Alcohols - Reactions
Combustion of alcohols
- Alcohols react with oxygen in the air when ignited and undergo complete combustion to form carbon dioxide and water
alcohol + oxygen → carbon dioxide + water
Complete combustion of alcohols to produce carbon dioxide and water
Conversions to halogenoalkanes
- These reactions involve replacing the hydroxyl group in an alcohol molecule with a halogen atom(known as halogenation)
- Different methods are required for each halogen
Chlorination
- Phosphorus(V) chloride is added to the alcohol resulting in a vigorous reaction at room temperature
- This means the mixture doesn't need heating
- This reaction is used as a qualitative test for the presence of the -OH group
- If you add PCl5 to an unknown liquid the evolution of steamy fumes (HCl gas) is evidence for the presence of the -OH group
- Two inorganic products are formed: phosphoryl chloride and hydrogen chloride
CH3CH2CH2OH + PCl5 → CH3CH2CH2Cl + POCl3 + HCl
- Chlorination of tertiary alcohols can be carried out in a different way by mixing (shaking) with hydrochloric acid at room temperature
- An example equation for the reaction of 2-methyl propan-2-ol is:
(CH3)3COH + HCl → (CH3)3CCl + H2O
- This reaction does not work for primary and secondary alcohols
Bromination
- This reaction is carried out using a warmed mixture of potassium bromide and 50% concentrated sulfuric acid with the reacting alcohol
- More concentrated sulfuric acid would would oxidise bromide ions to bromine resulting in different products
- The reaction can be written as two equations as the inorganic reactants first react together to form hydrogen bromide and potassium sulfate
2KBr + H2SO4 → K2SO4 +2HBr
- The resulting hydrogen bromide then reacts with the alcohol, for example the reaction with butan-1-ol would be as follows:
CH3CH2CH2CH2OH + HBr → CH3CH2CH2CH2Br + H2O
Iodination
- This reaction is carried out using a mixture of red phosphorus and iodine with the alcohol whilst heating under reflux
- Similar to bromination, the reaction can be written as two equations as the inorganic reactants first react to form phosphorus(III) iodide
2P + 3I2 → 2PI3
- The reaction for the iodination of ethanol would be:
3C2H5OH + PI3 → 3C2H5I + H3PO3
- This reaction results in the formation of phosphorous acid as shown above
Dehydration to Alkenes
- Dehydration is done by heating the alcohol with concentrated phosphoric acid
- The reaction is similar to the elimination reaction of a halogenoalkene
- The OH group and hydrogen of adjacent carbons are removed forming a C=C bond
- The equation for the dehydration of ethanol would be
CH3CH2OH → CH2=CH2 + H2O
- Phosphoric acid does not appear in the equation as the water formed dilutes the concentrated phosphoric acid