Chemistry Reactions

[Mao]

/posting in locked thread.

I don't think the question has been answered properly yet.

In deciding whether or not a reaction is SN or E, we have to think about the leaving group, the incoming group (if any), and the stability of the final product. The requirements are very different.

E:
- a group (typically halide) leaves
- a strong base abstracts an alpha proton (or another leaving group)
- forms double bond

If your initial compound cannot form a double bond like this (e.g. t-Butyl chloride), you don't get elimination reactions.

SN:
- a group (typically halide) leaves
- a group (typically a strong nucleophile) enters
- remains aliphatic

In determining whether a reaction is *1 or *2, it comes down to how well the original compound can support charge. Tertiary carbons can stabilise a cationic charge quite well (via short-range distortions of orbitals), and thus we tend to have 1st order reactions (dissociation: rate[A] --> subsequent steps). For less substituted carbons, carbocations tend to be much less stable, so dissociation is unlikely. In these cases, the nucleophile/base do their thing first (rate[R][A], hence 2nd order).

This is a very crude way to interpret it, the reality is there is no clear-cut distinction between these mechanisms, or a critical 'strength' of nucleophile or base, or a critical structural feature that will determine SN is favoured over E or vice versa. The only predictive method I have seen is via quantum-chemical calculation of the free-energy landscape of the reaction, and even then it's often a competition between different mechanisms.

TL;DR, SN1, SN2, E1, E2, they all take place to a certain extent. Look at the stability of products to see whether it is SN or E. Look at the stability of intermediates/transition state to see whether it is *1 or *2. If you don't see a clear-cut answer, the reality probably isn't so clear cut either.