Steric effects Hofmann elimination

Tertiary halides undergo elimination most easily. Eliminations of chlorides, bromides, and iodides follow Zaitsev s rule, except for a few cases where steric effects are important (for an example, see p. 1316). Eliminations of fluorides follow Hofmann s rule (p. 1316). 

The effect of Y on the mode of elimination may also involve a steric element. Thus it is found that increase in the size of Y and, more particularly branching in it, leads to an increasing proportion of Hofmann elimination with the same alkyl group, e.g. with (42) ... 

These several steric effects are explainable on the basis that any crowding, irrespective of its origin, will make the T.S. (44) that involves the removal of proton from (46a)—Saytzev elimination—relatively more crowded than the T.S. (45) that involves removal of proton (D from (46b)—Hofmann elimination. The differential will increase as the crowding increases (in R, Y or B), and Hofmann elimination will thus be progressively favoured over Saytzev ... 

Hofmann s Rule implies that steric effects have the greatest influence on the outcome of the Hofmann or similar eliminations. The loss of the (3-hydrogen occurs preferably from the most unhindered (least substituted) position [-CH3 > -CH2-R > -CH(R2)]. The product alkene with fewer substitutents will predominate. 

If steric hindrance is not significant, electronic effects will predominate. Finally, it should be noted that there is a statistical factor in favor of the Hofmann elimination, because there are usually more protons that can be eliminated on the Hofmann side. 

We can understand the regioselectivity of the Hofmann elimination by comparing steric effects in the E2 transition states for formation of 1-butene and trans-2-... 

The direction of elimination is also affected by steric effects. Highly hindered bases shift the orientation in dehydrohalogenations toward more Hofmann rule elimination. This effect can be reasonably attributed to the fact that the internal hydrogen that must be removed for Saytzeff rule elimination becomes inaccessible to very bulky bases, and abstraction of less-hindered protons is favored ... 

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