A Guidebook to Mechanism in Organic Chemistry

Heterocyclic Bases

between the o-N0 2 groups and the hydrogens of the NH 2 may help
to hold these groups in the required orientation: the p orbitals may
become parallel and interact; and the base is thus enormously
weakened by the very powerful electron-withdrawing mesomeric
effects of the three nitro groups:

(IX)

(iv) Heterocyclic bases

(pKb 8 • 96) is a very much weaker base than

the aliphatic tertiary amines (e.g. pKb of Et 3 N = 3-12) and this weak­
ness is found to be characteristic of bases in which the nitrogen atom
is multiply bonded. This is due to the fact that as the nitrogen atom
becomes progressively more multiply bonded its lone-pair of elec­
trons is accommodated in an orbital that has progressively more s
character; they are thus drawn closer to the nitrogen nucleus and
held more tightly by it, thereby becoming less available for forming
a bond with proton, with a consequent decline in the basicity of the
compound. As we go -^N -• - ==N in, for example, R 3 N -+