Organic Chemistry

848 CHAPTER 20 More About Oxidation–Reduction Reactions

The carbonyl group of an amide is reduced to a methylene group by lithium

aluminum hydride (Section 18.5). Primary, secondary, and tertiary amines are formed,

depending on the number of substituents bonded to the nitrogen of the amide. To obtain

the amine in its neutral basic form, acid is not used in the second step of the reaction.

Because sodium borohydride cannot reduce an ester, an amide, or a carboxylic acid,

it can be used to selectively reduce an aldehyde or a ketone group in a compound that

also contains a less reactive group. Acid is not used in the second step of the following

reaction, in order to avoid hydrolyzing the ester:

The multiply bonded carbon atoms of alkenes and alkynes do not possess a partial

positive charge and therefore will not react with reagents that reduce compounds by

donating a hydride ion.

Because sodium borohydride cannot reduce carbon–carbon double bonds, a carbonyl

group in a compound that also has an alkene functional group can be selectively

reduced, as long as the double bonds are not conjugated (Section 18.13). Acid is not

used in the second step of the reaction, in order to avoid addition to the double bond.

A chemoselective reactionis a reaction in which a reagent reacts with one func-

tional group in preference to another. For example, in isopropyl alcohol re-

duces aldehydes faster than it reduces ketones.

O O O

H OH

NaBH 4

isopropyl alcohol

NaBH 4

CH 3 CH

OH

CHCH 2

CH 3 CH CHCH 2 CHCH 3

1. NaBH 4
   CH 2. H^2 O
   3

C

O

CH 3 CH 2 CH CH 2 no reduction reaction

NaBH 4

CH 3 CH 2 CCH no reduction reaction

NaBH 4

O O

OCH 3

OH O

OCH 3

1. NaBH 4


2. H 2 O

CH 3 CH 2 CH 2 CH 2 NHCH 3

a secondary amine

CH 3 CH 2 CH 2 CH 2 NH 2

1. LiAlH 4
   2. H 2 O

CH 3

CH 3

CH 3 CH 2 CH 2 CH 2 NCH 3

a tertiary amine

1. LiAlH 4
   2. H 2 O


2. LiAlH 4


3. H 2 O
   CH 3 CH 2 CH 2 NH 2 a primary amine

CH 3 CH 2 CH 2 NHCH 3

CH 3 CH 2 CH 2 NCH 3

C

O

C

O

C

O

(CH 2 )

Tutorial:

Reductions

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