Download INTRODUCING ACYL CHLORIDES (acid

INTRODUCING ACYL CHLORIDES (acid
chlorides)
This page explains what acyl chlorides are and looks at their
simple physical properties such as boiling points. It introduces
their chemical reactivity in a general way, but details of specific
reactions are given on separate pages - see the acyl chlorides
menu (link at the bottom of the page).
What are acyl chlorides?
Acyl chlorides as "acid derivatives"
A carboxylic acid such as ethanoic acid has the structure:
There are a number of related compounds in which the -OH
group in the acid is replaced by something else. Compounds like
this are described as acid derivatives.
Acyl chlorides (also known as acid chlorides) are one example
of an acid derivative. In this case, the -OH group has been
replaced by a chlorine atom.
The acyl group
The acyl group is a hydrocarbon group attached to a carbon-
oxygen double bond:
For UK A level purposes, the "R" group is normally restricted to
an alkyl group. It could, however, equally well be a group based
on a benzene ring.
Naming acyl chlorides
The easiest way of thinking about the names is to see the
relationship with the corresponding carboxylic acid:
carboxylic acid
name
acyl chloride
name
acyl chloride
formula
ethanoic acid
ethanoyl chloride
CH3COCl
propanoic acid
propanoyl
chloride
CH3CH2COCl
butanoic acid
butanoyl chloride
CH3CH2CH2COCl
If you have something substituted into the hydrocarbon chain,
the carbon in the -COCl group counts as the number 1 carbon.
For example, 2-methylbutanoyl chloride is:
Note: Hardly anyone ever mentions methanoyl chloride,
HCOCl - derived from methanoic acid. That is because
methanoyl chloride is very unstable, decomposing to give
carbon monoxide and HCl.
Physical properties of acyl chlorides
Appearance
An acyl chloride like ethanoyl chloride is a colourless fuming
liquid. The strong smell of ethanoyl chloride is a mixture of the
smell of vinegar (ethanoic acid) and the acrid smell of hydrogen
chloride gas.
The smell and the fumes are because ethanoyl chloride reacts
with water vapour in the air. The reaction with water is given in
detail on another page. (Find it from the acyl chlorides menu link at the bottom of this page.)
Solubility in water
Acyl chlorides can't be said to dissolve in water because they
react (often violently) with it. The strong reaction means that it is
impossible to get a simple aqueous solution of an acyl chloride.
Boiling points
Taking ethanoyl chloride as typical:
Ethanoyl chloride boils at 51°C. It is a polar molecule, and so
has dipole-dipole attractions between its molecules as well as
van der Waals dispersion forces.
However, it doesn't form hydrogen bonds. Its boiling point is
therefore higher than, say, an alkane of similar size (which has
no permanent dipoles), but not as high as a similarly sized
alcohol (which forms hydrogen bonds in addition to everything
else.)
Note: If you aren't happy about intermolecular forces
(including van der Waals dispersion forces and hydrogen
bonds) then you really ought to follow this link before you go
on.
Use the BACK button on your browser to return to this page.
Reactivity of acyl chlorides
Substitution of the chlorine atom by other groups
Acyl chlorides are extremely reactive, and in their reactions the
chlorine atom is replaced by other things.
In each case, in the first instance, hydrogen chloride gas is
produced as steamy acidic fumes. However, in some cases the
hydrogen chloride goes on to react with one of the substances in
the reaction mixture.
Taking ethanoyl chloride as typical, the initial reaction is of this
kind:
The reactions involve things like water, alcohols and phenols, or
ammonia and amines. All of these particular cases contain a
very electronegative element with an active lone pair of
electrons - either oxygen or nitrogen.
Note: You can find details of all these reactions from the
acyl chlorides menu (link below).
If you are interested in exploring the general mechanism for
these reactions, you will find it by following this link to
another part of the site dealing with nucleophilic additionelimination reactions. If you want mechanisms for specific
reactions you could explore other pages from the
nucleophilic addition-elimination menu as well - but read the
general mechanism first.