In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, ...
, an acyl halide (also known as an acid halide) is a
chemical compound
A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atoms from more than one chemical element held together by chemical bonds. A molecule consisting of atoms of only one element ...
derived from an
oxoacid
An oxyacid, oxoacid, or ternary acid is an acid that contains oxygen. Specifically, it is a compound that contains hydrogen, oxygen, and at least one other element, with at least one hydrogen atom bonded to oxygen that can dissociate to produce ...
by replacing a
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
group () with a
halide group (, where X is a
halogen).
If the acid is a
carboxylic acid (), the compound contains a
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
, 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. A compound containi ...
group () singly bonded to a halogen atom.
The general formula for such an acyl halide can be written RCOX, where R may be, for example, an
alkyl
In organic chemistry, an alkyl group is an alkane missing one hydrogen.
The term ''alkyl'' is intentionally unspecific to include many possible substitutions.
An acyclic alkyl has the general formula of . A cycloalkyl is derived from a cycloa ...
group, CO is the carbonyl group, and X represents the halide, such as
chloride
The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...
.
Acyl chlorides are the most commonly encountered acyl halides, but
acetyl iodide
Acetyl iodide is an organoiodine compound with the formula CH3COI. It is a colourless liquid. It is formally derived from acetic acid. Although far rarer in the laboratory than the related acetyl bromide and acetyl chloride, acetyl iodide is p ...
is the one produced (transiently) on the largest scale. Billions of kilograms are generated annually in the production of
acetic acid.
Preparation
Aliphatic acyl halides
On an industrial scale, the reaction of
acetic anhydride with
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
produces a mixture of acetyl chloride and acetic acid:
:
(CH3CO)2O + HCl -> CH3COCl + CH3CO2H
Common syntheses of acyl chlorides also entail the reaction of
carboxylic acids with
phosgene,
thionyl chloride
Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
, and
phosphorus trichloride
Phosphorus trichloride is an inorganic compound with the chemical formula PCl3. A colorless liquid when pure, it is an important industrial chemical, being used for the manufacture of phosphites and other organophosphorus compounds. It is toxi ...
is used for acyl bromides, which are rarely of value.
Aromatic acyl chlorides
Benzoyl chloride is produced from
benzotrichloride
Benzotrichloride (BTC), also known as α,α,α-trichlorotoluene, phenyl chloroform or (trichloromethyl)benzene, is an organic compound with the formula C6H5CCl3. Benzotrichloride is an unstable, colorless (to yellowish), viscous, chlorinated hydr ...
using either water or
benzoic acid
Benzoic acid is a white (or colorless) solid organic compound with the formula , whose structure consists of a benzene ring () with a carboxyl () substituent. It is the simplest aromatic carboxylic acid. The name is derived from gum benzoin, ...
:
:
C6H5CCl3 + H2O -> C6H5COCl + 2 HCl
:
C6H5CCl3 + C6H5CO2H -> 2 C6H5COCl + HCl
As with other
acyl chlorides, it can be generated from the parent acid and other chlorinating agents
phosphorus pentachloride
Phosphorus pentachloride is the chemical compound with the formula PCl5. It is one of the most important phosphorus chlorides, others being PCl3 and POCl3. PCl5 finds use as a chlorinating reagent. It is a colourless, water-sensitive and moist ...
or
thionyl chloride
Thionyl chloride is an inorganic compound with the chemical formula . It is a moderately volatile, colourless liquid with an unpleasant acrid odour. Thionyl chloride is primarily used as a chlorinating reagent, with approximately per year bein ...
.
Representative laboratory routes to
aromatic
In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...
acyl halides are comparable to those for aliphatic acyl halides. For example,
chloroformylation, a specific type of
Friedel-Crafts acylation which uses
formaldehyde
Formaldehyde ( , ) (systematic name methanal) is a naturally occurring organic compound with the formula and structure . The pure compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde (refer to section ...
as a reagent, or by the
direct chlorination of
benzaldehyde derivative
In mathematics, the derivative of a function of a real variable measures the sensitivity to change of the function value (output value) with respect to a change in its argument (input value). Derivatives are a fundamental tool of calculus. ...
s.
