Carbonyl compounds Carbonyl compounds have the functional group C=O and form an homologous series with a general formula CnH2nO There are two types; 1) Aldehydes and 2) Ketones Aldehydes vs Ketones Aldehydes have a carbonyl group C=O, at one end. Ketones also have a carbonyl group, but in the interior of the molecule. Naming Carbonyl compounds • Aldehydes and ketones are named using the usual IUPAC rules with the addition of an appropriate suffix to the number of carbons. For aldehydes -al Eg; CH3(CH2)3CHO is pentanal For ketones -one Eg; CH3(CH2)3COCH3 is hexanone Eg; 3C molecule with the carbonyl group at one end. So this is propanal. 3C molecule with the carbonyl group in the interior. So this is propanone. • Name the following compounds; • CH3(CH2)8CHO • Decanal • C6H5CHO • Benzaldehyde • (aka benzenecarbaldehyde) • CH3CO(CH2)3CH3 • Hexan 2 one Testing for carbonyl compounds. Both aldehydes and ketones give an orange/red precipitate with 2,4 dinitro phenyl hydrazine. Identifying organic compounds 2,4 DNPH precipitates have very sharp, easily distinguishable melting points. So organic compounds were once identified by conversion to carbonyls and precipitation of the 2,4 dinitrophenylhydrazones. These were filtered, recrystalised and their MPs determined and compared with text book values. Eg; An unknown carbonyl compound has a MP of 123OC. Methanal Ethanal Propanal Butanal Benzaldehyde MP (OC) 167 164 156 123 237 • It must therefore have been Butanal. Distinguishing between aldehydes and ketones. Aldehydes are oxidised as orange chromium (vi) is reduced to green chromium (iii) when heated with acid. Only one organic product is possible; a Carboxylic Acids. Half equations Dichromate redution Cr2O7 2- + 14H+ + 6e → 2Cr3+ + 7H20 Aldehyde oxidation; CH3CHO + H2O→ CH3CO(OH) + 2H++ 2eEthanal Ethanoic acid •NB Instead of H+ [H] or ‘H’ can be used in writing half equations. Distinguishing between aldehydes and Ketones Ketones cannot be oxidised by acidified potassium dichromate. To oxidise them a much stronger oxidising agent is needed that can break C/C bonds. Reaction with Fehling’s Solution Aldehydes change the colour of Fehling’s Reagent upon heating from blue to red. Blue copper (ii) ions are reduced to red copper (i). Cu 2+ + e- → Cu+ Ketones do not react with Fehling’s Solution as they are not oxidised by mild oxidising agents. Reaction with Tollen’s Reagent Aldehydes react with Tollen’s Reagent (ammonical silver solution [Ag(NH3)2]+(aq), ), depositing a silver mirror on the side of the tube. Silver ions are reduced to metallic silver. Ag+(aq) + e- → Ag(s) But Ketones do not react with Tollen’s reagent as they are not oxidised by mild oxidising agents. The exception has a carbonyl group. If this is an aldehyde the sugar is an aldose. If in the ketone form the sugar is a ketose. Sugars and carbonyl groups. Sugars form unbranched chains in which each carbon atom, bar one, has an hydroxyl group. Eg; Glyceraldehyde is an aldose Dihydroxyacetone is a ketose Aldoses can be oxidised like any other aldehyde. In the process another chemical is reduced. So aldoses are referred to as reducing sugars. Ketoses, like ketones, cannot be easily oxidised. So they are known as nonreducing sugars. Reaction with Benedict’s Reagent Benedict’s Reagent is similar to Fehlings. So Aldoses change its colour of Fehling’s blue to red. Blue copper (ii) ions are reduced by the reducing sugar to red copper (i). Cu 2+ + e- → Cu+ Ketones do not react with Benedict’s Reagent as they are not oxidised by mild oxidising agents.