A Guidebook to Mechanism in Organic Chemistry

r

the unshared electron pair is again in an sp^3 orbital and its pJf 6

(3 *42) is almost identical with that of triethylamine (3-12).

Pyrrole (X) is found to exhibit some aromatic character (though

this is not so pronounced as with benzene or pyridine) and does not

behave like a conjugated diene as might otherwise have been expected:

HC CH

\../

• N
  
  
  • H
    — * (X)
  
  
  
  

For such aromaticity to be achieved, six n electrons of the ring atoms

must occupy stable delocalised orbitals. This necessitates the contri­

bution of two electrons by the nitrogen atom and, though the com­

pleted orbitals will be deformed towards nitrogen because of the

more electronegative nature of that atom as compared with carbon,

nitrogen's electron pair will thus not be available for taking up a

proton:

H

(X)

The situation resembles that already encountered with aniline (p. 52)

in that the cation (XI), obtained if protonation is forced upon pyrrole

(protonation is shown as taking place on nitrogen, but it may occur

on the a-carbon atom as happens with C-alkylated pyrroles),

C 5 H 5 N -»-R• C==N, the unshared pairs are in sp*, sp^3 and sp^1 orbitals,

respectively, and the declining basicity is reflected in the two pK 6

values quoted above and the fact that the basicity of alkyl cyanides is

too small to measure.

With quinuclidine, however,

The Strengths of Acids and Bases