Reactions of aldehydes and ketones

Question 1

Explain the reagents and conditions involved in reducing aldehydes and ketones to alcohols.

Answer 1

• A suitable reducing agent is needed such as sodium tetrahydridoborate(III), NaBH₄ (sometimes called sodium borohydride). Water is often used as a solvent. The reaction is usually carried out by warming the carbonyl compound with the reducing agent.

Question 2

Write the equation for the reduction of propanal to propan-1-ol.

Answer 2

CH₃CH₂CHO + 2[H] → CH₃CH₂CH₂OH

Question 3

Write the equation for the reduction of pentan-2-one to pentan-2-ol.

Answer 3

CH₃COCH₂CH₂CH₃ + 2[H] → CH₃CHOHCH₂CH₂CH₃

Question 4

Explain how the discovery of the compound sodium borohydride came about.

Answer 4

Hermann Irving studying chemistry at the Univeristy of Chicago found that NaBH₄ was less reactive than other reducing agets. NaBH₄ would only reduce the carbonyl group present, this means that NaBH₄ could be used to reduce carbonyl compouns in a mixture of other compounds with diffirent functional groups.

Question 5

What is a hydride ion?

Answer 5

• A hydride ion is a negatively charged hydrogen ion with a lone pair of electrons, :H^-.
• NaBH₄readily generates hydride ions.

Question 6

Write and describe the mechanism for the nucleophilic addition reaction of an aldehyde with a hydride ion.

Answer 6

• Alehydes and ketones react with NaBH₄ in a nucleophilic addition reaction. Essentially the BH₄^- ion acts as a source of hydride ions, :H^-.
  1) The electron deficient carbon atom in the polar C=O double bond is attacked by the hydride ion, :H^-, which acts as a nucleophile. The lone pair of electrons from the :H^- ion forms a bond with the carbon atom.
  2) At the same time, the π-bond in the C=O bond breaks to produce a negatively charged intermediate.
  3) The intermediate donates an electron pair to the hydrogen atom of an H₂O molecules, forming a dative covalent bond and a hydroxide ion, OH^-.
• The organic addition product is an alcohol.