COMBUSTION AND OXIDATION OF ALCOHOLS:
Combustion of alcohols:
In a plentiful supply of oxygen, alcohols burn completely to form carbon dixoide and water.
C2H5OH(l) + 302(g) -------> 2CO2(g) + 3H2O(l)
Oxidation of alcohols:
Primary and secondary alcohols can be oxidised using an oxidising agent; a suitable oxidising agent is a solution containing acidified dichromate ions, H+/Cr2O7,2-
This oxidisng mixture can be made from potassium dichromate, K2Cr2O7 and sulfuric acid, H2SO4.
During the reaction the acidified potassium dichromate turns from orange to green.
On gentle heating with acidified potassium dchromate, a primary alchohol can be oxidised to produce and aldehyde.
Propan-1-ol + [O] ------K2Cr2O7/H2SO4(distill immediately)------> Propanal + H2O
On stronger heating with excess acidified potassium dichromate, the alcohol will be completely oxidised, passing through the aldehyde stage, to form a carboxylic acid.
Propan-1-ol + 2[O] ------K2Cr2O7/H2SO4 (reflux)-----> Propanoic acid + H2O
When making the carboxylic acid, the reaction mixture is usually heated under reflux before distilling off the product.
KEY DEFINTION: Reflux is the continual boiling and condensing of a reaction misture to ensure that the reaction takes place without the contents of the flask drying out.
EXAMINER TIP: If you connect a condenser the wrong way around, the water cannot fill the outer water jacket of the condenser and it will not cool vapour efficiently. Water always enters lower than it leaves.
Functional group on aldehye= C=O, H on C also.
Functional group on carboxylic acid= C=O, OH on C also.
Secondary alcohols are oxidised by acidified dichromate ions to produce ketones. Ketones cannot be oxidise futher.
Butan-2-ol + [O] -----K2Cr2O7/H2SO4(heat)-----> Butanone + H2O
Ketone functional group= CC(=O)C
Tertiary alcohols are resistant to oxidation.
ESTERIFICATION AND DEHYDRATION OF ALCOHOLS:
An ester is formed when alcohol is warmed with carboxylic acid in the presence of an acid catalyst.
Conc. sulphuric acid is often used as the acid catalyst.
In the reaction the OH bond in the alcohol is broken and water is formed. The water molecule comes from the OH of the carboxlic acid group and the H in the alcohol gorup.
CH3CH2COOH + CH3OH -------> CH3CH2COOCH3 + H2O
Propanoic acid +methanol ------> methyl propanoate + water
Esters are used as adhesives and solvents in the chemical industry. The flavours and fragrances of different esters are widely used to produce food flavourings and perfumes.
Pentyl ethanoate= banana smell
Octyl ethanoate= orange smell
Preparing an ester= 1cm acid + 1cm alcohol; add a few drops of conc. sulphuric acid; place boiling tube in hot water bath at about 80 degrees for 5min; pour product into cold beaker; will see an oil floating on surace of the water, the oil is the sweet smelling ester.
Dehydration of an alcohol:
An alcohol can be dehyrated to form an alkene in the presence of an acid catalyst.
- This called an elimination reaction.
- Conc H3PO4 or H2SO4 are suitable acid catalysts.
The alcohol is heated under reflux, in the presense of the acid for about 40mins.
Ethanol -------Conc.H2SO4(heat)-----> Ethane + H2O