INTRODUCING CARBOXYLIC ACIDS
This page explains what carboxylic acids are, and looks at the
ions that they form in their salts. It also considers their simple
physical properties such as solubility and boiling points. Details
of the chemical reactions of carboxylic acids are described on
separate pages.
What are carboxylic acids?
Carboxylic acids contain a -COOH group
Carboxylic acids are compounds which contain a -COOH group.
For the purposes of this page we shall just look at compounds
where the -COOH group is attached either to a hydrogen atom
or to an alkyl group.
Note: There is no very significant reason for this. For simplicity, I
am just trying to avoid making it look complicated by having either
another active group present in the molecule as well as the COOH, or the presence of a benzene ring.
Benzoic acid (benzenecarboxylic acid) has the -COOH group
attached to a benzene ring. Its physical and chemical properties
are in line with those of any other carboxylic acid of a similar size,
so I haven't felt it necessary to write about it separately.
If you are interested in amino acids, you could follow this link to
the amino acids and proteins menu.
Examples of carboxylic acids
The name counts the total number of carbon atoms in the
longest chain - including the one in the -COOH group. If you
have side groups attached to the chain, notice that you always
count from the carbon atom in the -COOH group as being
number 1.
Note: If you aren't confident about naming organic compounds,
then you might like to follow this link at some point.
Use the BACK button on your browser if you want to return to this
page.
Salts of carboxylic acids
Carboxylic acids are acidic because of the hydrogen in the COOH group. When the acids form salts, this is lost and
replaced by a metal. Sodium ethanoate, for example, has the
structure:
Depending on whether or not you wanted to stress the ionic
nature of the compound, this would be simplified to CH3COONa+ or just CH3COONa.
Notice:


The bond between the sodium and the ethanoate is ionic.
Don't draw a line between the two (implying a covalent
bond). That's absolutely wrong!
Although the name is written with the sodium first, the
formula is always written in one of the ways shown. This
is something you just have to get used to.
Note: We often write the formula of the ion showing the negative
charge on one of the oxygen atoms (as above). This is OK for
many purposes, but is technically wrong. In fact the negative
charge is delocalised over the whole of the -COO end of the ion
and the two carbon-oxygen bonds are identical - not one single
and one double.
This is discussed in more detail elsewhere on the site about halfway down a page about the acidity of organic compounds,
although you would probably have to refer to other pages as well
to understand this properly.
This isn't essential for the purposes of the present page, but if you
choose to follow this link use the BACK button (or HISTORY file
or GO menu) on your browser to return to this page.
Physical properties of carboxylic acids
The physical properties (for example, boiling point and solubility)
of the carboxylic acids are governed by their ability to form
hydrogen bonds.
Boiling points
Before we look at carboxylic acids, a reminder about alcohols:
The boiling points of alcohols are higher than those of alkanes of
similar size because the alcohols can form hydrogen bonds with
each other as well as van der Waals dispersion forces and
dipole-dipole interactions.
Note: Hydrogen bonding in alcohols is discussed in detail in the
introduction to alcohols. If you aren't confident about hydrogen
bonding and other intermolecular forces and their relationship to
physical properties it would be a good idea to read this before you
go on. It is all done in more detail than I have used on this present
page.
Use the BACK button on your browser to return to this page.
The boiling points of carboxylic acids of similar size are higher
still.
For example:
propan-1-ol
CH3CH2CH2OH
97.2°C
ethanoic acid
CH3COOH
118°C
These are chosen for comparison because they have identical
relative molecular masses and almost the same number of
electrons (which affects van der Waals dispersion forces).
The higher boiling points of the carboxylic acids are still caused
by hydrogen bonding, but operating in a different way.
In a pure carboxylic acid, hydrogen bonding can occur between
two molecules of acid to produce a dimer.
This immediately doubles the size of the molecule and so
increases the van der Waals dispersion forces between one of
these dimers and its neighbours - resulting in a high boiling
point.
Solubility in water
In the presence of water, the carboxylic acids don't dimerise.
Instead, hydrogen bonds are formed between water molecules
and individual molecules of acid.
The carboxylic acids with up to four carbon atoms will mix with
water in any proportion. When you mix the two together, the
energy released when the new hydrogen bonds form is much
the same as is needed to break the hydrogen bonds in the pure
liquids.
The solubility of the bigger acids decreases very rapidly with
size. This is because the longer hydrocarbon "tails" of the
molecules get between water molecules and break hydrogen
bonds. In this case, these broken hydrogen bonds are only
replaced by much weaker van der Waals dispersion forces.
Note: The similar case with the solubility of alcohols is discussed
in detail in the introduction to alcohols. If you aren't happy about
the effect of chain length on solubility then it would definitely be
worth following this link.
Use the BACK button on your browser to return to this page.
The energetics of dissolving carboxylic acids in water is made
more complicated because some of the acid molecules actually
react with the water rather than just dissolving in it. This is the
basis for the acidity of these compounds and is discussed on
another page.