Acyl fluorides
Of commercial interest,
acyl chlorides react with HF to give
acyl
In chemistry, an acyl group is a moiety derived by the removal of one or more hydroxyl groups from an oxoacid, including inorganic acids. It contains a double-bonded oxygen atom and an alkyl group (). In organic chemistry, the acyl group (IUPAC ...
fluorides. Aromatic (as well as aliphatic) acyl fluorides are conveniently prepared directly from carboxylic acids, using stable, inexpensive commodity chemicals: , NBS and in a bench-top protocol.
Cyanuric fluoride converts carboxylic acids to acyl fluorides.
Acyl bromides and iodides
Acyl bromides and iodides are synthesized accordingly but are less common.
Reactions
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 REACT or React may refer to:
Science and technology
*REACT (telescope), a telescope at Fenton Hill Observatory, New Mexico, US
Computing
* React (JavaScript library) , a JavaScript library for building user interfaces, from Facebook
** React Nati ...
with:
*
water
Water (chemical formula ) is an Inorganic compound, inorganic, transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living ...
, to form a
carboxylic acid. This hydrolysis is the most heavily exploited reaction for acyl halides as it occurs in the industrial synthesis of
acetic acid.
:
*an
alcohol to form an
ester
In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...
:
*an
amine
In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen
Hydrogen is the chemical element wi ...
to form an
amide
In organic chemistry, an amide, also known as an organic amide or a carboxamide, is a compound with the general formula , where R, R', and R″ represent organic groups or hydrogen atoms. The amide group is called a peptide bond when it i ...
:
*an
aromatic compound
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past grouping ...
, using a
Lewis acid catalyst such as AlCl
3, to form an aromatic
ketone.
''See''
Friedel-Crafts acylation.
*carboxylic acids to form an
organic acid anhydride
An organic acid anhydride is an acid anhydride that is an organic compound. An acid anhydride is a compound that has two acyl groups bonded to the same oxygen atom. A common type of organic acid anhydride is a carboxylic anhydride, where the pa ...
s.
In the above reactions, HX (
hydrogen halide
In chemistry, hydrogen halides (hydrohalic acids when in the aqueous phase) are diatomic, inorganic compounds that function as Arrhenius acids. The formula is HX where X is one of the halogens: fluorine, chlorine, bromine, iodine, or astatine. ...
or hydrohalic acid) is also formed. For example, if the acyl halide is an acyl chloride, HCl (
hydrogen chloride
The compound hydrogen chloride has the chemical formula and as such is a hydrogen halide. At room temperature, it is a colourless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric water vapor. Hydrogen chloride ga ...
or
hydrochloric acid
Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid
Acid strength is the tendency of an acid, symbol ...
) 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, has two acyl chloride
functional group
In organic chemistry, a functional group is a substituent or moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions regardless of the re ...
s; see the structure at right. It is the dichloride (i.e., double chloride) of the 6-carbon dicarboxylic acid
adipic acid
Adipic acid or hexanedioic acid is the organic compound with the formula (CH2)4(COOH)2. From an industrial perspective, it is the most important dicarboxylic acid: about 2.5 billion kilograms of this white crystalline powder are produced annuall ...
. An important use of adipoyl chloride is
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are many fo ...
with an organic di-amino compound to form a polyamide called
nylon
Nylon is a generic designation for a family of synthetic polymers composed of polyamides ( repeating units linked by amide links).The polyamides may be aliphatic or semi-aromatic.
Nylon is a silk-like thermoplastic, generally made from pe ...
or polymerization with certain other organic compounds to form
polyesters.
Phosgene (carbonyl dichloride, Cl–CO–Cl) is a very toxic gas that is 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.
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
4-Toluenesulfonyl chloride (''p''-toluenesulfonyl chloride, toluene-''p''-sulfonyl chloride) is an organic compound with the formula CH3C6H4SO2Cl. This white, malodorous solid is a reagent widely used in organic synthesis. Abbreviated TsCl or ...
) are
irritants to the eyes, skin and
mucous membrane
A mucous membrane or mucosa is a membrane that lines various cavities in the body of an organism and covers the surface of internal organs. It consists of one or more layers of epithelial cells overlying a layer of loose connective tissue. It i ...
s.
References
External links
{{Authority control
Functional groups