple starting materials.

The Nature of Acid Chlorides

The acid chlorides, also known as acyl chlorides, are characterized by the presence of a carbonyl functional group (C=O) connected by a single bond to a chlorine atom. Acid chlorides are acid derivatives of either alkyl or aryl carboxylic acids, and in fact the acid chloride functional group (−COCl) is essentially a carboxyl group (−COOH) in which the hydroxyl group (−OH) has been replaced by chlorine. The general formula of an acid chloride shows the acid chloride group attached to either an R− or an Ar− placeholder, where R indicates an alkyl group (a basic hydrocarbon) and Ar represents an aryl group (a hydrocarbon derived from an aromatic ring):

Acid chlorides are the most reactive derivatives of carboxylic acids. This relatively high reactivity and the ease with which acid chlorides can be prepared make them extremely versatile reagents in many applications. They are especially prone to reactions with nucleophiles, which typically replace the chlorine atom with another atom or group.

Nomenclature of Acid Chlorides

Acid chlorides are named by replacing the "-ic acid" ending of the parent acid name with "-yl chloride." The acid chloride of benzoic acid, for example, is benzoyl chloride, while the acid chloride of oxalic acid is oxalyl chloride. The naming convention for acid chlorides of more complex acids, in which other functional groups take priority, uses the prefix "chlorocarbonyl-" ahead of the proper name of the parent acid.

Preparation of Acid Chlorides

Acid chlorides can be readily prepared from essentially any carboxylic acid. The standard method of preparation is via reaction with thionyl chloride (SOCl₂), phosphorus trichloride (PCl₃), or phosphorus pentachloride (PCl₅). The reaction with thionyl chloride is generally preferred, as it is easier to obtain the acid chloride in a pure form using this reactant than with either of the phosphorus-based chlorinating agents. Any volatile thionyl chloride can be easily removed by distillation, whereas the phosphorus-based by-products are reactive solids that are not as readily eliminated from the reaction mixture.

The inorganic chlorinating agents used to prepare acid chlorides are themselves acid chlorides of their corresponding acids. Thionyl chloride is the acid chloride of sulfurous acid (H₂SO₃), while phosphorus trichloride and phosphorus pentachloride are acid chlorides of phosphoric acid (H₃PO₄).

Reactions of Acid Chlorides

Acid chlorides are the reagents of choice for acylation reactions, which add an acyl group (RCO−) to a compound. The reaction of an alcohol with an acid chloride can replace the hydrogen atom of the alcohol’s hydroxyl group with the acyl group to form the corresponding ester (RCOOR'), with hydrogen chloride (HCl) as a by-product. Similarly, the reaction of an acid chloride with ammonia (NH₃) or an amine will readily form the corresponding amide compounds. For example, the reaction of octanoyl chloride (C₇H₁₅COCl), a derivative of octanoic acid, with dimethylamine (CH₃NHCH₃) produces N,N-dimethyloctanamide (C₇H₁₅CON(CH₃)₂), where the N,N- prefix indicates that the two methyl groups (−CH₃) are attached to the nitrogen atom.

Perhaps the most important use of acid chlorides is in the formation of ketones by the acylation of benzene and benzene-like aromatic compounds via Friedel-Crafts acylation reactions, which use a Lewis acid such as aluminum trichloride (AlCl₃) or ferric chloride (FeCl₃) to catalyze the reaction. The reaction between propanoyl chloride (C₂H₅COCl) and benzene (C₆H₆), for example, produces the ketone propiophenone, officially named 1-phenyl-1-propanone and also known as ethyl phenyl ketone, according to the following diagram:

The by-product of this reaction is again hydrogen chloride, which is usually given off as a gas. The by-products of such reactions are not a consideration and are seldom, if ever, included in the equations.

PRINCIPAL TERMS

• acid derivative: a compound formed by modifying the structure of an acid, such as an ester created by the reaction between a carboxylic acid and an alcohol.
• functional group: a specific group of atoms having a characteristic structure and corresponding chemical behavior within a molecule.
• nucleophile: a chemical species that tends to react with positively charged or electron-poor species.
• reactivity: the propensity of a chemical species to undergo a reaction under applied conditions.
• R (generic placeholder): a symbol used primarily in organic chemistry to represent a hydrocarbon side chain or other unspecified group of atoms in a molecule; can be used specifically for an alkyl group, with Ar used to represent an aryl group.

Bibliography

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Jones, Mark M., et al. Chemistry and Society. 5th ed. Philadelphia: Saunders Coll., 1987. Print.

Morrison, Robert Thornton, and Robert Neilson Boyd. Organic Chemistry. 6th ed. Englewood Cliffs: Prentice, 1992. Print.

Wuts, Peter G. M., and Theodora W. Greene. Greene’s Protective Groups in Organic Synthesis. 4th ed. Hoboken: Wiley, 2007. Print.