L4 - Reduction

objectives

Understandings

• Carboxylic acids can be reduced to primary alcohols (via the aldehyde). Ketones can be reduced to secondary alcohols. Typical reducing agents are lithium aluminium hydride (used to reduce carboxylic acids) and sodium borohydride.

Applications and Skills

• Writing reduction reactions of carbonyl containing compounds: aldehydes and ketones to primary and secondary alcohols and carboxylic acids to aldehydes, using suitable reducing agents.
• Conversion of nitrobenzene to phenylamine via a two-stage reaction.

Reduction of Carbonyls

Reagent

sodium tetrahydridoborate(III) (sodium borohydride), NaBH₄ or LiAlH4

Conditions

aqueous or alcoholic solution

Mechanism

Reduction

Nucleophile

H¯ (hydride ion)

Product(s)

Alcohols

Aldehydes are REDUCED to primary (1°) alcohols.

Ketones are REDUCED to secondary (2°) alcohols.

Carboxylic Acids can be Reduced to Aldehydes and then Primary Alcohols

Equation(s)

CH₃CHO + 2[H] ——> CH₃CH₂OH

CH₃COCH₃ + 2[H] ——> CH₃CHOHCH₃

Notes The water provides a proton

Reduction of Carboxylic Acids

Reagent/catalyst:

lithium tetrahydridoaluminate(III) LiAlH₄

Conditions

reflux in ethoxyethane

Product

aldehyde

Equation

e.g. CH₃COOH(l) + 2[H] ———> CH₃CHO(l) + H₂O(l)

Reduction of Aldehydes

Reagent

sodium tetrahydridoborate(III) NaBH₄

Conditions

warm in water or ethanol

Product

primary alcohol

Equation

e.g. C₂H₅CHO(l) + 2[H] ———> C₃H₇OH(l)

Reduction of Ketones

Reagent

sodium tetrahydridoborate(III) NaBH₄

Conditions

warm in water or ethanol

Product

secondary alcohol

Equation

e.g. CH₃COCH₃(l) + 2[H] ———> CH₃CH(OH)CH₃(l)

Reduction of Nitrobenzene

Nitrobenzene to phenylamine

The conversion is done in two main stages:

Stage 1: conversion of nitrobenzene into phenylammonium ions

Nitrobenzene is reduced to phenylammonium ions using a mixture of tin and concentrated hydrochloric acid. The mixture is heated under reflux in a boiling water bath for about half an hour.

Under the acidic conditions, rather than getting phenylamine directly, you instead get phenylammonium ions formed. The lone pair on the nitrogen in the phenylamine picks up a hydrogen ion from the acid.

The electron-half-equation for this reaction is:

Stage 2: conversion of the phenylammonium ions into phenylamine

All you need to do is to remove the hydrogen ion from the -NH₃⁺group.

Sodium hydroxide solution is added to the product of the first stage of the reaction.