Organic Chemistry

Section 20.1 Reduction Reactions 847

Aldehydes, ketones, and acyl halides can be reduced by sodium borohydride.

The metal–hydrogen bonds in lithium aluminum hydride are more polar than the

metal–hydrogen bonds in sodium borohydride. As a result, is a stronger

reducing agent than Consequently, both and reduce

aldehydes, ketones, and acyl halides, but is not generally used for this pur-

pose since is safer and easier to use. is generally used to reduce only

compounds—such as carboxylic acids, esters, and amides—that cannot be reduced

by the milder reagent.

If diisobutylaluminum hydride (DIBALH) is used as the hydride donor at a low

temperature instead of the reduction of the ester can be stopped after the

addition of one equivalent of hydride ion. Therefore, the final products of the reaction

are an aldehyde and an alcohol (Section 18.5).

Replacing some of the hydrogens of with OR groups decreases the reactivity

of the metal hydride. For example, lithium tri-tert-butoxyaluminum hydride reduces

an acyl chloride to an aldehyde, whereas reduces the acyl chloride all the way

to an alcohol.

1. LiAl[OC(CH 3 ) 3 ] 3 H, −78 ºC


2. H 2 O

an acyl chloride an aldehyde

CH 3 CH 2 CH 2 CH 2 Cl CH 3 CH 2 CH 2 CH 2 H

C

O

C

O

LiAlH 4

LiAlH 4

LiAlH 4 ,

CH 3 CH 2 CH 2 OH

a primary alcohol

an ester

CH 3 CH 2 CH 2 CH 2 OH H 2 O

1. LiAlH 4


2. H 3 O+


3. LiAlH 4


4. H 3 O+
   a primary alcohol
   a carboxylic acid

+

+ CH 3 OH

C

CH 3 CH 2 CH 2 OH

O

C

O

CH 3 CH 2 OCH 3

NaBH 4 LiAlH 4

LiAlH 4

NaBH 4. LiAlH 4 NaBH 4

LiAlH 4

OH

CH 3 CH 2 CH 2 CHCH 3

a secondary alcohol

a ketone

CH 3 CH 2 CH 2 CH 2 OH

1. NaBH 4


2. H 3 O+


3. NaBH 4


4. H 3 O+
   a primary alcohol
   an aldehyde

C

CH 3 CH 2 CH 2 H

O

C

CH 3 CH 2 CH 2

an acyl chloride

CH 3

O

CH 3 CH 2 CH 2 CH 2 OH

a primary alcohol

1. NaBH 4
   CH C 2. H^3 O+
   3 CH 2 CH 2 Cl

O

Remember, the numbers in front of the

reagents above or below a reaction

arrow indicate that the second reagent

is not added until reaction with the first

reagent is completed.

1. [(CH 3 ) 2 CHCH 2 ] 2 AlH, − 78 ºC
   2. H 2 O

an ester an aldehyde

C

CH 3 CH 2 CH 2 OCH 3

O

C

O

CH 3 CH 2 CH 2 H + CH 3 OH

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