Steric Effects
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Previously, we discussed the effects of electron availability on the reactivity of molecules.  However, another set of separate and sometimes contradictory effects are steric effects.

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 cation PhN2+:

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.

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