carboxylic acid synthesis

Synthesis of carboxylic acids

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!

Oxidation of alcohols

primary and secondary alcohols are 
 oxidised by acidified potassium dichromate

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:

The definition which is used simply depends on the actual chemistry taking place, bear in mind that no matter what definition is used they all amount to the same thing, we just use the definition that helps explain most clearly the chemistry taking place. Here it is simplest to think about oxidation as the loss of hydrogen. To oxidise an alcohol two atoms of hydrogen are removed by the oxidising agent. To help you grasp what is happening when an alcohol is oxidised I have colour coded all the hydrogen atoms in the different types of alcohols brown, as shown in the image opposite.

Looking at the image you can see that primary alcohols are oxidised to form aldehydes, while secondary alcohols are oxidised to form ketones. Tertiary alcohols are not able to be oxidised with acidified potassium dichromate.

The oxidising agent which is most commonly used to oxidise alcohols is potassium dichromate (K2Cr2O72-). Potassium dichromate is an orange coloured solid which is a powerful oxidising agent, the chromium atoms in the solid dichromate have an oxidation state of +6 and this means they make excellent electron acceptors or oxidising agents. However in order to act as an oxidising agent it needs to be dissolved in acid, 1M sulfuric acid is normally used for this purpose.

Mild oxidation of alcohols

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!

apparatus set-up for the oxidation of alcohols

The equation for this oxidation reaction is often written as shown below, here the symbol [O] is used to represent the oxidising agent.

equation for the oxidation of ethanol to ethanal

Further oxidation

further oxidation of ethanal to ethanoic acid

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|:

overall equation for the oxidation of ethanol to ethanoic acid


apparatus for hydrolysis of an ester

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

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.

equations for the acid hydrolysis of the ester 
 methyl ethanoate

Base hydrolysis

Potassium or sodium hydroxide are refluxed with an esther to hydrolyse it. Obviously it is not possible to form a carboxylic acid when the ester is refluxed in an alkaline solution. Instead the carboxylate anion of the carboxylic acid is produced. Since this carboxylate anion cannot react with the alcohol to reform the ester this reaction unlike the acid hydrolysis reaction is NOT reversible. Once the base catalysed reflux reaction is complete the solution can then be acidified to form the carboxylic acid and the alcohol.

The equation below shows the product of the base catalysed hydrolysis of methyl ethanoate using sodium hydroxide solution.

equations for the base hydrolysis of the ester
 methyl ethanoate

Amide hydrolysis

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.

equations to show the products of the acidic and alklaine hydrolysis of ethanamide

Nitrile hydrolysis

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:

equations to show acidic and alklaine hydrolysis of nitriles to give carboxylic acids

Key Points