Page 154 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements

NITROGEN
134
The overall synthesis may be represented as follows:

2NH + NaOCl → N H + NaCl + H O (5A.12)
2
3
4
2
The logic underlying the synthesis is that, since the formation of hydrazine involves the
formation of a N–N bond, we should first transform ammonia to a nitrogen electrophile
of the form NH X, which can then be attacked by ammonia. With a low N–Cl bond
2
dissociation energy (BDE) of about 200 kJ/mol, chloramine (NH Cl) fits the bill for
2
such an intermediate perfectly. To form NH Cl, however, we must first create an elec-
2
trophile that can chlorinate ammonia. Hypochlorous acid (HOCl) appears well suited for
that role:
−
O
− H H − OH Cl OH
Cl O H
−
HO
H Cl (5A.13)
H
− HOH
Cl OH N Cl Cl N
N +
H H
H H H
H
Once NH Cl has been formed, hydrazine formation should occur via a straightforward
2
nitrogen-on-nitrogen S 2 attack, followed by a final deprotonation:
N
Cl H − H + H
H H
− Cl + − H H
N N N N N N (5A.14)
H H H
H H H H H

A final point: Is it reasonable to invoke un-ionized hypochlorous acid (HOCl) as an
active intermediate in a solution of bleach, which is an alkaline medium? With a pK of
a
7.53, hypochlorous acid is a weak acid, about three orders of magnitude weaker than acetic
acid, so it’s not an unreasonable proposal. On the other hand, it is possible to envision
concerted pathways, such as the following, where the hypochlorite anion acts as the actual
chlorinating agent:

H
H
O
O
H
H H H (5A.15)
− +
+ O N H + O Cl N H
Na Cl H Na H