- Alcohols can be classified into three groups; primary, secondary and tertiary.
- Primary alcohols are attached to a carbon with only one R-group (carbon chain or other atoms other than hydrogen).
- Secondary alcohols are attached to a carbon with two R-groups.
- Tertiary alcohols are a carbon with three R-groups.
- The –OH group can be oxidised by strong oxidising agents (e.g. potassium dichromate):
- Two hydrogen atoms are being removed from the molecule (one attached to the carbon and the other from the –OH group).
- The product is a Carbonyl group (either a carbonyl or aldehyde.
- The product depends upon the kind of alcohol you start with.
- Primary alcohols make aldehydes, which can then go on to form carboxyl acid.
- Secondary alcohols make ketones.
- Tertiary alcohols cannot be oxidised.
Aldehydes and Ketones
- Aldehydes and Ketones contain a carbonyl group:
- In an aldehyde, the carbonyl group is at the end of the alkane chain:
- Aldehydes are named using the suffix “-al”:
CH3 –CHO (ethanal)
- In a ketone, the carbonyl group is inside the alkyl chain:
- Ketones are named using the suffix “-one”:
- Primary alcohols are oxidised to aldehydes and then carboxyl acids. For example, ethanol is oxidised to ethanal, which is then oxidised to ethanoic acid:
- Secondary alcohols are oxidised to ketones. For example, propan-2-ol is oxidised to propanone:
- Tertiary alcohols are difficult to oxidise, as they do not have a hydrogen atom on the carbon atom to which the –OH is attached.
Dehydration of alcohols
- Alcohols can lose a molecule of water to become an alkene.
- Propene is formed when propan-1-ol vapour is passed over a hot catalyst of alumina (300oC).
- It is a dehydration reaction because it involves the removal of a water molecule from a molecule of the reactant.
- Dehydration can also occur when heated with concentrated sulphuric acid.
- Dehydration is an example of an elimination reaction (reverse of an addition reaction).
Useful books for revision:
Revise AS Chemistry for Salters (Written by experienced examiners and teachers of Salter's chemistry)
Revise AS Chemistry for Salters (OCR) (Salters Advanced Chemistry)