Aldehydes and ketones 

In a nutshell

Aldehydes and ketones contain a carbonyl group. They can both be reduced to form alcohols. Carbonyls can undergo nucleophilic addition reactions. Any experiments involving potassium cyanide should be performed in a fume cupboard.

Equations 

These are general chemical equations. You will need to be able to use these when you write equations for specific substances.

$$aldehyde \, + [O] \, \rightarrow carboxylic \space acid \\ \ \\ketone + [O] \, \rightarrow no \space reaction \\ \ \\ RCHO \, + 2[H] \rightarrow RCH_2OH \space (primary \space alcohol) \\ \ \\RCOR' \, + 2[H] \rightarrow RCH(R)OH \space (secondary \space alcohol)$$​​

Carbonyl group

Aldehydes and ketones contain a carbonyl group $$(C=O)$$. However, the position of the carbonyl group is different. 

• Aldehydes have a carbonyl group at the end of the carbon chain.
• Ketones have a carbonyl group in the middle of the chain.

Example

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Oxidation

Aldehydes can be oxidised to carboxylic acids. Ketones are not oxidised.  

General equation:

$$aldehyde \, + [O] \, \rightarrow carboxylic \,acid \\ ketone + [O] \, \rightarrow no \, reaction$$​ 

Identifying aldehydes and ketones

Tollens' reagent is a silver nitrate solution dissolved in ammonia. Upon heating with an aldehyde the $$Ag^+$$ ions in Tollens' reagent are reduced to $$Ag$$ atoms, leading to the formation of a silver mirror. There is no reaction or observation with ketones.

Fehling's solution contains copper ($$II$$) ions dissolved in sodium hydroxide. Upon heating with an aldehyde it goes from a blue solution to a brick-red precipitate. This is because copper $$(II)$$ ions are reduced to copper $$(I)$$ oxide. There is no reaction or observation with ketones.

Reduction to alcohols 

A reducing agent, such as $$NaBH_4$$, dissolved in methanol and water can reduce carbonyls back to alcohols. $$[H]$$ is used to represent the reducing agent in an equation.​

• Aldehydes can be reduced to a primary alcohol. 
  
• Ketones can be reduced to a secondary alcohol. 

​$$RCHO \, + 2[H] \rightarrow RCH_2OH \space (primary \, alcohol) \\ RCOR' \, + 2[H] \rightarrow RCH(R)OH \space (secondary \, alcohol)$$

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Nucleophilic addition

Hydroxynitriles are produced when acidified potassium cyanide reacts with a carbonyl compound. The cyanide acts as a nucleophile and attacks the partially positive carbon and adds itself to the molecule. 

General Mechanism

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Risk assessment

Potassium cyanide is an irritant. It can react with moisture to produce hydrogen cyanide (toxic gas). To reduce risks when working with potassium cyanide, perform the experiment in a fume cupboard.  

Important: Always wear safety goggles, a lab coat and gloves when carrying out a chemistry experiment.