Previously, we discussed the effects of electron availability on
the reactivity of molecules. However, another set of separate
and sometimes contradictory effects are steric
For example, mesomeric effects will only work if the pi systems
involved can become planar (or at least very nearly); if they are
not, the delocalisation will not be effective. In some cases,
steric effects can prevent the pi-system from attaining planarity,
and thereby disrupt the mesomeric effect.
This can be seen if we compare the molecules N,N-dimethylaniline,
and the 2,6, dimethyl derivative of this. Firstly let us look
at the behaviour of N,N-dimethylaniline in relation to the diazonium
We can see that the overall reaction leads to substitution
at the para position. However, the 2,6 dimethyl derivative
does not display this behaviour.
|2,6 dimethyl N,N-diemthylaniline
As we can see from the diagram above, the presence
of the two extra methyl groups forces the methyls attached to the
nitrogen atom to adopt a position almost perpendicular to the plane
of the ring. This means that there is no longer effective
overlap between the nitrogen lone pair and the ring system, as there
was before. In this case, the mesomeric effect has been totally
disrupted, and the dimethyl derivative does not perform the substitution
reaction seen above.