A Guidebook to Mechanism in Organic Chemistry

Structure, Reactivity and Mechanism 0

often being indicated by curved arrows (e.g. (IV) ->-(XII) or (XIII)),
the tail of the curved arrow indicating where an electron pair moves
from and the head where it moves to:

CH^CH-l^CH^CHj «-» CH^CH^CH^-CHj

(IV) (XII)

CH 2 =^CH^CH=Qh 2 ~ CH 2 -^CH^=CH-^CH 2

av) (xiii)

This is the basis of the concept ^f resonance. The individual

classical structures that may readily be written down are referred to

as canonical structures and the real, unique structure of the com­

pound, somewhere 'in between' all of them, being referred to as a

resonance hybrid. The term mesoWierism is also used for the pheno­

menon, though less widely, to avoid the semantic difficulty mentioned

above, emphasising that the compound <fbes not have several

possible structures which-are rapidly interconverted (i.e. it is^jp/

a sort of extra rapid and reversing tautomerism!), but one structure

only, 'in between' the classical structures that can more readily be

written (meso implying 'in betweea').

A certain number of limitations must be borne in mind, however,

whjg considering delocalisation and its representation through two

or more classical structures as above. Broadly speaking, the more

canonical structures that can be written foj a compound, the greater

the delocalisation of electrons and the more stable the compound <qjU

be. These structures must not vary too widely from each other in

energy content, however, or those of higher energy will contribute so

little to the hybrid as to make their contribution virtually irrelevant.

Structures involving separation of charge (e.g. XII and XIII) may be

written but, other things being equal, these are usually of higher

energy content than those in which such separation has not taken

place (e.g. IV), and hence contribute correspondingly less to the

hybrid. The structures written must all contain the same number of

paired electrons and the constituent atoms must all occupy essentially

the same positions relative to each other in each canonical structure.

If delocalisation is to be significant, all atoms attached to unsatur­

ated centres must lie in the same plane or nearly so. This requirement

has already been referred to for butadiene (p. 9), for if the molecule

takes up a position such as (XIV)