Chemical properties of Aldehydes and Ketones Reactions common to both Aldehydes and Ketones

Chemical properties of Aldehydes and Ketones Reactions common to both Aldehydes and Ketones The difference between an Aldehyde and a Ketone is the position of the C=O (Carbonyl group). Aldehyde – always on a carbon at the end of the chain Ketone – Always on a carbon at middle carbon of the chain 1. BOTH Aldehydes and Ketones can be reduced Aldehyde reduced to Primary Alcohol Reducing agent = Sodium tetrahydroborate III (NaBH4) Ketone reduced to Secondary Alcohol Reduction Oxidisation Gain of Electrons Loss of Electrons H O H3C H + 2[H]  H3C C OH H True for all secondary alcohols Carbonyl + 2[H] ─(NaBH4) Alcohol CH3CH2CHO + 2[H]  CH3CH2CH2OH Propanal + NaBH4  Propan-1-ol CH3COCH3 +2[H]  CH3CH(OH)CH3 Propanone + NaBH4  Propan-2-ol 2. BOTH Aldehydes and Ketones undergo nucleophilic substitution using HCN Def. A nuclophile is an electron pair donor E.g. NH3 – 1 lone pair H2O – 2 lone pairs OH– – 3 lone pairs Cl– – 4 lone pairs Therefore nucleophiles attack E.g. C δ+=Oδ– 1 Due to a difference in electronegativity the O is electronegative than the C so attracts the electron pair more strongly C = C O NOTE: – A C=C will undergo electrophillic addition reactions and the reagents are added across the double bond – A C=O double bond undergo nucleophillic addition reactions and the reagents are added across O the double bond Mechanism of the reaction between Ethanal and HCN Conditions: K +CN– = catalyst Reagents: δ + O H3C C O H + HCN(g) δ + δ – H3C C H H CN O H N C KCN Ethanal  Intermediate O H3C C H CN  H3C C H CN 2 Hydroxylpropanenitrile Nitrile group Mechanism of the reaction between Propanone and HCN Conditions: K +CN– = catalyst δ + δ – H CN Reagents: O H3C C CH3 O H C C CH O H H C C CH 2 Propanone  3 3  3 3 CN CN N C KCN Intermediate 2 Hydroxyl 2 methylpropanenitrile C C C C C = C + 7 7 3. BOTH react with 2,4, Dinitrophenylhydrazine (2,4, DNPH)  2,4,DNPH is an orange solution  The reaction is used as a test for the C=O double bond  Reacts to give an orange/yellow precipitate H C C O H2N H3C C N + H2O 3 NH O H NH O H Ethanal + N + O O N 2,4, DNPH O + O O N Orange/Yellow O precipitate Example exam Q. Q. How can you distinguish between an Aldehyd and a Ketone? (4 marks) A.  Dissolve in 2,4 DNPH if precipitate is found the C=O bond present  Orange precipitate is purified by recrystalisation  Dry the crystals and heat them until their melting point is found  check the melting point found against data  Identify the exact Aldehyde or Ketone found Reactions for Aldeydes only 1. ONLY Aldeydes can be oxidised with Acidified Dichromate (H+ /Cr2O 2- ) O H3C H H O + [O]  H C C H OH 3 H + /Cr2O 2- Ethanal Acidified Dichromate Ethanoic Acid