Page 150 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

NITROGEN
130
of P and As are 2.19 and 2.18, respectively) and significantly more than both carbon
(2.55) and sulfur (2.58). Against this backdrop, we can appreciate that ammonia and
organic amines are considered hard bases in the HSAB sense as well as fairly good
–
nucleophiles. The amide ion NH – and its organic analogs R N are very strong bases
2 2
(far too strong to exist in aqueous solution) and are widely used as such in organic
chemistry.
• As suggested above, part of the richness of p-block chemistry derives from the fact
that a given element can act as either a nucleophile or an electrophile depending on
its valence and coordination state. This is particularly true of nitrogen. Many of the
reactions discussed in this chapter involve a nitrogen nucleophile attacking a nitrogen
electrophile, thereby forming a N–N bond. In some cases, subsequent elimination
leads to N–N multiple bonds.
• Like carbon and oxygen, nitrogen forms highly stable multiple bonds, most notably
with carbon, oxygen, and other nitrogen atoms. Not surprisingly, the great thermody-
namic stability of dinitrogen (N ) dictates the course of a large number of reactions
2
involving nitrogen compounds.

5A.1 AMMONIA AND SOME OTHER COMMON NITROGEN NUCLEOPHILES

Ammonia is a base and a nucleophile. It reacts with alkyl halides to give alkylamines, as
shown below for methyl iodide:
H H H H
H − H
− l +
N
C l N C
H
H H H H H
H (5A.1)
H H H + H
N H H H
− NH 4
H N C N C
H +
H H H H
H H H
Polyalkylation is common and difficult to prevent, and with an excess of methyl iodide, the
final product is the tetramethylammonium cation.
H
H
H H H H
H − H C H
I C − I +
N C N C
H
H H H H
H H
H (5A.2)
H H
C H CH 3
H + +
H N C H C N
3
H H 3 C CH 3
N H H
H
H