Introduction to Soil Chemistry

In Figure 2.5 the path to the right involves lower energy transition states
and reactive intermediates, and so the reaction will initially follow this path,
although the product (Pr) is at an energy higher than the one obtained by following
the left path (P l). The formation of this product (P r) will be controlled
by kinetics, will be produced first, and will initially be at the highest concentration.
After some time, reaction will take place and the product on the right
(P r) will rereact to form the more stable, thermodynamic, product on the left
(P l). This means that on standing, the product on the right will decrease to a
low concentration while the one on the left will dominate the product mix.
In soil it is very common to find that a chemical added to soil becomes
increasingly difficult to remove with time. The mechanism described above is
one that would account for this observation. A compound added to soil crystallizes
to a less stable crystal structure that is easily dissolved and extracted
from soil. With time its crystalline structure rearranges to a less soluble, lower
energy, and thus less easily extractable form [14].
2.6. STERIC FACTORS
Large bulky groups will slow or prevent nearby reactive sites from
being attacked. They may also cause a reactive species to assume a shape that
is not accessible to attack. The following two common bulky groups, which
have a high degree of steric hindrance, are used extensively in organic
synthesis:
CH 3
C
CH
3
CH 3
rate factors 39
t-butyl 2,2-dimethylpropyl group
(2.2)
The position of such groups can affect the reactivity, access to reactive sites,
and the conformation of the species to which they are attached. Thus, in considering
the extractability of a component, its form, reactivity, and the occurrence
of steric hindrances must all be considered [15].
2.7. RATE FACTORS
The rate of a reaction can also be an important factor in both the amount
and longevity of a component in soil. A very common and interesting
example is the microbial oxidation of ammonia first to nitrite and then to
nitrate:
H
C
H
CH 3
C CH 3
CH 3