10.4 – Alcohols
Alcohols have the general formula:
10.4.1 – Describe, using equations, the complete combustion of alcohols
Alcohols will not affect the pH of a solution as they do not behave as bases and are weaker acids than water.
Alcohols are useful fuels because they have high enthalpies of combustion, create little pollution because there is no sulfur and can be obtained from renewable resources
They will undergo complete combustion:
However, if there is limited supply of oxygen, they will produce carbon monoxide.
10.4.2 – Describe, using equations, the oxidation reactions of alcohols
If the oxidation of alcohols is controlled by the reagents used, then the skeleton can kept intact and a variety of products can be formed. In most cases, acidified potassium dichromate(VI) – K2Cr2O7 can be used. However, acidified potassium permanganate – KMnO4 – can also be used. The oxidising agent can be written as +[O]
Primary Alcohols
The oxidation of alcohols must take place in two steps – first going to an aldehyde, then to a carboxylic acid.
To obtain the aldehyde before it reacts further, distillation can be used to remove it due to its lower boiling point. It will go into the gaseous state and enter condensing tube. It will no longer be in contact with the oxidising agent and will not be converted to carboxylic acid.
To obtain the carboxylic acid as it is formed, heating in a reflux condenser is used instead.
Secondary Alcohols
When secondary alcohols are oxidised, they form ketones.
Tertiary Alcohols
These do not react readily under normal conditions as a very large amount of energy is needed to break the bonds.
10.4.3 – Determine the products formed by the oxidation of primary and secondary alcohols
Primary Alcohols (1°)
These are easily oxidised to aldehydes and then to carboxylic acids.
It is essential to distil off the aldehyde before it is oxidised to the carboxylic acid. Aldehydes do not have hydrogen bonding, and so have lower boiling points than alcohols and acids. The alcohol is dripped into a warm solution of acidified K2Cr2O7.
To oxidise an alcohol straight to the acid, reflux the mixture. Reflux is when the mixture is heated in a flask with a condenser in it. This way, the aldehyde will be put back into the liquid state so that it will remain in contact with the oxidising agent.
Secondary Alcohols (2°)
These are easily oxidised to ketones.
The alcohol is refluxed with acidified K2Cr2O7
Note that tertiary alcohols are not oxidised by potassium dichromate(VI) under normal condition