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Aromatic chemistry and carbonyls

Aldehydes and ketones

Aldehydes and ketones

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Explainer Video

Tutor: Alisha

Summary

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] →carboxylic acid ketone+[O] →no reaction RCHO +2[H]→RCH2OH (primary alcohol) RCOR′ +2[H]→RCH(R)OH (secondary alcohol)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)aldehyde+[O]→carboxylic acid ketone+[O]→no reaction RCHO+2[H]→RCH2​OH (primary alcohol) RCOR′+2[H]→RCH(R)OH (secondary alcohol)​​



Carbonyl group

Aldehydes and ketones contain a carbonyl group (C=O)(C=O)(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.
Chemistry; Organic chemistry II; KS5 Year 12; Aldehydes and ketones
1. Aldehyde; 2. Ketone


Example
Chemistry; Organic chemistry II; KS5 Year 12; Aldehydes and ketones
1. Methanal; 2. Pentan-333-one​


​

Oxidation

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

General equation:


aldehyde +[O] →carboxylic acidketone+[O] →no reactionaldehyde \, + [O] \, \rightarrow carboxylic \,acid \\ ketone + [O] \, \rightarrow no \, reactionaldehyde+[O]→carboxylicacidketone+[O]→noreaction​ 


​[O][O][O]​​
An oxidising agent



Identifying aldehydes and ketones

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


Fehling's solution contains copper (IIIIII) 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)(II)(II) ions are reduced to copper (I)(I)(I) oxide. There is no reaction or observation with ketones.



Reduction to alcohols 

A reducing agent, such as NaBH4NaBH_4NaBH4​, dissolved in methanol and water can reduce carbonyls back to alcohols. [H][H][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]→RCH2OH (primary alcohol)RCOR′ +2[H]→RCH(R)OH (secondary alcohol)RCHO \, + 2[H] \rightarrow RCH_2OH \space (primary \, alcohol) \\ RCOR' \, + 2[H] \rightarrow RCH(R)OH \space (secondary \, alcohol)RCHO+2[H]→RCH2​OH (primaryalcohol)RCOR′+2[H]→RCH(R)OH (secondaryalcohol)


​

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

​


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. 

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FAQs - Frequently Asked Questions

How are hydroxynitriles produced from carbonyls?

Hydroxynitriles are be produced when acidified potassium cyanide reacts with a carbonyl compound.

What is Fehling's solution?

Fehling's solution contains copper (II) ions dissolved in sodium hydroxide. It helps to identify aldehydes and ketones.

What is Tollens' reagent?

Tollens' reagent is a silver nitrate solution dissolved in ammonia. It helps to identify aldehydes and ketones.

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