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NPTEL – Chemistry – Reagents and Organic reactions

Module II
Lecture 14

Reduction Reactions

2.1.1 Lithium Aluminum Hydride (LAH)

2.1.1.1 Introduction
Lithium aluminum hydride (LAH) is a strong reducing agent with chemical formula LiAlH4. It can reduce a variety of functional groups such as aldehydes, esters, acids, ketones, nitriles, epoxides and azides. It vigorously reacts with water and all the reactions are performed in polar aprotic solvents.

2.1.1.2 Preparation
It was first prepared by treating lithium hydride (LiH) with aluminum chloride (AlCl3) (Scheme 1). 4LiH + AlCl3
Scheme 1

LiAlH4 +

3LiCl

In industrial scale, it is prepared from sodium aluminum hydride which is prepared by reaction of sodium, aluminum and hydrogen at high temperature and pressure (Scheme 2). Na + Al + 2H2 NaAlH4 LiAlH4

Scheme 2

NaAlH4 + LiCl

+ NaCl

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NPTEL – Chemistry – Reagents and Organic reactions

2.1.1.3 Reduction of Aldehydes and Ketones
The aldehydes and ketones are reduced to give the corresponding alcohols when treated with LAH (Scheme 3). For the reduction of aldehydes and ketones, the reaction is generally performed in ether solvents at low temperature. O R or O R R' R H LiAlH4 R OH or H

OH R'

O Br Me

LiAlH4 Et2O, -78 C
Scheme 3

OH Br Me

Mechanism The hydride from LAH is transferred to the carbonyl group to form alkoxy anion which then forms complex with aluminum (Scheme 4). Similarly, four hydride atoms are replaced by four alkoxy groups and the aluminum complex thus formed is then hydrolyzed by dilute acid to give the desired alcohol. O O R H H H Al H H H O Al H R H H H R H H H R R O H R H O H O Al O R H H H O H R R H O H H O Al O R H O H H H R H R H H R

O O Al H R H H H

OH 4 R H H

H3O

Scheme 4

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NPTEL – Chemistry – Reagents and Organic reactions

2.1.1.4 Reduction of Acids
The acids are often reduced with LAH to prepare the corresponding alcohols (Scheme 5). The other comparable method for reduction of acid is using BH 3 as a reducing agent. The three hydride ions are consumed for reducing one acid group.

O R
Me O OH

LiAlH4 OH

R

OH
Me

LiAlH4 Et2O
Scheme 5

OH

2.1.1.5 Reduction of Esters and Amides
The esters and amides are reduced to the corresponding alcohols and amines, respectively, when treated with LAH (Scheme 6). Each molecule of ester or amides consumes two molecules of hydride ions. O R or O R N R' R' R N R' O R' LiAlH4 R or R' OH + R' OH

Me O O O O Me O Me

Me O LiAlH4 Et2O HO O Me O Me OH

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NPTEL – Chemistry – Reagents and Organic reactions

LiAlH4 NH O OH THF NH OH

Me
Scheme 6

Me

2.1.1.6 Reduction of Azides, Nitriles and Nitro Compounds
The azides, nitriles and nitro compounds are reduced to the corresponding amines in presence of LAH (Scheme 7). The aromatic nitro compounds do not give the desired amines. R or R C N or R NO2
Scheme 7

N3 LiAlH4 R NH2

2.1.1.7 Reduction of Epoxides
The epoxides are reduced to the corresponding alcohols (Scheme 8). The hydride ion is transferred to the less hindered side of the epoxides. Me LiAlH 4 O Me OH

O

Me H CO2Me O O

HO LiAlH4 Et2O/THF 0 C

Me H O OH O

Scheme 8

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NPTEL – Chemistry – Reagents and Organic reactions

2.1.2 Lithium Trialkoxyaluminum Hydride [LiAlH(OR)3] 2.1.2.1 Introduction Though LAH is a powerful reducing agent but is less selective. The reactivity and selectivity can be modified by replacing the three hydride ion with alkoxy groups such as tert-butoxy or ethoxy group. The resulting reagents are less reactive but more selective than LAH.

2.1.2.2 Preparation
The lithium trialkoxyaluminum hydride can be prepared by treating LAH with the alcohol and...
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