Reduction of Carbonyls (AQA A Level Chemistry)

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Reduction of Carbonyls

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:

7-2-2-reduction-of-a-carboxylic-acid

  • Aldehyde to a primary alcohol:downloadable AS & A Level Chemistry revision notes
  • Ketone to a secondary alcohol:

Reduction of a ketone, downloadable AS & A Level Chemistry revision notes

Examiner Tip

In theory the reduction of a carboxylic acid is a two stage process, from the carboxylic acid to the aldehyde and then further reduction from the aldehyde to the primary alcohol. In reality however, the reaction would really go from the carboxylic acid straight to the primary alcohol. Be careful and check the wording of the question when asked about the reduction of a carboxylic acid!

Examiner Tip

Lithium tetrahydridoaluminate, LiAlH4, in a non-aqueous solvent can also be used as a reducing agent for the reduction of carbonyls.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.