Advanced Higher Chemistry Unit 3 Aldehydes and Ketones

Contain the carbonyl group (C=O) Uses Methanal – Phenol methanal resins (bakelite) for computer circuit boards, worktops and heat shields. Propanone (acetone) – Use in manufacture of epoxy resins. Industrial solvents (ketones especially in nitro-cellulose, adhesive, plastics and pharmaceutical industries)

Systematic and trivial names are common methyl ethyl ketone butanone acetonepropanone acetaldehydeethanal formaldehydemethanalTrivialSystematic CH 3 COCH 2 CH 3 CH 3 COCH 3 CH 3 CHO H 2 C=O

Synthesis of Aldehydes and Ketones Oxidation of appropriate alcohol primary alcohol  aldehyde secondary alcohol  ketone » Possible oxidising agents acidified potassium permanganate acidified sodium dichromate hot copper oxide Preparation of the aldehyde requires care to prevent further oxidation to the carboxylic acid. Preparation of the aldehyde requires care to prevent further oxidation to the carboxylic acid.

 bond Bonding in Aldehydes and Ketones The carbon and the oxygen bond is formed in a similar manner to the carbon to carbon double bond. The carbon and oxygen atoms are sp 2 hybridised (see bonding in alkenes) A  bond and  are formed between the carbon and oxygen.

The bond angles are 120 o and the functional group is planar. Unlike the carbon to carbon double bond, the carbonyl group is significantly polar. The physical and chemical properties of both aldehydes and ketones are governed by the polar nature of the carbonyl group.

∞ ∞ ∞ propanone propanal propan-1-ol butane Solubility (g/100g H 2 O) b.p. (°C) m.p. (°C) RFM Polarity of C=O bond has a significant effect on physical properties.

Hydrogen bond Dipole 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 ketones increases, the non-polar part of the molecule dominates and the solubility decreases. Hence, only the smaller aldehydes and ketones are soluble in water.

Differences in properties The attachment of a hydrogen atom to the carbonyl group in aldehydes make these compounds behave differently in two ways: 1. aldehydes are easily oxidised whereas ketones are oxidised with great difficulty. 2.aldehydes are more susceptible to nucleophilic attack at the carbonyl group. The polarity of the carbonyl group in a ketone is less than in an aldehyde due to the electron donating effect of the two alkyl groups in the ketone. This reduces the partial positive charge on the carbon atom of the group.

Reactions of Aldehydes and Ketones Both undergo addition and condensation reactions. They can be distinguished from one another by their reaction with oxidising agents. Aldehyde oxidation can be carried out by using: Tollen’s Reagent – silver mirror test (complexed Ag +  Ag) (complexed Ag +  Ag) Fehling’s solution (blue) – orange ppt. (complexed Cu 2+  Cu 2 O) (complexed Cu 2+  Cu 2 O)

Aldehydes and Ketones - Reduction Both can be reduced to corresponding alcohol using the reducing agent lithium aluminium hydride (LiAlH 4 ). LiAlH 4 contains the AlH 4 - ion which is able to transfer a hydride ion (H - ) to the partially positive carbon atom of the carbonyl group. This reaction must be carried out in anhydrous conditions as LiAlH 4 reacts explosively with water.

This step is repeated until all 4 hydrogens on the aluminium have been replaced. The resultant complex is hydrolysed to obtain the alcohol. (R-CH 2 -O) 4 Al - + 4H 2 O  4RCH 2 OH + Al OH - The reduction of aldehydes produces primary alcohols whereas the reduction of ketones gives rise to secondary alcohols.

Aldehydes and Ketones - Addition Polarity of the C=O bond means » Carbon atom susceptible to nucleophilic attack. » Oxygen atom susceptible to electrophilic attack. Therefore reaction can be nucleophilic 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 make the carbon atom more susceptible to nucleophilic attack. This can be achieved by solvation of the oxygen atom by water or alcohol solvent. Aldehydes are more reactive than ketones as the partial +ve bond in ketones is stabilised by the electron donating effect of the 2 alkyl groups. Aldehydes are more reactive than ketones as the partial +ve bond in ketones is stabilised by the electron donating effect of the 2 alkyl groups. NB – the mechanism is VERY DIFFERENT from alkene addition reactions.

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

Addition of sodium hydrogensulphite Products are crystalline. Carbonyl compounds can be purified using this reaction. 1)Crude (impure) carbonyl reacted with conc. sodium hydrogensulphite. 2)Crystals are then removed by filtration. 3)Carbonyl compound regenerated by heating with dilute acid. NB the reaction can also be used to remove carbonyl impurities from non-carbonyl compounds.

Addition of ammonia derivatives Some compounds related to ammonia can add to the carbonyl group because of the nucleophilic nature of the lone pair of electrons on the nitrogen atom. Addition is followed by elimination of a water molecule to produce a compound containing a carbon to nitrogen double bond.

Aldehydes and Ketones - Condensation First step of the reaction is the addition of a hydrazine compound (i.e. NH 2 -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. They are always crystalline solids with characteristic melting points once purified. This allows identification of the original carbonyl compound. 2,4-dinitrophenylhydrazine is one of the most commonly used reagents and is used as the test for the carbonyl group. (Brady’s Reagent – yellow ppt forms)

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