Carboxylic acids are usually prepared either by the oxidation of alcohols or by the hydrolysis of ester, amides or nitriles. Details on these oxidation and hydrolysis reactions are given below. Most of it should already be familiar to you from your earlier work on organic chemistry!
One thing that is often confusing in chemistry is the use of the words oxidation and reduction, this is simply because there are so many different definitions that chemists use for these two words, for example oxidation can be:
As an example of the oxidation process consider the oxidation of the primary alcohol ethanol to the aldehyde ethanal, the apparatus set-up is shown below can be used to carry out this reaction. The set-up is simple distillation, the alcohol ethanol has a boiling point of 780C while the ethanal has a boiling point of only 230C. The oxidising agent, the dichromate and sulfuric acid are added to the pear shaped flask first then the ethanol is dripped in slowly. Gently warming will cause the ethanal vapour to enter the Liebig condenser where it will liquefy and collect in the flask. It is good practice to cool the flask in iced water since the boiling point of the ethanal is close to room temperature (250C) and from my experience ethanal is very good at giving you a thumping headache!
The equation for this oxidation reaction is often written as shown below, here the symbol [O] is used to represent the oxidising agent.
The aldehyde ethanal produced by the oxidation
of the alcohol ethanol can be further oxidised
to a carboxylic acid, in this case ethanoic acid. The same potassium
or sodium dichromate can be used to oxidise
the ethanal but this time the conditions are made more severe. Concentrated
sulfuric acid is used along with more dichromate in order to carry out this
second oxidation reaction.
This time we will define oxidation as the addition of oxygen, slightly
different from the example above where we defined oxidation as a loss
of hydrogen, but remember at the end of the day it all amounts to exactly to same thing! Ethanal can be
oxidised to
ethanoic acid. However the apparatus set-up will have to be modified.
The ethanol as above is oxidised to
the aldehyde ethanal, however ethanal is very volatile and evaporates easily, so
we will have to set-up a reflux experiment.
The set-up is shown opposite. Here the ethanol is oxidised to ethanal which
will evaporate and enter the Liebig condenser where
it will be liquefied and simply drip back down into the oxidising mixture
of sulfuric acid and dichromate. After around 20
minutes or so the oxidation reaction should be complete. Simply rearrange
the apparatus back into a distillation set-up and distil off
the ethanoic acid (boiling point 1180C) produced.
Overall we combine the two oxidation equations above to get|:
Hydrolysis is the breaking up of a substance using water or the elements found in water,
that is H+ or OH-. Hydrolysis using water alone is usually very slow
but hydrogen ion (H+) or hydroxide ions (OH-) can be used to
speed up or catalyse the hydrolysis reaction. That is we can use dilute acids or alkalis to speed up
the break down or hydrolysis of a compound.
Hydrolysis of an ester is usually carried out by
simply heating an ester under reflux conditions with either dilute sulfuric or hydrochloric acid in the
case of acid hydrolysis or with dilute sodium or potassium hydroxide in the case of alkaline hydrolysis.
The ester is broken down by water with the hydrogen ions (H+) in the acid or the hydroxide ions
(OH-) in the alkali acting as catalysts.
Acid hydrolysis is carried out by simply refluxing the ester with a dilute acid such as sulfuric or hydrochloric acids. Acid hydrolysis of an ester simply splits the ester back up into the alcohol and carboxylic acid it was made from. This hydrolysis is simply the reverse of the esterification reaction that created the ester. Like the esterification reaction that created the ester this reaction is reversible and a large excess of water is needed to force the position of equilibrium to the right-hand side, that is produce more alcohol and carboxylic acid. The example below shows the products of the hydrolysis of the ester methyl ethanoate.
Amides like ester can be hydrolysed using acids or bases to yield carboxylic acids. The amides are refluxed with either concentrated acid or alkali. The equations below show the hydrolysis of the amide ethanamide but they apply equally to all amides.
Nitriles like amides above can refluxed with either concentrated acid or alkalis solutions to yield carboxylic acids or their salts. Equations for the hydrolysis of ethanenitrile are outlined below:
