bulky bases in elimination reactions

[yoyohomieg5432]

Normally when I see talk of bulky bases it's in context of E2. Bulky bases favor formation of non-zaitsev product. It also is used to favor E2 over Sn2 reactions (correct?)

Why is talk of bulky bases not really discussed with respect to E1? I.e. wouldn't using a bulky base favor E1 over Sn1? Since the base can't act as a nucleophile at the carbocation? E1 is known for always making the zaitsev product, but is there ever cases where you would want to use a bulky base for E1 so you form a non-zaitsev?

---

Members don't see this ad.

 

[Jesus1]

Generally bulky bases act in E2 reactions.They are electron rich but their really large size restricts them from acting as nucleophiles for steric reasons.
An easy way to avoid getting confused with these reactions is to use the following rubric;
Primary alkyl halides-always do only SN2 or E2.With a small base or nucleophile like ethoxide,SN2 will always be preferred as elimination is slowest in primaries.With bulky bases (like tBuOK,DBU,DBN) however E2 is always preferred.
For secondaries-Small bases will generally give a mixture of SN2 and E2 products.Bulky bases only give E2 products.Alcohols and water almost always give an SN1/E1 mixture.In fact SN1/E1 typically occur together.
For tertiaries-Small bases will do exclusively E2 (as SN2 does not occur in tertiaries).Bulkies will do only E2 as well.Water or an alcohol will do E1 and SN1.
Heat favors elimination
In a ring (like cyclohexane),bulky bases usually go for the exocyclic elimination product (which is typically the least substituted alkene in effect disobeying Zaitsev's rule).
Hope this helps.