OF ALDEHYDES AND KETONES A guide for A level students THE CHEMISTRY OF ALDEHYDES AND KETONES A guide for A level students

ALDEHYDES & KETONES CONTENTS Prior knowledge Bonding in carbonyl compounds Structural differences Nomenclature Preparation Identification Oxidation Nucleophilic addition Reduction 2,4-dinitrophenylhydrazine

Before you start it would be helpful to… ALDEHYDES & KETONES Before you start it would be helpful to… know the functional groups found in organic chemistry know the arrangement of bonds around carbon atoms recall and explain the polarity of covalent bonds

CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120° Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar PLANAR WITH BOND ANGLES OF 120°

CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120° Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these P ORBITAL PLANAR WITH BOND ANGLES OF 120°

CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120° Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond P ORBITAL PLANAR WITH BOND ANGLES OF 120°

CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120° Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120°

CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120° Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120° NEW ORBITAL

CARBONYL COMPOUNDS - BONDING PLANAR WITH BOND ANGLES OF 120° Bonding the carbon is sp2 hybridised and three sigma (s) bonds are planar the unhybridised 2p orbital of carbon is at 90° to these it overlaps with a 2p orbital of oxygen to form a pi (p) bond as oxygen is more electronegative than carbon the bond is polar P ORBITAL ORBITAL OVERLAP PLANAR WITH BOND ANGLES OF 120° NEW ORBITAL

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 Skeletal 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 O O

CARBONYL COMPOUNDS - NOMENCLATURE Aldehydes C2H5CHO propanal Ketones CH3COCH3 propanone CH3CH2COCH3 butanone CH3COCH2CH2CH3 pentan-2-one CH3CH2COCH2CH3 pentan-3-one C6H5COCH3 phenylethanone

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 Method 1 strong peak around 1400-1600 cm-1 in the infra red spectrum Method 2 formation of an orange precipitate with 2,4-dinitrophenylhydrazine Although these methods identify a carbonyl group, they cannot tell the difference between an aldehyde or a ketone. To narrow it down you must do a second 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

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 • powerful oxidising agents oxidise ketones to a mixture of carboxylic acids ALDEHYDES easily oxidised to acids RCHO(l) + [O] ——> RCOOH(l) CH3CHO(l) + [O] ——> CH3COOH(l) KETONES oxidised under vigorous conditions to acids with fewer carbons C2H5COCH2CH3(l) + 3 [O] ——> C2H5COOH(l) + CH3COOH(l)

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Mechanism 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 - NUCLEOPHILIC ADDITION Reagent hydrogen cyanide - HCN (in the presence of KCN) Conditions reflux in alkaline solution Nucleophile cyanide ion CN¯ Product(s) hydroxynitrile (cyanohydrin) Equation CH3CHO + HCN ——> CH3CH(OH)CN 2-hydroxypropanenitrile Notes HCN is a weak acid and has difficulty dissociating into ions HCN H+ + CN¯ the reaction is catalysed by alkali which helps produce more of the nucleophilic CN¯

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Mechanism Nucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O STEP 1

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Mechanism Nucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Mechanism Nucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Mechanism Nucleophilic addition Step 1 CN¯ acts as a nucleophile and attacks the slightly positive C One of the C=O bonds breaks; a pair of electrons goes onto the O Step 2 A pair of electrons is used to form a bond with H+ Overall, there has been addition of HCN STEP 1 STEP 2

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Watch out for the possibility of optical isomerism in hydroxynitriles CN¯ attacks from above CN¯ attacks from below

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION Watch out for the possibility of optical isomerism in hydroxynitriles CN¯ attacks from above CN¯ attacks from below

CARBONYL COMPOUNDS - NUCLEOPHILIC ADDITION ANIMATED MECHANISM

CARBONYL COMPOUNDS - REDUCTION WITH NaBH4 Reagent sodium tetrahydridoborate(III) (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

CARBONYL COMPOUNDS - REDUCTION WITH HYDROGEN Reagent hydrogen Conditions catalyst - nickel or platinum Reaction type Hydrogenation, reduction Product(s) Alcohols Aldehydes are REDUCED to primary (1°) alcohols. Ketones are REDUCED to secondary (2°) alcohols. Equation(s) CH3CHO + H2 ——> CH3CH2OH CH3COCH3 + H2 ——> CH3CHOHCH3 Note Hydrogen also reduces C=C bonds CH2 = CHCHO + 2H2 ——> CH3CH2CH2OH

CARBONYL COMPOUNDS - REDUCTION Introduction Functional groups containing multiple bonds can be reduced C=C is reduced to CH-CH C=O is reduced to CH-OH CN is reduced to CH-NH2 Hydrogen H• H2 H+ (electrophile) H¯ (nucleophile) Reactions Hydrogen reduces C=C and C=O bonds CH2 = CHCHO + 4[H] ——> CH3CH2CH2OH Hydride ion H¯ reduces C=O bonds CH2 = CHCHO + 2[H] ——> CH2=CHCH2OH Explanation C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯

CARBONYL COMPOUNDS - REDUCTION Example What are the products when Compound X is reduced? COMPOUND X H2 NaBH4

CARBONYL COMPOUNDS - REDUCTION Example What are the products when Compound X is reduced? COMPOUND X H2 NaBH4 C=O is polar so is attacked by the nucleophilic H¯ C=C is non-polar so is not attacked by the nucleophilic H¯

2,4-DINITROPHENYLHYDRAZINE C6H3(NO2)2NHNH2 Structure Use reacts with carbonyl compounds (aldehydes and ketones) used as a simple test for aldehydes and ketones makes orange crystalline derivatives - 2,4-dinitrophenylhydrazones derivatives have sharp, well-defined melting points also used to characterise (identify) carbonyl compounds. Identification / characterisation A simple way of characterising a compound (finding out what it is) is to measure the melting point of a solid or the boiling point of a liquid. C6H3(NO2)2NHNH2

2,4-DINITROPHENYLHYDRAZINE C6H3(NO2)2NHNH2 The following structural isomers have similar boiling points because of similar van der Waals forces and dipole-dipole interactions. They would be impossible to identify with any precision using boiling point determination. Boiling point 213°C 214°C 214°C Melting point of 2,4-dnph derivative 209°C 248°C 265°C By forming the 2,4-dinitrophenylhydrazone derivative and taking its melting point, it will be easier to identify the unknown original carbonyl compound. CHO Cl

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 and understand the mechanism of nucleophilic addition Recall the products from the reduction of carbonyl compounds CAN YOU DO ALL OF THESE? YES NO

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