Reactions of Carbonyl Compounds (OCR A Level Chemistry A): Revision Note

Oxidation of Aldehydes

• Aldehydes and ketones contain the carbonyl functional group, C=O

• This is why aldehydes and ketones are also known as carbonyls

• The difference between aldehydes and ketones is the groups bonded to the carbon of the carbonyl group

• The carbonyl group in an aldehyde is always situated at the end of the chain

  • When naming aldehydes, you do not include the '1' in the name, the carbonyl carbon is always number 1 on the chain

  • The simplest aldehyde is methanal, HCHO, with the only carbon being that of the carbonyl group

• The carbonyl group in a ketone is always situated in the middle of the chain

  • The simplest ketone is propan-2-one, CH₃COCH₃, as you need an alkyl group either side of the carbonyl carbon in a ketone

• During the oxidation of a primary alcohol to an aldehyde, the apparatus must be set up to distill off the aldehyde as it is produced

• Further oxidation of primary alcohols can then take place

  • Aldehydes can be easily oxidised to form carboxylic acids

• To oxidise a primary alcohol straight to a carboxylic acid, you would heat the reaction mixture under reflux

  • The aldehyde would still be produced, but as it evaporates it would condense and drop back into the reaction mixture, to be further oxidised to the carboxylic acid

• The oxidising agent used for all of the oxidation reactions be acidified potassium dichromate

  • K₂Cr₂O₇ with sulfuric acid, H₂SO₄

• Ketones are very resistant to being oxidised, so no further oxidation reaction will take place with secondary alcohols

  • This is because ketones do not have a readily available hydrogen atom, in the same way that aldehydes (or alcohols) do

  • An extremely strong oxidising agent would be needed for oxidation of a ketone to take place

  • The oxidation will likely oxidise a ketone in a destructive way, breaking a C-C bond

Nucleophilic Addition Reactions

• Many of the reactions which carbonyl compounds undergo are nucleophilic addition reactions

• The carbonyl group -C=O, in aldehydes and ketones is polarised

• The oxygen atom is more electronegative than carbon drawing electron density towards itself

• This leaves the carbon atom slightly positively charged and the oxygen atom slightly negatively charged

• The carbonyl carbon is therefore susceptible to attack by a nucleophile, such as the cyanide ion

   The carbonyl group here has a dipole with a delta positive carbon and a delta negative oxygen 

General Mechanism with an aldehyde:

General Mechanism with a ketone: 

In both reactions, the nucleophile (Nu) attacks the carbonyl carbon to form a negatively charged intermediate which quickly reacts with a proton

Addition of HCN to carbonyl compounds

The nucleophilic addition of hydrogen cyanide to carbonyl compounds is a two-step process, as shown below

• In step 1, the cyanide ion attacks the carbonyl carbon to form a negatively charged intermediate

• In step 2, the negatively charged oxygen atom in the reactive intermediate quickly reacts with aqueous H⁺ (either from HCN, water or dilute acid) to form 2-hydroxynitrile compounds,

  • e.g. 2-hydroxypropanenitrile

• This reaction is important in organic synthesis, because it adds a carbon atom to the chain, increasing the chain length

• The products of the reaction are hydroxynitriles

  • The nitrile group is the priority functional group so it is attached to carbon 1 and results in the suffix -nitrile

  • The hydroxyl group is not the priority functional group so the hydroxyl group is named using the hydroxy- prefix, rather than the -ol suffix

Reduction of Carbonyls

• There are a large number of reducing agents which will reduce both an aldehyde and a ketone to an alcohol

• Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols

• Possibly the most common reducing agent for this is sodium tetrahydridoborate, NaBH₄

  • You may also see this named as sodium borohydride in some sources

• In an aqueous solution, NaBH₄ generates the hydride ion nucleophile, :H^-

• The hydride ion will reduce a carbonyl group in an aldehyde or a ketone, but is not strong enough to reduce a C=C double bond

  • This is because it is attracted to the C in the C=O bond, but is repelled by the high electron density of the C=C bond

• When this reaction takes place, it is an example of a nucleophilic addition reaction

Reduction Reactions

• Carboxylic acid to a primary alcohol:

  

• Aldehyde to a primary alcohol:

  

• Ketone to a secondary alcohol: