Organic Chemistry

842 CHAPTER 20 More About Oxidation–Reduction Reactions

Reduction at carbon increases the

number of bonds or decreases

the number of

or bonds.C¬X

C¬O,C¬N,

C¬H

Oxidation at carbon decreases the

number of bonds or increases

the number of

or bonds.C¬X

C¬O,C¬N,

C¬H

at reactions where oxidationor reductionhas taken place on carbon: If the reaction in-

creases the number of bonds or decreases the number of or

bonds (where X denotes a halogen), the compound has been reduced. If the

reaction decreases the number of bonds or increases the number of

or bonds, the compound has been oxidized. Notice that the oxidation

stateof a carbon atom equals the total number of its and bonds.

Let’s now take a look at some examples of oxidation–reduction reactions that take

place on carbon. You have seen these reactions in previous chapters. Notice that in each

of the following reactions, the product has more bonds than the reactant has: The

alkene, aldehyde, and ketone, therefore, are being reduced (Sections 4.11, 18.5, and

15.15). Hydrogen, sodium borohydride, and hydrazine are the reducing agents.

In the next group of reactions, the number of bonds increases in the first reac-

tion. In the second and third reactions, the number of bonds decreases and the

number of bonds increases. This means that the alkene, the aldehyde, and

the alcohol are being oxidized. Bromine and chromic acid are the oxidizing

agents. Notice that the increase in the number of bonds in the third reaction

results from a carbon–oxygen single bond becoming a carbon–oxygen double bond.

C¬O

(H 2 CrO 4 )

C¬O

C¬H

C¬Br

1. NaBH 4
   RHC 2. H^3 O+ RCH^2 OH

O

an aldehyde

H 2 NNH 2

HO−, ∆ RCH^2 R

a ketone

C

RR

O

RCH CHR RCH 2 CH 2 R

an alkene

H 2

Pt

C¬H

C¬O,C¬N, C¬X

C¬N, C¬X

C¬H C¬O,

C¬X

C¬H C¬O,C¬N,

Tutorial:

Changes in oxidation state

CH 4 CH 3 OH

CH 3 OCH 3

HCH O CO

O

CH 3 CCH 3

O

CH 3 CCH 3

NCH 3

CH 3 CCH 3 (H)

OCH 3

OCH 3

CH 3 CNH 2

O

CH 3 OCNHCH 3

O

CH 3 CCl

O

ClCCl

O

CH 3 COCH 3

O

CH 3 OCOCH 3

O

HCOH

O

OXIDATION STATE

number of C Z bonds

(Z = O, N, or halogen)

01 2 3 4

oxidation reactions

reduction reactions

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