CIE A Level Chemistry Hydroxy Compounds Exam Questions

About CIE A Level Chemistry Hydroxy Compounds

Hydroxy compounds in Cambridge International 9701 covers alcohols at AS and phenol at A Level. You need to describe how alcohols are made, classify them as primary, secondary or tertiary, and learn their reactions: combustion, substitution to halogenoalkanes, reaction with sodium, oxidation with acidified potassium dichromate(VI), dehydration to alkenes and esterification. The tri-iodomethane (iodoform) test identifies the CH3CH(OH) group. At A Level you compare the acidity of water, ethanol and phenol, and study the reactions of phenol with sodium, sodium hydroxide, bromine water and nitric acid, and its use in making azo dyes. Examiners often ask for reagents, conditions and observations, and explanations of relative acidity.

What you need to know

• Making alcohols Electrophilic addition of steam to alkenes (H3PO4 catalyst), hydrolysis of halogenoalkanes with aqueous NaOH, reduction of aldehydes and ketones with NaBH4, and hydrolysis of esters.
• Oxidation Heating with acidified K2Cr2O7 turns orange to green. Primary alcohols give aldehydes (distil off) then carboxylic acids (reflux), secondary alcohols give ketones and tertiary alcohols are not oxidised.
• Other reactions of alcohols With sodium they give hydrogen and an alkoxide. They react with HX, PCl3, PCl5 or SOCl2 to form halogenoalkanes, dehydrate to alkenes with heated Al2O3 or concentrated H2SO4, and form esters with carboxylic acids and a concentrated H2SO4 catalyst.
• Tri-iodomethane test Alcohols containing CH3CH(OH)- give a pale yellow precipitate of CHI3 when warmed with alkaline aqueous iodine. Ethanol is the only primary alcohol to give a positive test.
• Relative acidity (A Level) Phenol is more acidic than water, which is more acidic than ethanol. In the phenoxide ion the negative charge is delocalised into the ring, stabilising it, while the ethyl group in ethoxide pushes electrons onto the oxygen.
• Reactions of phenol (A Level) Phenol reacts with NaOH and Na but not carbonates. With bromine water it gives a white precipitate of 2,4,6-tribromophenol, and with dilute HNO3 at room temperature it gives a mixture of 2- and 4-nitrophenol.
• Why phenol is more reactive than benzene A lone pair on the oxygen is delocalised into the ring, increasing its electron density so it attracts electrophiles more strongly.

Common mistakes in Hydroxy Compounds

• Saying tertiary alcohols are oxidised to ketones; tertiary alcohols are not oxidised by acidified dichromate.
• Describing the colour change of dichromate as green to orange; it goes from orange to green.
• Forgetting that only alcohols with a CH3CH(OH) group give a positive tri-iodomethane test.
• Saying phenol reacts with sodium carbonate to give CO2; phenol is too weakly acidic.
• Writing that bromine reacts with phenol by addition; it is electrophilic substitution, with three Br atoms substituted.

Try three Hydroxy Compounds exam-style questions