Advanced Higher ChemistryAdvanced Higher Chemistry

Unit 3Unit 3

Aldehydes and KetonesAldehydes and Ketones

Aldehydes and KetonesAldehydes and KetonesContain the carbonyl group (C=O)Contain the carbonyl group (C=O)

UsesUsesMethanal – Phenol methanal resins Methanal – Phenol methanal resins (bakelite) for computer circuit boards, (bakelite) for computer circuit boards, worktops and heat shields.worktops and heat shields.

Propanone (acetone) – Use in Propanone (acetone) – Use in manufacture of epoxy resins.manufacture of epoxy resins.

Industrial solvents (ketones especially in Industrial solvents (ketones especially in nitro-cellulose, adhesive, plastics and nitro-cellulose, adhesive, plastics and pharmaceutical industries)pharmaceutical industries)

Systematic and trivial names are commonSystematic and trivial names are common

methyl ethyl methyl ethyl ketoneketone

butanonebutanone

acetoneacetonepropanonepropanoneacetaldehydeacetaldehydeethanalethanalformaldehydeformaldehydemethanalmethanal

TrivialTrivialSystematicSystematic

CHCH33COCHCOCH22CHCH33

CHCH33COCHCOCH33

CHCH33CHOCHO

HH22C=OC=O

Synthesis of Aldehydes and Synthesis of Aldehydes and KetonesKetones

Oxidation of appropriate alcoholOxidation of appropriate alcoholprimary alcoholprimary alcohol aldehyde aldehydesecondary alcohol secondary alcohol ketone ketone

»Possible oxidising agentsPossible oxidising agentsacidified potassium permanganateacidified potassium permanganateacidified sodium dichromateacidified sodium dichromatehot copper oxidehot copper oxide

Preparation of the aldehyde requires care to Preparation of the aldehyde requires care to prevent further oxidation to the carboxylic acid.prevent further oxidation to the carboxylic acid.

bond

Bonding in Aldehydes and KetonesBonding in Aldehydes and KetonesThe carbon and the oxygen bond is formed in a similar manner to The carbon and the oxygen bond is formed in a similar manner to the carbon to carbon double bond. The carbon and oxygen atoms the carbon to carbon double bond. The carbon and oxygen atoms are spare sp22 hybridised (see bonding in alkenes) hybridised (see bonding in alkenes)

A A bond and bond and are formed between the carbon and oxygen. are formed between the carbon and oxygen.

The bond angles are 120The bond angles are 120oo and the and the functional group is planar.functional group is planar.

Unlike the carbon to carbon double bond, Unlike the carbon to carbon double bond, the carbonyl group is significantly polar.the carbonyl group is significantly polar.

The physical and chemical properties of The physical and chemical properties of both aldehydes and ketones are governed both aldehydes and ketones are governed by the polar nature of the carbonyl group.by the polar nature of the carbonyl group.

∞∞5656-95-955858∞∞4949-81-815858

∞∞9797-127-1276060

00-1-1-137-1375858

propanonepropanone

propanalpropanalpropan-1-olpropan-1-ol

butanebutane

Solubility Solubility

(g/100g H(g/100g H22O)O)b.p. b.p. ((°C)°C)

m.p. m.p. ((°C)°C)RFMRFM

Polarity of C=O bond has a significant effect on Polarity of C=O bond has a significant effect on physical properties.physical properties.

Hydrogen bondDipole

attraction

• The intermolecular attractions are stronger than normal van der Waals, but not as strong as hydrogen bonding.

• As with ethers no hydrogen bonding is possible within ketones and aldehydes, but it is possible in water.

As chain length of the aldehydes and As chain length of the aldehydes and ketones increases, the non-polar part of ketones increases, the non-polar part of the molecule dominates and the solubility the molecule dominates and the solubility decreases. Hence, only the smaller decreases. Hence, only the smaller aldehydes and ketones are soluble in aldehydes and ketones are soluble in water.water.

Differences in propertiesDifferences in properties

The attachment of a hydrogen atom to the carbonyl The attachment of a hydrogen atom to the carbonyl group in aldehydes make these compounds group in aldehydes make these compounds behave differently in two ways:behave differently in two ways:1. 1. aldehydes are easily oxidised whereas aldehydes are easily oxidised whereas ketones are oxidised with great difficulty.ketones are oxidised with great difficulty.2.2. aldehydes are more susceptible to aldehydes are more susceptible to nucleophilic nucleophilic attack at the carbonyl group. The attack at the carbonyl group. The polarity of the polarity of the carbonyl group in a ketone is less carbonyl group in a ketone is less than in an than in an aldehyde due to the electron donating aldehyde due to the electron donating effect of effect of the two alkyl groups in the ketone. This the two alkyl groups in the ketone. This reduces reduces the partial positive charge on the the partial positive charge on the carbon atom carbon atom of the group.of the group.

Reactions of Aldehydes and KetonesReactions of Aldehydes and Ketones

Both undergo addition and condensation reactions.Both undergo addition and condensation reactions.

They can be distinguished from one another by their They can be distinguished from one another by their reaction with oxidising agents.reaction with oxidising agents.

Aldehyde oxidation can be carried out by using:Aldehyde oxidation can be carried out by using:

Tollen’s Reagent – silver mirror testTollen’s Reagent – silver mirror test

(complexed Ag(complexed Ag++ Ag)Ag)

Fehling’s solution (blue) – orange ppt.Fehling’s solution (blue) – orange ppt.

(complexed Cu(complexed Cu2+2+ CuCu22O)O)

Aldehydes and Ketones - ReductionAldehydes and Ketones - ReductionBoth can be reduced to corresponding alcohol using the reducing agent Both can be reduced to corresponding alcohol using the reducing agent lithium aluminium hydride (LiAlHlithium aluminium hydride (LiAlH44).).

