6.2.1 Reactions of Carbonyl Compounds

• 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

• 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: