Preparation of Carboxylic Acids 2

This page looks at ways of making carboxylic acids in the lab by the oxidation of primary alcohols or aldehydes, and by the hydrolysis of nitriles.

Chemistry of the reactions

Primary alcohols and aldehydes are normally oxidised to carboxylic acids using potassium dichromate(VI) solution in the presence of dilute sulfuric acid. During the reaction, the potassium dichromate(VI) solution turns from orange to green. The potassium dichromate(VI) can just as well be replaced with sodium dichromate(VI). Because what matters is the dichromate(VI) ion, all the equations and color changes would be identical.

Primary alcohols are oxidized to carboxylic acids in two stages - first to an aldehyde and then to the acid. We often use simplified versions of these equations using "[O]" to represent oxygen from the oxidizing agent. The formation of the aldehyde is shown by the simplified equation:

"R" is a hydrogen atom or a hydrocarbon group such as an alkyl group. The aldehyde is then oxidised further to give the carboxylic acid:

If you start with an aldehyde, you are obviously just doing this second stage. Starting from the primary alcohol, you could combine these into one single equation to give:

$RCH_2OH + 2[O] \rightarrow RCOOH + H_2O$

For example, if you were converting ethanol into ethanoic acid, the simplified equation would be:

$CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O$

It is possible that you might want to write proper equations for these reactions rather than these simplified ones. You can work these out from electron-half-equations. How you do this is described in detail elsewhere on the site.

The complete equation for the conversion of a primary alcohol to a carboxylic acid is:

$3RCH_2OH + 2Cr_2O_7^{2-} + 16H^+ \rightarrow 3RCOOH + 4Cr^{3+} + 11H_2O$

or if you were starting from an aldehyde is:

$3RCHO + Cr_2O_7^{2-} + 8H^+ \rightarrow 3RCOOH + 3Cr^{3+} + 4H_2O$

Doing the reactions

It would actually be quite uncommon to make an acid starting from an aldehyde, but very common to start from a primary alcohol. The conversion of ethanol into ethanoic acid would be a typical example. The alcohol is heated under reflux with an excess of a mixture of potassium dichromate(VI) solution and dilute sulphuric acid. Heating under reflux (heating in a flask with a condenser placed vertically in it) prevents any aldehyde formed escaping before it has time to be oxidised to the carboxylic acid.

Using an excess of oxidising agent is to be sure that there is enough oxidising agent present for the oxidation to go all the way to the carboxylic acid. When oxidation is complete, the mixture can be distilled. You end up with an aqueous solution of the acid.

Making carboxylic acids by hydrolysing nitriles

Nitriles are compounds which contain -CN attached to a hydrocarbon group. Some common examples include:

The name is based on the total number of carbons in the longest chain - including the one in the -CN group. Where you have things substituted into the chain (as in the third example), the -CN carbon counts as number 1.

Nitriles are produced in two important reactions - both of which result in an increase in the length of the carbon chain because of the extra carbon in the -CN group. They are formed in the reaction between halogenoalkanes (haloalkanes or alkyl halides) and cyanide ions. For example:

$CH_3CH_2Br + CN^- \rightarrow CH_3CH_2CN + Br^-$

or during the reaction between aldehydes or ketones and hydrogen cyanide. For example, the reaction between ethanal and hydrogen cyanide to make 2-hydroxypropanenitrile is:

Converting the nitrile into a carboxylic acid

There are two ways of doing this, both of which involve reacting the carbon-nitrogen triple bond with water. This is described as hydrolysis. The two methods produce slightly different products - you just have to be careful to get this right.

Acid hydrolysis

The nitrile is heated under reflux with a dilute acid such as dilute hydrochloric acid. A carboxylic acid is formed. For example, starting from ethanenitrile you would get ethanoic acid. The ethanoic acid could be distilled off the mixture.

$CH_3CN + 2H_2O + H^+ \rightarrow CH_3COOH + NH_4^+$

Alkaline hydrolysis

The nitrile is heated under reflux with an alkali such as sodium hydroxide solution. This time you would not, of course, get a carboxylic acid produced - any acid formed would react with the sodium hydroxide present to give a salt. You also wouldn't get ammonium ions because they would react with sodium hydroxide to produce ammonia.

Starting from ethanenitrile, you would therefore get a solution containing ethanoate ions (for example, sodium ethanoate if you used sodium hydroxide solution) and ammonia.

$CH_3CN + H_2O + OH^- \rightarrow CH_3COO^- + NH_3$

You have to remember to convert the ions into the free carboxylic acid, because that's what we are trying to make. To liberate the weak acid, ethanoic acid, you just have to supply hydrogen ions from a strong acid such as hydrochloric acid. You add enough hydrochloric acid to the mixture to make it acidic.

$CH_3COO^- + H^+ \rightarrow CH_3COOH$

Now you can distil off the carboxylic acid.

Contributors

Jim Clark (Chemguide.co.uk)