A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism

compared with isomeric compounds in which it is, i.e. (XXIX) in

which there are nine a-hydrogen atoms compared with (XXX) in

which there are only five:.

CHa CH 3

I I

CH 3 —C=CH—CH 3 Me— CH 2 —C=CH 2

(XXIX) (XXX)

This leads to their preferential formation in reactions which could

lead to either compound on introduction of the double bond and

even to the fairly ready isomerisation of the less into the more stable

compound. ' '

Although hyperconjugation has proved useful on a number of

occasions, its validity is not universally accepted and a good deal of

further work needs to be done on, its theoretical justification.

STERIC EFFECTS
We have to date been discussing/actors that may influence the rela­
tive availability of electrons in bonds or at particular atoms ina
compound, and hence influeaQ that compound's reactivity. The
working or influence of these factors may, however, be modified or
even nullified by the operation of steric factors; thus effective delocali-
sajjjjp via n orbitals can only take place if the p or n orbitals on the
atoms involved in the delocalisation can become parallel or fairly
nearly so. If this is prevented, signifi^n^ overlapping cannot take
place and delocalisation fails to occur. A good example of thisjjs'r
provided by dimethylaniline (XXXI) and its 2,6-dialkyl derivatives^
e.g. (XXXII). The NMe 2 group in (XXXI), being electron-donating
(due to the unshared electron pair on nitrogen interacting with the
delocalised «• orbitals of the nucleus), activates the nucleus towards
attack by the diazonium cation PhN 8 ®, i.e. towards azb-coupling,
leading to preferential substitution at o- and, more particularly,
/^-positions (cf. p. H9):

(XXXI)