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Nitration and aromatic reactivity - Hoggett J.G.

Hoggett J.G., Moodie R.B., Penton J.R. Nitration and aromatic reactivity - Cambridge, 1971. - 252 p.
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The most accurate method of deriving &2fb. from &2obs. is to use the
equation &2fb. = &2obs.(i +1); the ionisation ratio of the compound under
study being determined directly at the required acidity and temperature.
In the cases where the temperature at which rates are measured is not 25
°C the way in which &2fb. depends upon acidity will be given correctly,
but again there will remain the difficulty that the slope to be expected
at this temperature other than 25 °C is not known.
In concentrated sulphuric acid. The way in which the rate of nitration of
some non-basic compounds depends upon acidity in the region above that of
maximum rate (~ 90% sulphuric acid) has been discussed (§2.3.2). Cationic
species behave similarly (table 2.4, fig. 2.1).
Values of d(log10&2obs.)/d(%H2SO4) at acidities greater than 90%
sulphuric acid for some free bases and cations are given in table 8.2,
and in table 8.3. Curvature of the rate profile often makes accurate
estimation of the slope difficult. However, in general the values of this
quantity for compounds reacting via the free bases are larger than those
given by cations.
Acidity function plots have been used in another way.9 In 65-90 %
sulphuric acid the concentration of the nitronium ion is not equal to the
concentration of nitric acid; the quantity
kt = A2obs.[HN03]stolch/[N02+]
has been used to allow for incomplete ionisation of nitric acid in this
region. Values of k* for a number of compounds have been calculated up to
95 % sulphuric acid, assuming that nitric acid is half-ionised in
8.2] Recognition of the reacting species
88% sulphuric acid and that the ionisation follows - (#jj + l°gioaH2o)-
From the following schemes
k2 obs
[S]stoieh + [HN03]stoIch------> product,
[S]stoicb + [N02+]------> product,
[S] + [N02+]-------> product,
we have U = k2KJ(Ka + hx), where Ka is the ionisation constant of the
conjugate acid SH+, and hx the acidity function appropriate to the
ionisation of this acid. When hx > Ka, then
log10k* = Hx + constant.
Similarly, if nitration involves the conjugate acid, k* = k2hJ(Ka + hx),
and when > K" , , * , ,
°gio 2 = constant.
Thus, when nitration involves a free base, a plot of log10&2 against
- H0 should have a slope of a (where a = HJH0), but if it involves the
conjugate acid the slope should be zero.
It is found in practice that for a number of compounds reacting via the
predominant species an almost horizontal plot is obtained. For compounds
presumed to be nitrated via the free bases, such as 2,6-lutidine i-oxide
and 3- and 5-methyl-2-pyridone, slopes of approximately unity are
obtained.9 Since this type of plot allows for the incomplete ionisation
of nitric acid, it can be used at higher acidities than plots using - (HR
+ log10 eIai) which break down when the condition hRlai2i > E is no
longer true.
8.2.2 Comparison of the rate of nitration of the base with that of a
necessarily cationic derivative
This method is exemplified by its application to quinoline,1
isoquinoline,1 cinnoline,6 and isoquinoline 2-oxide,10 which are nitrated
as their conjugate acids. The rate profiles for these compounds and their
N- or
O-methyl perchlorates show closely parallel dependences upon acidity
(fig. 2.4). Quaternisation had in each case only a small effect upon the
rate, making the criterion a very reliable one. It has the additional
advantage of being applicable at any temperature for which kinetic
measurements can be made (table 8.1, sections A and D).
The criterion has also been applied qualitatively. Thus, 2,4,6-collidine
and 1,2,4,6-tetramethylpyridinium cation can be nitrated under identi-
Aromatic reactivity: A. Bases
cal conditions, suggesting that the protonated collidine is probably the
form being nitrated in the first case.12 Rate profiles for the nitration
of these compounds in oleum support this conclusion.13 In the case of
pyridine i-oxide, conditions which effected nitration with a half-life of
20 min gave none of the nitro-compound from i-methoxypyridinium in 144 h;
the free base is presumably being nitrated in the first case.10 A similar
observation was made with 2,6-lutidine 1-oxide and its quaternary
derivative.10 In the case of quinoline x-oxide the behaviour of
i-methoxyquinolinium is consistent with the view that 5- and 8-nitration
of the former involves the conjugate acid.2
8.2.3 Comparison of the observed rate of nitration with the calculated
encounter rate
The rate constant for the collision of two species is given by the
following approximate formula, and corresponds to a maximum value of the
rate constant of reaction (§§2.5, 3.3):
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