Once you have identified the functional groups in a molecule, you can predict how it behaves. Each group reacts in much the same way whatever else is in the molecule, so a molecule with several groups shows the reactions of all of them.Physical properties
• -OH groups can form hydrogen bonds, which raise boiling points and make small molecules soluble in water.
• Hydrocarbon chains are non-polar, so the longer the chain, the less soluble the molecule is in water.Chemical reactions
• C=C: decolourises bromine water (electrophilic addition); also adds H2, HBr and steam.
• primary or secondary alcohol: oxidised by acidified dichromate(VI), which turns from orange to green; a tertiary alcohol is not oxidised.
• alcohol: dehydrated to an alkene with an acid catalyst; converted to a haloalkane.
• haloalkane: nucleophilic substitution with NaOH(aq) to form an alcohol.Worked example: predict the reactions of CH2=CHCH2OH (prop-2-en-1-ol).
• The C=C decolourises bromine water, forming CH2BrCHBrCH2OH.
• The primary alcohol turns acidified dichromate(VI) from orange to green.
• The -OH forms hydrogen bonds with water, so this small molecule is soluble in water.A reagent may change only one of the groups. Bromine water reacts with the C=C but leaves the -OH unchanged, while acidified dichromate(VI) oxidises the alcohol group. Choose reagents so that you change only the group you intend to.
• -OH groups can form hydrogen bonds, which raise boiling points and make small molecules soluble in water.
• Hydrocarbon chains are non-polar, so the longer the chain, the less soluble the molecule is in water.Chemical reactions
• C=C: decolourises bromine water (electrophilic addition); also adds H2, HBr and steam.
• primary or secondary alcohol: oxidised by acidified dichromate(VI), which turns from orange to green; a tertiary alcohol is not oxidised.
• alcohol: dehydrated to an alkene with an acid catalyst; converted to a haloalkane.
• haloalkane: nucleophilic substitution with NaOH(aq) to form an alcohol.Worked example: predict the reactions of CH2=CHCH2OH (prop-2-en-1-ol).
• The C=C decolourises bromine water, forming CH2BrCHBrCH2OH.
• The primary alcohol turns acidified dichromate(VI) from orange to green.
• The -OH forms hydrogen bonds with water, so this small molecule is soluble in water.A reagent may change only one of the groups. Bromine water reacts with the C=C but leaves the -OH unchanged, while acidified dichromate(VI) oxidises the alcohol group. Choose reagents so that you change only the group you intend to.