4.4 - Aldehydes and ketones Formation Aldehydes and ketones are formed by the oxidation of primary and secondary alcohols respectively with acidified dichromate (Cr 2 O 7 2- / H + ). The formation of the aldehyde requires gentle warming. The aldehyde is distilled off as it forms to prevent complete oxidation to the carboxylic acid. This is not required for the ketone, as the ketone cannot be oxidised any further. Distinguishing between aldehydes and ketones Aldehydes and ketones may be distinguished from each other as aldehydes can be oxidised, whereas ketones cannot. Two common reagents used to distinguish between an aldehyde and a ketone are Fehling’s solutions and Tollens’ reagent. Tollens’ reagent is ammoniacal silver nitrate which produces a silver mirror if an aldehyde is present. Silver ions oxidise the aldehyde and are themselves reduced to silver atoms. Ag + + e – → Ag colourless silver mirror There is no change with a ketone. Fehling’s solution is a Cu 2+ complex which turns from blue to red when an aldehyde is present. Cu 2+ ions oxidise the aldehyde and are themselves reduced to Cu + ions. Cu 2+ + e – → Cu + blue red Again there is no change with a ketone. Primary aldehyde carboxylic acid ketone RCH 2 OH RCHO H OH R R C C O R 1 R 1 RCOOH Secondary Reduction Aldehydes and ketones (carbonyl group) can be reduced using sodium tetrahydridoborate(III), NaBH 4 , in aqueous solution. The aldehyde reduces to a primary alcohol and the ketone to a secondary alcohol. H 3 C H H H O O propanal propan- l -ol a primary alcohol a secondary alcohol butan- 2 -ol CH 2 C NaBH 4 NaBH 4 H 3 C H O C C C O CH 2 H H H 3 C H 3 C H 3 C CH 2 H 3 C CH 2 Identifying aldehydes and ketones 2,4-DNP (2,4-dinitrophenylhydrazine) can be used to test for an aldehyde or ketone. The reaction which takes place is an addition-elimination or condensation reaction. When a carbonyl compound reacts with 2,4-DNP, a yellow or orange solid forms. This solid can be isolated and purified and the original aldehyde or ketone identified from its melting temperature. O 2 N NO 2 NO 2 NH NH 2 O C R 1 R + O 2 N NH + N C R 1 R O 4 5 6 O O H H The product is called a 2,4 - dinitrophenylhydrazone. 2,4 - dinitrophenylhydrazine N 3 2 1 O O N NH N H H Nucleophilic addition of HCN Carbonyl compounds will undergo a nucleophilic addition reaction with hydrogen cyanide, HCN. The rate is slow but is greatly increased by addition of alkali or cyanide ions. The mechanism for this nucleophilic addition reaction is as follows. Iodoform reaction This is a reaction which is used to identify a carbonyl compound that contains CH 3 CO— (ethanal and methyl ketones) and alcohols that contain CH 3 CHOH— (ethanol and methyl secondary alcohols). The reagents required are either iodine and aqueous sodium hydroxide or aqueous potassium iodide and aqueous sodium chlorate(I), NaClO. A positive result will produce a yellow precipitate of triiodomethane, CHI 3 , which has a characteristic antiseptic smell. N H H C C H 3 C O C O H H 3 C e.g. CH 3 CHO + HCN CH 3 CH(OH)CN HCN 2-hydroxypropanenitrile Nucleophilic attack by cyanide ion at the carbon atom of the polar carbonyl group The resulting negative ion gains a proton from water (solvent) or any other available molecule such as HCN O C δ+ δ– H 3 C H 3 C H – C N O – C C H H N C N H O H 3 C C H HO – δ– δ+ δ+ H O +