Chemistry- aldehydes and ketones

Describe the bonding in carbonyl compounds

(Aldehydes, ketones, carboxylic acids, esters, amides, acyl chlorides and acid anhydrides)

• Carbonyl functional group

• Planar carbon-oxygen double bond with bond angles of 120º

Why is the carbon-oxygen bond polar

• due to the difference in electronegativities between carbon and oxygen

• The more electronegative oxygen atom draws electrons in the double bond towards itself resulting in a delta positive charge on the carbon atom and delta negative on the oxygen

• This leaves the delta positive carbon atom susceptible to attack by nucleophiles

Why are all carbonyl compounds unsaturated?

What reactions can they undergo?

• Due to the C=O bond

• Nucleophilic addition reactions

• The nucleophile attacks the delta positive carbon and an addition reactions occurs

• The carbon-oxygen bond becomes single (and H+ adds from acidified solution/water? to form neutral compound)

Steps of reduction of aldehydes and ketones

Reduction of carbonyls reagents and conditions

Reagent- NaBH4 in aqueous ethanol

Conditions- room temp and pressure

Reduction of aldehydes eg propanal reaction- reagents, conditions, equation

Aldehydes will be reduced to primary alcohols

Reduction of ketones eg propanone reaction- reagents, conditions, equation

Nucleophilic addition mechanism for reduction of aldehydes eg ethanal

Nucleophilic addition mechanism for reduction of propanone

Reagent, conditions and equation nucleophilic addition reaction addition of HCN

• KCN followed by dilute acid

• Aqueous

Steps of reaction of aldehydes and ketones with HCN

• The lone pair of electrons in the carbon atom in the cyanide ion attacks the delta positive carbon atom of the aldehyde/ketone

• The intermediate then takes a proton from the reaction solvent to form the hydroxynitrile

• Extends the carbon chain by 1 carbon atom

• As C=O group is planar, the CN- ion has an equal chance of attacking the delta positive carbon from above or below so a racemic mixture would be formed

Why dont we use HCN in the lab

What do we use instead and why

• Highly toxic

• HCN is a weak acid so only partially ionises into its ions so gives a low yield of :CN-

• HCN rev → :CN- + H+

• Instead use KCN as fully ionises into :CN- giving high yield of nucleophile (add H+ via sulfuric acid solvent)

• KCN → K+ and :CN-

Mechanism for the nucleophilic addition of cyanide to aldehydes and ketones

Optical activity of symmetrical vs asymmetrical ketones

• Symertrical ketone will not be optically active as there is no chiral centre in the product

• Asymmetrical ketones will contain a chiral centre and show optical isomerism Eg butanone