CIE A Level Chemistry Carbonyl Compounds Exam Questions

About CIE A Level Chemistry Carbonyl Compounds

Carbonyl compounds in Cambridge International 9701 deals with aldehydes and ketones. You need to describe how they are formed by oxidising primary and secondary alcohols, how they are reduced to alcohols by NaBH4 or LiAlH4, and their reaction with HCN in the presence of KCN to form hydroxynitriles by nucleophilic addition. A big part of the topic is identification: 2,4-dinitrophenylhydrazine for any carbonyl, Tollens' reagent and Fehling's solution to distinguish aldehydes from ketones, and alkaline iodine for methyl ketones. This topic links to hydroxy compounds, carboxylic acids, nitrogen compounds and organic synthesis, and the nucleophilic addition mechanism is examined frequently.

What you need to know

• Formation Aldehydes form when primary alcohols are heated with acidified K2Cr2O7 and distilled off. Ketones form from secondary alcohols heated under reflux with the same reagent.
• Reduction NaBH4 in aqueous or alcoholic solution, or LiAlH4 in dry ether, reduces aldehydes to primary alcohols and ketones to secondary alcohols. The reducing agent is written as [H] in equations.
• Nucleophilic addition of HCN HCN with a KCN catalyst at about 10 to 20 °C. The CN- ion attacks the delta positive carbonyl carbon, then the O- intermediate takes H+ from HCN. The product is a hydroxynitrile with one extra carbon.
• Optical activity of products The planar C=O can be attacked from either side, so if the product has a chiral centre a racemic mixture forms, which is not optically active.
• 2,4-DNPH test Gives an orange or yellow precipitate with aldehydes and ketones (a condensation reaction). The melting point of the purified precipitate can be used to identify the compound.
• Distinguishing aldehydes and ketones Aldehydes give a silver mirror with Tollens' reagent and a brick-red precipitate with Fehling's solution. Ketones give no change.
• Tri-iodomethane test Carbonyl compounds containing CH3CO- give a pale yellow precipitate with alkaline aqueous iodine; ethanal is the only aldehyde that does.

Common mistakes in Carbonyl Compounds

• Drawing the first curly arrow from the carbonyl carbon or from H+; it starts at the lone pair or negative charge on CN- and goes to the carbonyl carbon.
• Saying ketones are oxidised by Tollens' reagent; only aldehydes are oxidised by mild oxidising agents.
• Forgetting the KCN catalyst; HCN alone is a weak acid with too low a concentration of CN- ions.
• Claiming the hydroxynitrile product is optically active; a racemic mixture forms because attack is equally likely from both sides.
• Describing Fehling's solution as turning blue; it starts blue and gives a brick-red precipitate with aldehydes.

Try three Carbonyl Compounds exam-style questions