Acyl halide
An acyl halide is a chemical compound
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...

 derived from an oxoacid
An oxoacid is an acid that contains oxygen. To be more specific, it is an acid that:#contains oxygen#contains at least one other element#has at least one hydrogen atom bound to oxygen#forms an ion by the loss of one or more protons....

 by replacing a hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...

 group with a halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...


If the acid is a carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

, the compound contains a –COX functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

, which consists of a carbonyl
In organic chemistry, a carbonyl group is a functional group composed of a carbon atom double-bonded to an oxygen atom: C=O. It is common to several classes of organic compounds, as part of many larger functional groups....

 group singly bonded to a halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...

 atom. 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 halide, such as chloride
The chloride ion is formed when the element chlorine, a halogen, picks up one electron to form an anion Cl−. The salts of hydrochloric acid HCl contain chloride ions and can also be called chlorides. The chloride ion, and its salts such as sodium chloride, are very soluble in water...

. Acyl chloride
Acyl chloride
In organic chemistry, an acyl chloride is an organic compound with the functional group -CO-Cl. Their formula is usually written RCOCl, where R is a side chain. They are usually considered to be reactive derivatives of carboxylic acids. A specific example of an acyl chloride is acetyl chloride,...

s are the most commonly encountered acyl halides, but acetyl iodide is the one produced (transiently) on the largest scale. Billions of kilograms are generated annually in the production of acetic acid.

The hydroxyl group of a sulfonic acid
Sulfonic acid
Sulfonic acid usually refers to a member of the class of organosulfur compounds with the general formula RS2–OH, where R is an alkyl or aryl. The formal part of acid, HS2–OH, are formally derivatives of the "parent" inorganic compound with the formula HSO2.-Preparation:Sulfonic acid is...

 may also be replaced by a halogen to produce the corresponding sulfonyl halide. In practical terms this is almost always chloride to give the sulfonyl chloride.


A common laboratory method for the synthesis of acyl halides entails reaction of carboxylic acid
Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

s with reagent
A reagent is a "substance or compound that is added to a system in order to bring about a chemical reaction, or added to see if a reaction occurs." Although the terms reactant and reagent are often used interchangeably, a reactant is less specifically a "substance that is consumed in the course of...

s such as thionyl chloride
Thionyl chloride
Thionyl chloride is an inorganic compound with the formula SOCl2. It is a reactive chemical reagent used in chlorination reactions. It is a colorless, distillable liquid at room temperature and pressure that decomposes above 140 °C. Thionyl chloride is sometimes confused with sulfuryl...

 or phosphorus pentachloride for acyl chlorides, phosphorus pentabromide
Phosphorus pentabromide
Phosphorus pentabromide is a reactive, yellow solid of formula PBr5, which has the structure PBr4+ Br− in the solid state but in the vapor phase is completely dissociated to PBr3 and Br2. Rapid cooling of this phase to 15 K leads to formation of an ionic species [PBr4]+[Br3]-.It can be used in...

 for acyl bromides and
cyanuric fluoride
Cyanuric fluoride
Cyanuric fluoride or 2,4,6-trifluoro-1,3,5-triazine is a chemical compound with the formula 3. It is acolourless, pungent liquid. It has been used as a precursor for fibre-reactive dyes, as a specific reagent for tyrosine residues in enzymes, and as a fluorinating agent.-Preparation and...

 for acyl fluorides.

Aromatic acyl chlorides can be prepared by chloroformylation, a specific type of Friedel-Crafts acylation using formaldehyde
Formaldehyde is an organic compound with the formula CH2O. It is the simplest aldehyde, hence its systematic name methanal.Formaldehyde is a colorless gas with a characteristic pungent odor. It is an important precursor to many other chemical compounds, especially for polymers...

 as the reagent.


Acyl halides are rather reactive compounds often synthesized to be used as intermediates in the synthesis of other organic compounds. For example, an acyl halide can react
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

  • water, to form a carboxylic acid
    Carboxylic acid
    Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...

    . This hydrolysis is the most heavily exploited reaction for acyl halides as it occurs in the industrial synthesis of acetic acid
    Acetic acid
    Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...

