Chemical properties of 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. Cδ+=Oδ– 1 Due to a difference in electronegativity the O is electronegative than the C so attracts the electron pair more strongly C O C = 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  + N Primary Alcohol  Aldehyde  Carboxylic Acid Secondary Alcohol  Ketone Tertiary Alcohol