LiAlHLiAlH4 4 contains the AlHcontains the AlH44-- ion which is able to transfer a hydride ion (H ion which is able to transfer a hydride ion (H--) )

to the partially positive carbon atom of the carbonyl group.to the partially positive carbon atom of the carbonyl group.This reaction must be carried out in anhydrous conditions as LiAlHThis reaction must be carried out in anhydrous conditions as LiAlH4 4 reacts explosively with water.reacts explosively with water.

This step is repeated until all 4 hydrogens on the This step is repeated until all 4 hydrogens on the aluminium have been replaced.aluminium have been replaced.

The resultant complex is hydrolysed to obtain the The resultant complex is hydrolysed to obtain the alcohol.alcohol.(R-CH(R-CH22-O)-O)44AlAl- - + 4H + 4H22O O 4RCH 4RCH22OH + AlOH + Al3+3+ + 4OH + 4OH--

The reduction of aldehydes produces primary alcohols whereas the reduction of ketones gives rise to secondary alcohols.

Aldehydes and Ketones - AdditionAldehydes and Ketones - Addition

Polarity of the C=O bond meansPolarity of the C=O bond means

»Carbon atom susceptible to nucleophilic attack.Carbon atom susceptible to nucleophilic attack.

»Oxygen atom susceptible to electrophilic attack.Oxygen atom susceptible to electrophilic attack.

Therefore reaction can be nucleophilic addition Therefore reaction can be nucleophilic addition (most common) or electrophilic addition.(most common) or electrophilic addition.

Using a general reagent X-Y in which the bond is

strongly polarised:• The nucleophilic X atom attacks the carbon.• The X-Y bond breaks heterolytically to leave Y+.• Since there is no satisfactory leaving group Y+

couples with the negative oxygen.• Overall result is the addition of X-Y across the

carbon to oxygen double bond.

• In some reactions the initial step will be electrophilic attack of Y+ on the oxygen atom. In others, it will be nucleophilic attack by X- on the carbon atom.

• Y+ is frequently a hydrogen ion and so the process is often subject to general acid catalysis. In such cases, the initial step could involve protonation of the oxygen atom of the group prior to attack by X-.

Increasing the polarity of the C=O bond will Increasing the polarity of the C=O bond will make the carbon atom more susceptible to make the carbon atom more susceptible to nucleophilic attack.nucleophilic attack.

This can be achieved by solvation of the oxygen This can be achieved by solvation of the oxygen atom by water or alcohol solvent.atom by water or alcohol solvent.

Aldehydes are more reactive than ketones as Aldehydes are more reactive than ketones as the partial +ve bond in ketones is stabilised by the partial +ve bond in ketones is stabilised by the electron donating effect of the 2 alkyl the electron donating effect of the 2 alkyl groups.groups.

NB – the mechanism is VERY DIFFERENT NB – the mechanism is VERY DIFFERENT from alkene addition reactions.from alkene addition reactions.

Addition of Hydrogen CyanideAddition of Hydrogen Cyanide

Useful for the synthesis of 2-hydroxy Useful for the synthesis of 2-hydroxy acids, which can then be dehydrated to acids, which can then be dehydrated to unsaturated acids.unsaturated acids.

Addition of sodium hydrogensulphiteAddition of sodium hydrogensulphite

Products are crystalline.Products are crystalline.

Carbonyl compounds can be purified using this Carbonyl compounds can be purified using this reaction.reaction.

1)1) Crude (impure) carbonyl reacted with conc. sodium Crude (impure) carbonyl reacted with conc. sodium hydrogensulphite.hydrogensulphite.

2)2) Crystals are then removed by filtration.Crystals are then removed by filtration.

3)3) Carbonyl compound regenerated by heating with Carbonyl compound regenerated by heating with dilute acid.dilute acid.

NB the reaction can also be used to remove carbonyl NB the reaction can also be used to remove carbonyl impurities from non-carbonyl compounds.impurities from non-carbonyl compounds.

Addition of ammonia derivativesAddition of ammonia derivatives

Some compounds related to ammonia can Some compounds related to ammonia can add to the carbonyl group because of the add to the carbonyl group because of the nucleophilic nature of the lone pair of nucleophilic nature of the lone pair of electrons on the nitrogen atom.electrons on the nitrogen atom.

Addition is followed by elimination of a Addition is followed by elimination of a water molecule to produce a compound water molecule to produce a compound containing a carbon to nitrogen double containing a carbon to nitrogen double bond.bond.

Aldehydes and Ketones - CondensationAldehydes and Ketones - Condensation

• First step of the reaction is the addition of a hydrazine compound (i.e. NH2-NHR).

• Second step is the elimination of a water molecule and the production of a hydrozone.

• Overall the reaction is a condensation reaction

Examples -

The products are known as DERIVATIVES.The products are known as DERIVATIVES.

They are always crystalline solids with characteristic They are always crystalline solids with characteristic melting points once purified.melting points once purified.

This allows identification of the original carbonyl This allows identification of the original carbonyl compound.compound.

2,4-dinitrophenylhydrazine is one of the most commonly 2,4-dinitrophenylhydrazine is one of the most commonly used reagents and is used as the test for the carbonyl group. used reagents and is used as the test for the carbonyl group. (Brady’s Reagent – yellow ppt forms)(Brady’s Reagent – yellow ppt forms)

ExerciseExercise

Now try the exercise on page 39 of your Now try the exercise on page 39 of your Unit 2(c) notes.Unit 2(c) notes.