The Foundations of Chemistry

pKblog Kb

The weaker a base, the higher its pKb

value.

OXIDATION–REDUCTION REACTIONS

Oxidationof an organic molecule usually corresponds to increasingits oxygencontent

or decreasingits hydrogencontent. Reductionof an organic molecule usually corre-

sponds to decreasingits oxygencontent or increasingits hydrogencontent.

For example, the oxygen content increases when an aldehyde is converted to a carboxylic

acid, so this process is an oxidation.

Converting a primary alcohol to an aldehyde or a secondary alcohol to a ketone is also

an oxidation; the hydrogen content decreases.

oxidation

RC

O

H R (ketone)

R

2 ° alcohol R C

OH

Hyd

rogen

contentdec

reas

es

oxidation

RC

O

H H (aldehyde)

H

1 ° alcohol R C

OH

Oxy

gencontentincrease

s

oxidation

RC

O

R C H OH

O

28-5

We can rank the base strengths of common organic bases as:

alkoxidesaliphatic amines
phenoxidescarboxylates
aromatic amines
heterocyclic amines

TABLE 28-2 Basicities of Ammonia and Some Amines in Water

Name Formula Kb pKb

ammonia NH 3 1.8 10 ^5 4.74

methylamine CH 3 NH 2 5.0 10 ^4 3.30

dimethylamine (CH 3 ) 2 NH 7.4 10 ^4 3.13

trimethylamine (CH 3 ) 3 N 7.4 10 ^5 4.13

ethylamine CH 3 CH 2 NH 2 4.7 10 ^4 3.33

aniline C 6 H 5 NH 2 4.2 10 ^10 9.38

ethylenediamine H 2 NCH 2 CH 2 NH 2 8.5 10 ^5 (Kb1) 4.07

pyridine C 5 H 5 N 1.5 10 ^9 8.82

The notation Remphasizes that
the two R groups may be the same
(RR, e.g., in the formation of

acetone, or different

(R R, e.g., in the formation of
methyl ethyl ketone,

CH 3 C CH 2 CH 3 ).

O

CH 3 C CH 3 )

O

We could describe the oxidation and
reduction of organic compounds in
terms of changes in oxidation numbers,
just as we did for inorganic compounds
in Sections 4-4 through 4-5. Formal
application of oxidation number rules
to organic compounds often leads
to fractional oxidation numbers for
carbon. For organic species, the
descriptions in terms of increase or
decrease of oxygen or hydrogen are
usually easier to apply.