3. Introduction
Acid halides are the derivatives of acids in which the OH of carboxyl group has been
replaced by a halogen atom
The acid halides are also referred to as acyl halides; the acyl group is R-CO-.
The acid chlorides are by far the most important of all the acid halides and will be discussed in detail. Their
type formula may be represented as,
R C C l o r R C O C l o r R C O C l
O
4. Nomenclature
They are named by dropping the ending -ic acid from either the common name or
the IUPAC name and adding the suffix -yl chloride or halide.
5.
6. Methods Of Preparation
Acid chlorides are prepared from appropriate carboxylic acids:
1) By reaction with thionyl chloride (SOCI2):
Thionyl Chloride reacts with carboxylic acids to produce an acyl chloride, giving
off sulfur dioxide and hydrogen chloride gases. For example:
This method is superior to others as the by-products being gases escape leaving the acid
chloride in the pure state.
7. Cont…
2) By reaction with phosphorus tri-chloride (PCI3)
When a carboxylic acid reacts with phosphorus tri-chloride ,a mixture of the acyl chloride and
phosphorus acid is obtained.
For example :
8. 3) By reaction with phosphorus pentachloride (PCI5)
This reaction leaves a liquid mixture of acyl chloride and a phosphorus compound, phosphorus
tri-chloride oxide (phosphorus oxychloride, POCl3).
For example:
It is possible to prepare acid bromides and iodides by reactions analogous to those used in the
preparation of chlorides, However, in comparison to the chlorides, they are seldom used.
9. PHYSICAL PROPERTIES
(1)All simple acid chlorides are colorless liquids of lower boiling point than the correspondingacid
(2) They have sharp pungent smell, and are lachrymators (tear-producing).
(3) They emit pale fumes of hydrogen chloride when exposed in air on account of their reactionwith
water vapor.
(4) They are insoluble in water but slowly dissolve in it because of hydrolysis.
(5) The C=O stretching band in IR Spectrum occurs at frequency range 1780-1850
cm¹.
10. Chemical Properties
In acyl chloride (RCOCI), we have a carbonyl group attached to a highly
electronegative chlorine atom. Due to the electron-withdrawing effect of the Cl atom,
the electron density on the carbonyl carbon is diminished further. As a result, acid
chlorides are readily attacked by nucleophiles, whereby the Cl atom is replaced by a
nucleophile.
11. Cont….
Thus, acid chlorides are very reactive compounds, their characteristic reactions
being nucleophilic substitution reactions are as:
They react with weak nucleophiles as water, alcohols, ammonia and amines to produce
the corresponding acid, ester, amide or substituted amides
12. Cont…
Some of their important reactions are listed below:
Hydrolysis: Acid chlorides react with water to form carboxylic acids.
13. Cont…
Reaction with Alcohol. They react with an alcohol when the chlorine atom is
displaced by alkoxy group.-OR', to produce an ester:
H
3
C C C
l
O
+H
O
C
2
H
5 H
3
C C O
C
2
H
5 + H
C
l
O
14. Cont…
Reaction with Ammonia:
Acyl chloride reacts violently with a cold concentrated solution of ammonia. A white
solid product is formed which is a mixture of ethanamide (an amide) and ammonium
chloride.
15. Cont..
Reaction with 1 ° and 2 ° Amines. Acid halides react with 1° and 2° amines to
produce substituted amides.
16. Cont…
Reaction with Salts of Carboxylic Acids
Acid chlorides react with carboxylic acids to form anhydrides.
17. Cont…
Friedel-Crafts Reaction:
Acylation is the term given to substituting an acyl group such as CH3CO- onto another
molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double
bond. The most common example of an acyl group is the ethanoyl group, CH3CO-, and
this group is used throughout this article. If benzene is reacted with ethanoyl chloride
in the presence of an aluminium chloride catalyst, the equation for the reaction is as
follows:
