C14 - Alcohols Flashcards
What are the reasons for different physical properties between alcohols and alkanes?
Alkanes have non-polar bonds and intermolecular forces between non-polar molecules are very weak London forces.
Alcohols have polar O-H bonds so are polar and have weak London forces as well as strong hydrogen bonds.
What are primary, secondary and tertiary alcohols?
Primary - the -OH is attached to a carbon atom with 2 H atoms and 1 alkyl group
Secondary - the -OH is attached to a carbon atom with 1 H atoms and 2 alkyl group
Tertiary- the -OH is attached to a carbon atom with no H atoms and 3 alkyl group
What is the oxidising agent used for the oxidisation of alcohols?
Potassium dichromate, acidified with dilute sulphuric acid.
(K2Cr2O7)
It becomes reduced from +6 to +3
What happens when primary alcohols are oxidised?
They form aldehydes.
Further oxidation produces carboxylic acids.
What happens when secondary alcohols are oxidised?
They form ketones
What happens when tertiary alcohols are oxidised?
Nothing - they don’t oxidise
How can you tell if an alcohol is oxidised?
Add acidified potassium dichromate and, if it oxidises the alcohol, it turns from orange to green.
How can aldehydes be obtained?
By the oxidation and distillation of primary alcohols.
It must be distilled to prevent further oxidisation.
How can carboxylic acids be obtained?
By strongly heated a primary alcohol under reflux with excess acidified potassium dichromate to ensure its fully oxidised.
What is reflux?
Continuously boiling and condensing.
Heating with a vertical condenser to avoid the loss of volatile substances.
How are ketones obtained?
From the reflux of secondary alcohols.
What is the dehydration of alcohols?
When water is removed.
An alcohol is heated under reflux in the presence of an acid catalyst, producing an alkene.
This is an elimination reaction.
How do alcohols undergo substitution reactions?
Alcohols react with hydrogen halides to form halo alkanes.
When preparing a haloalkane, the alcohol is heated under reflux with acid and a sodium halide.
What occurs during the dehydration of alcohols?
Reflux is necessary and a concentrated acid (sulphuric or phosphoric) catalyst is needed.
1) Sulphuric acid is a polar molecule so the lone pair of electrons on the oxygen of the alcohol is attracted to he slight positive charge on one of the hydrogens of the acid.
The hydrogen acts as an electrophile.
2) A bond forms between the oxygen on the ethanol and hydrogen on the sulphuric acid.
The ethanol is said to be protonated.
The acid loses the H atom and forms a hydrogen sulphate ion.
3) The carbon-oxygen bond of the protonated ethanol is broken, resulting in the creation of a water molecule and a carbocation.
4) One of the hydrogens on the non-positively charged carbon of the carbocation is attacked by / attracted to the lone pair on the oxygen in the HSO4- ion.
