AQA A Level Chemistry Aldehydes and Ketones Questions and Answers

AQA A Level Chemistry Aldehydes and Ketones Questions and Answers What is a carbonyl group? C=O What type of bond is C=O? Polar What are the effects of the carbonyl group being planar? There is no steric hindrance, so the bond can be attacked from any angle Why do nucleophiles attack carbonyl groups? The carbon is slightly positive How do you turn a primary alcohol into an aldehyde? Oxidise it How to you turn a carboxylic acid into an aldehyde? Reduce it What is the main reaction involving nucleophiles which occurs with carbonyl groups? Nucleophilic addition What 2 chemicals can be used as reducing agents for aldehydes and ketones? 1. Lithium Aluminium Hydride (LiAlH4) 2. Sodium Borohydride (NaBH4) What is the only situation in which NaBH4 decomposes? In acidic conditions How is LiAlH4 prepared? As a solution in dry ether What reduction of ions occurs when you use potassium dichromate to oxidise alcohols/aldehydes? Orange Cr +6 ions are reduced to green Cr 3+ ions What is a chiral carbon? Carbon atom with four different groups attached When can two molecules show optical isomerism? When they contain a chiral carbon Can a ketone ever be chiral? No What are enantiomers? Optical isomers that have the same molecular and structural formulas, except that one is a non-superimposable mirror image of the other What is a racemic mixture? 50/50 mixture of two enantiomers What are the 2 exceptions to the chemical and physical properties of enantiomers being the same? 1. Reflect plane-polarised light in opposite directions 2. React with other chiral molecules differently What direction do light wave vibrations travel in relation to the direction of travel of the wave? Perpendicular What is special about plane-polarised light? It only vibrates in one direction What direction does light normally vibrate in? All directions How are hyrdroxynitriles formed from ketones or aldehydes? You react the ketone or alcohol with KCN followed by dilute acid, producing a hydroxynitrile in a nucleophilic addition reaction How are alcohols formed from aldehydes or ketones? You react the ketone or alcohol with NaBH4 or LiAlH4, producing an alcohol by nucleophilic addition Under what conditions do you reduce ketones or aldehydes? Aqueous conditions What provides the H+ ions in the reduction of a ketone using NaBH4, given that there is no acid used? Water What type of mixture is created when you make hydroxynitriles via reduction of ketones or aldehydes? Racemic mixture How do you seperate off the hydroxynitrile after synthesising it from a ketone? Distillation, as it has hydrogen bonding and thus a higher boiling point than the reactants Why are hydroxynitriles formed as a racemic mixture? Due to the planar nature of the carbonyl group in the reactants there is an equal chance of attack from top or bottom What is a hydroxynitrile? Molecules containing an alcohol and a nitrile group bonded to the same carbon (nitrile being CN) How does the Tollens' test work? Aldehyde is oxidized by Ag+ and positive result shows a silvery mirror as Ag+ is reduced to metallic (elemental) silver Does a ketone give a positive Tollen's test? No What is Fehling's solution? Blue solution of complexed copper (II) ions dissolved ion sodium hydroxide What is Tollens' reagant? Colourless silver nitrate solution dissolved in aqueous ammonia How does Fehling's test work? The aldehyde reduces the copper (II) ions to brick-red copper (I) oxide