OF ALDEHYDES AND KETONES THE CHEMISTRY OF ALDEHYDES AND KETONES

ALDEHYDES & KETONES CONTENTS Prior knowledge Structural differences Nomenclature Preparation Identification Oxidation Nucleophilic addition Reduction

CARBONYL COMPOUNDS - STRUCTURE Structure carbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity Difference ALDEHYDES - at least one H attached to the carbonyl group C = O H C = O H CH3

CARBONYL COMPOUNDS - STRUCTURE Structure carbonyl groups consists of a carbon-oxygen double bond the bond is polar due to the difference in electronegativity Difference ALDEHYDES - at least one H attached to the carbonyl group KETONES - two carbons attached to the carbonyl group C = O H C = O H CH3 C = O CH3 C = O C2H5 CH3

CARBONYL COMPOUNDS - FORMULAE Molecular C3H6O Structural C2H5CHO CH3COCH3 Displayed C = O H C2H5 C = O CH3 H C C C H H O H H H H C C C O H H H H H

CARBONYL COMPOUNDS - NOMENCLATURE Aldehydes C2H5CHO propanal Ketones CH3COCH3 propanone CH3CH2COCH3 butanone CH3COCH2CH2CH3 pentan-2-one CH3CH2COCH2CH3 pentan-3-one

CARBONYL COMPOUNDS - FORMATION ALDEHYDES Oxidation of primary (1°) alcohols RCH2OH + [O] ——> RCHO + H2O beware of further oxidation RCHO + [O] ——> RCOOH Reduction of carboxylic acids RCOOH + [H] ——> RCHO + H2O KETONES secondary (2°) alcohols RCHOHR + [O] ——> RCOR + H2O

CARBONYL COMPOUNDS - IDENTIFICATION Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent Tollen’s Reagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+ the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s) the test is known as THE SILVER MIRROR TEST

CARBONYL COMPOUNDS - IDENTIFICATION Differentiation to distinguish aldehydes from ketones, use a mild oxidising agent Tollen’s Reagent ammoniacal silver nitrate mild oxidising agent which will oxidise aldehydes but not ketones contains the diammine silver(I) ion - [Ag(NH3)2 ]+ the silver(I) ion is reduced to silver Ag+(aq) + e¯ ——> Ag(s) the test is known as THE SILVER MIRROR TEST Fehling’s Solution contains a copper(II) complex ion giving a blue solution on warming, it will oxidise aliphatic (but not aromatic) aldehydes the copper(II) is reduced to copper(I) a red precipitate of copper(I) oxide, Cu2O, is formed The silver mirror test is the better alternative as it works with all aldehydes Ketones do not react with Tollen’s Reagent or Fehling’s Solution

CARBONYL COMPOUNDS - CHEMICAL PROPERTIES OXIDATION • provides a way of differentiating between aldehydes and ketones • mild oxidising agents are best • aldehydes are easier to oxidise ALDEHYDES easily oxidised to acids RCHO(l) + [O] ——> RCOOH(l) CH3CHO(l) + [O] ——> CH3COOH(l) KETONES are not readily oxidised

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Nucleophilic addition: occurs with both aldehydes and ketones involves addition to the C=O double bond unlike alkenes, they are attacked by nucleophiles attack is at the positive carbon centre due to the difference in electronegativities alkenes are non-polar and are attacked by electrophiles undergoing electrophilic addition Group Bond Polarity Attacking species Result ALKENES C=C NON-POLAR ELECTROPHILES ADDITION CARBONYLS C=O POLAR NUCLEOPHILES ADDITION

CARBONYL COMPOUNDS - REDUCTION WITH NaBH4 Reagent sodium borohydride , NaBH4 Conditions aqueous or alcoholic solution Mechanism Nucleophilic addition (also reduction as it is addition of H¯) Nucleophile H¯ (hydride ion) Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Equation(s) CH3CHO + 2[H] ——> CH3CH2OH CH3COCH3 + 2[H] ——> CH3CHOHCH3 Notes The water provides a proton Question NaBH4 doesn’t reduce C=C bonds. WHY? CH2 = CHCHO + 2[H] ———> CH2 = CHCH2OH

What should you be able to do? REVISION CHECK What should you be able to do? Recall the structure of and bonding in the carbonyl group Explain the difference in structure between aldehydes and ketones Recall the different response to oxidation of aldehydes and ketones Recall the products from the reduction of carbonyl compounds CAN YOU DO ALL OF THESE? YES NO

OF ALDEHYDES AND KETONES THE CHEMISTRY OF ALDEHYDES AND KETONES THE END