A Guidebook to Mechanism in Organic Chemistry

Aliphatic Bases

usually expressed as pKb (pKb = -log 10 Kb), then the smaller the
numerical value of pKb, the stronger is the base to which it refers.
The strengths of bases may also be expressed in terms of pKa, thus
providing a continuous scale for acids and bases. Ka (and hence pKj
©
for a base R 3 N is a measure of the readiness with which R 3 NH will
part with a proton
R 3 NH+HaO ^ R 3 N + HsO®
e
i.e. of the acidity of R 3 NH, an<l^ defined by:
„ [R 3 N][H 3 Q®]
Ao ~ i
[R 3 NH]

pKb values in water may be converted into pJK„ by use of the relation:

pKa + pKb = 14-00 (at 25°)
(ii) Aliphatic bases

its increasing strength in nitrogenous bases is related to the readiness
with which they are prepared to take up protons and, therefore, to the
availability of the unshared electron pair on nitrogen, we should
expect to see an increase in basic strength as we ge: NH 3 -+RNH,
-t-RjNH-t-RgN, due to the increasing inductive, effect of succes^ve
alkyl groups making the nitrogen atom more negative. An actual
series of amines have pK 6 values as follows, however:

Me Me

\ \

Me-»-NHa NH Me->-N

3-36 y y

Me Me

3-23 4-20

NHS

4-75

Et Et

\. \

Et-»—NHa NH Et->-N

333 y y

Et» Et

3-07 3-12

It will be seen that the introduction of an alkyl group into ammonia
increases the basic strength markedly as expected, ethyl having a
very slightly greater effect than methyl. The introduction of a second