Photochemistry
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
Joint initiative of IITs and IISc – Funded by MHRD
Page 1 of 55
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
Joint initiative of IITs and IISc – Funded by MHRD
Page 2 of 55
NPTEL – Chemistry – Reagents and Organic reactions
2.1.1.4
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
Joint initiative of IITs and IISc – Funded by MHRD
Page 1 of 55
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
Joint initiative of IITs and IISc – Funded by MHRD
Page 2 of 55
NPTEL – Chemistry – Reagents and Organic reactions
2.1.1.4