Carbonium Ions, Electron-deficient N and O AtomsiMeT \ Me
fOm^vn; C=CH—Me (IX> \
* /
Me (XI)Such reactions in which a rearrangement of carbon skeleton takes
place are known collectively as Wagner-Meerwein rearrangements.
The rearranged car oonium ion (IX) is also able to eliminate H® to yield
an olBfine and some 2-methylbut-2-ene (XI) is, in fact, obtained. The
rearrangement, with its attendant consequences, can be avoided if the
displacement is carried out under conditions to promote an
reaction path but, as has already been mentioned, the reaction is then
very slow.
The possible occurrence of such rearrangements of a compound's
carbon skeleton during the course of apparently unequivocal
reactions is clearly of the utmost significance in interpreting the
results of degradative and synthetic experiments aimed at structure
elucidation. Some rearrangements of this type are highly complex,
e.g. in the field of natural products such as the terpenes, and have
often rendered the unambiguous assignment of structure extremely
difficult. •
It is interesting that if the halide Me 3 C-CHCl-Ph is hydrolysed
under SNl conditions,no rearrangement like the above takes place for
the first formed carbonium ion (XII) can stabilise itself by delocalisa
tion via the n orbitals of the benzene nucleus, and rearrangement such
as the above is thus no longer energetically advantageous:
In fact no neopentyl alcohol (VIII) is obtained, the only alcoholic
product is found to be t-amyl alcohol (X); this is due to the initial
carbonium ion (VII) rearranging to yield a second one (IX). It will be
seen that the latter is a tertiary carbonium ion whereas the former is a
primary one, and it is an interesting reflection that the tertiary ion is so
much more stable than the primary as to make it energetically worth
while for a carbon-carbon bond to be broken and for a methyl group
to migrate:
Me
Me—C—CH,—Me
H,Cy* |
Me Me / OH
I © I / (X)
Me—C—CH 2 -> Me—C—CH,—Me