  • an alcohol
    In chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....

     to form an ester
    Esters are chemical compounds derived by reacting an oxoacid with a hydroxyl compound such as an alcohol or phenol. Esters are usually derived from an inorganic acid or organic acid in which at least one -OH group is replaced by an -O-alkyl group, and most commonly from carboxylic acids and...

  • an amine
    Amines are organic compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are derivatives of ammonia, wherein one or more hydrogen atoms have been replaced by a substituent such as an alkyl or aryl group. Important amines include amino acids, biogenic amines,...

     to form an amide
    In chemistry, an amide is an organic compound that contains the functional group consisting of a carbonyl group linked to a nitrogen atom . The term refers both to a class of compounds and a functional group within those compounds. The term amide also refers to deprotonated form of ammonia or an...

  • an aromatic compound, using a Lewis acid
    Lewis acid
    ]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...

     catalyst such as AlCl3, to form an aromatic ketone
    In organic chemistry, a ketone is an organic compound with the structure RCR', where R and R' can be a variety of atoms and groups of atoms. It features a carbonyl group bonded to two other carbon atoms. Many ketones are known and many are of great importance in industry and in biology...

    . See Friedel-Crafts acylation.

In the above reactions, HX (hydrogen halide
Hydrogen halide
Hydrogen halides are acids resulting from the chemical reaction of hydrogen with one of the halogen elements , which are found in Group 17 of the periodic table. Astatine is not included in the list because it is very rare, unstable and not found as the acid in substantial quantities...

 or hydrohalic acid) is also formed. For example, if the acyl halide is an acyl chloride, HCl (hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...

 or hydrochloric acid
Hydrochloric acid
Hydrochloric acid is a solution of hydrogen chloride in water, that is a highly corrosive, strong mineral acid with many industrial uses. It is found naturally in gastric acid....

) is also formed.

Multiple functional groups

A molecule can have more than one acyl halide functional group. For example, "adipoyl dichloride", usually simply called adipoyl chloride
Adipoyl chloride
Adipoyl chloride is a di-acyl chloride, with formula C6H8Cl2O2.It is a hazardous chemical that evolves HCl when reacted with water...

, has two acyl chloride functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...

s; see the structure at right. It is the dichloride (i.e., double chloride) of the 6-carbon dicarboxylic acid adipic acid
Adipic acid
Adipic acid is the organic compound with the formula 42. From the industrial perspective, it is the most important dicarboxylic acid: About 2.5 billion kilograms of this white crystalline powder are produced annually, mainly as a precursor for the production of nylon...

. An important use of adipoyl chloride is polymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...

 with an organic di-amino compound to form a polyamide called nylon
Nylon is a generic designation for a family of synthetic polymers known generically as polyamides, first produced on February 28, 1935, by Wallace Carothers at DuPont's research facility at the DuPont Experimental Station...

 or polymerization with certain other organic compounds to form polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...


Phosgene is the chemical compound with the formula COCl2. This colorless gas gained infamy as a chemical weapon during World War I. It is also a valued industrial reagent and building block in synthesis of pharmaceuticals and other organic compounds. In low concentrations, its odor resembles...

 (carbonyl dichloride, Cl–CO–Cl) is a very toxic gas that is the dichloride of carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 (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
PolycarbonatePhysical PropertiesDensity 1.20–1.22 g/cm3Abbe number 34.0Refractive index 1.584–1.586FlammabilityV0-V2Limiting oxygen index25–27%Water absorption – Equilibrium0.16–0.35%Water absorption – over 24 hours0.1%...

 polymers, among other industrial applications.

General hazards

Volatile acyl halides are lachrymatory 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. In general, acyl halides (even non-volatile compounds such as tosyl chloride) are irritant
Irritation or exacerbation, in biology and physiology, is a state of inflammation or painful reaction to allergy or cell-lining damage. A stimulus or agent which induces the state of irritation is an irritant...

s to the eyes, skin and mucous membrane
Mucous membrane
The mucous membranes are linings of mostly endodermal origin, covered in epithelium, which are involved in absorption and secretion. They line cavities that are exposed to the external environment and internal organs...

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