CHEMISTRY 3.5 ANSWERSWORKSHEET SEVEN ORGANIC CHEMISTRY Aldehydes and ketones1. (a) carbonyl group (b) In aldehydes the carbon of the carbonyl group is attached to a hydrogen, in ketones it is attached to two other carbon atoms. (c) (i) pentan-3-one (ii) butanal (iii) propanone (d) (i) CH3CHOCH2CH2CH3 (ii) CH3CH2CHO (iii) CH3CHOCH2CH32. Aldehydes are produced by the oxidation of primary alcohols using acidified potassium dichromate solution. The aldehyde must be distilled off as it is made or it will oxidise further, up to the carboxylic acid. Ketones are made by the same oxidation reaction with secondary alcohols, but they do not need to be distilled of as the reaction proceeds.3. Although the carbonyl group is polar and this allows the lower members of these series to have permanent dipole interactions, they are unable to form hydrogen bonds between their molecules. Alcohols have an O-H bond and so can form hydrogen bonds with themselves producing higher boiling points than aldehydes and ketones.4. Because of the C=O bond in aldehydes and ketones, water molecules are able to hydrogen bond to them and this allows them to dissolve readily in water. O H O C H Hydrogen Bond5. If a little acidified potassium dichromate was added to propanal and warmed, the orange colour of the solution would turn green as the propanal was oxidised up to propanoic acid. When the same procedure was followed with propanone, there would be no colour change at all because it cannot be oxidised further.
6. (a) Tollen’s reagent contains the silver diammine complex ion [Ag(NH3)2]+ (b) (i) If the reaction is carried out in a clean test tube a silver mirror forms on the glass. Otherwise the silver produced appears as a black ppt. (ii) Metallic silver and a carboxylic acid. (c) Tollens reagent will oxidise aldehydes but will not oxidise ketones or alcohols so it can be used to identify aldehydes from these two other compounds.7. (a) Blue (b) A brick-red ppt forms. (c) Cu2O8. Step 1. Add Tollens reagent to samples of each chemical and warm. The mixture that either makes a silver mirror or a black ppt is an aldehyde. Step 2. Add a little acidified potassium dichromate to samples of the remaining two chemicals. The mixture that turns from orange to green is an alcohol. The remaining liquid is the ketone.