Types of Reaction Undergone by Carbonium IonsAll these possibilities are nicely illustrated in the reaction of nitrous
acid with n-propylamine:
© Ha.
MeCH 2 CH 2 N 2 < MeCHaCH 2 NHa
NaNO,
MeCH,CH 2 OH + H®
M/H.O
Na+Me CH 2 CHa (^^MeCH=CH 2 (LTJ)
(I) \ f
* (a)
Me CH Me > Me • CH(OH) • Me+ H®- (TV) H,o (V)
Thus reaction of the n-propyl cation (I) with water as nucleophile,
i.e. (a), yields n-propanol (II), elimination of a proton from the
adjacent carbon atom, (b), yields propylene (III), while rearrange
ment, (c), in this case migration of hydrogen, yields the isopropyl
cation (IV), which can then undergo (b) or (a) to yield more propy
lene (III) or isopropanol (V), respectively. The products obtained in a
typical experiment were n-propanol, 7 per cent, propylene, 28 per
cent and isopropanol, 32 per cent; the greatenstability of the iso-,
rather than the n-, propyl cation being reflected in the much«»jreater
amount of the secondary alcohol produced.
This has not exhausted the possibilities however for reaction of
either carbonium ion with other nucleophiles present in the system
can obviously lead to further products. Thus NOf from sodium
nitrite may lead to the formation of R-N0 2 and R- ONO (the latter
may also arise from direct esterification of first formed R- OH), Cle
from the acid may lead to R- CI, first formed R* OH be converted to
ROR and as yet unchanged R-NH 2 to R-NH-R. The mixture of
products actually obtained is, hardly surprisingly, greatly influenced
by the conditions under which the reaction is carried out but it will
come as no surprise that this reaction is, in the aliphatic series, seldom a
satisfactory preparative method for the conversion of R-NH 2 -»-
ROH!
An analogous situation is observed in the Friedel-Crafts alkylation
of benzene witk n-propyl bromide in the presence of gallium bromide.
Here the attacking species, if not an actual carbonium ion, is a highly
polarised complex (p. 109) RGaBr 4 , and the greater stability of the