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I am studying organic chemistry for quite a time now, but this is the general problem I come across. Both seems likely. I only know that heat causes elimination, but this does not appear to be the only reason always. So I want a complete reference.

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    $\begingroup$ I edited your question, also “You should never ask anyone for anything. Never- and especially from those who are more powerful than yourself.” ― Mikhail Bulgakov, The Master and Margarita $\endgroup$
    – Mithoron
    Jul 8, 2015 at 11:22
  • $\begingroup$ @Mith If that were right, then Chem.SE wouldn't exist ;) $\endgroup$
    – paracetamol
    Jun 29, 2017 at 16:15
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    $\begingroup$ @paracetamol There would be no homework, where OP are asking for answer, not posting good questions for everyone. $\endgroup$
    – Mithoron
    Jun 29, 2017 at 16:25
  • $\begingroup$ @Mith Point... but there wouldn't be any questions here to begin with :D $\endgroup$
    – paracetamol
    Jun 29, 2017 at 16:59
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    $\begingroup$ Stating a question and asking for answer are two different things. There's a lot difference between homework dump and Q&A. Also comments are not for prolonged discussions ;) We can continue in Chat $\endgroup$
    – Mithoron
    Jun 29, 2017 at 17:06

1 Answer 1

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A Google search provides several sites that address this challenging topic in organic chemistry in detail. Additionally, most organic chemistry textbooks spend several pages on the matter.

Here's a summary of the conditions that favor each mechanism:

  • $S_N 2$ reactions work best with methyl and primary substrates in polar aprotic solvents. Good nucleophiles are needed.
  • $S_N 1$ reactions work best with tertiary substrates in polar protic solvents. Nucleophiles can be anything, but strong base or heat tends to suggest elimination.
  • $E1$ reactions work best with tertiary substrates in polar protic solvents. Generally, no other reagent but the solvent is given (solvolysis) and the reaction is heated.
  • $E2$ reactions work best with tertiary substrates in polar protic solvents. A strong base is required, and generally it is the conjugate base of the solvent. These reactions are often heated.

Or we can break it down by substrate type:

  • Methyl substrates can only do $S_N 2$ reactions.
  • Primary substrates will do $S_N 2$ with strong nucleophiles in aprotic solvents and $E2$ reactions with strong bases in protic solvents. All other substitution/elimination conditions react so slowly as to be no reaction.
  • Tertiary substrates will do $S_N 1$ in protic solvents with weak bases, $E1$ in protic solvents with weak bases when heated, and $E2$ in protic solvents with strong bases. Substrates that generate resonance stabilized cations will behave like tertiary substrates.

What about secondary?

Secondary substrates are where you need to start looking for clues:

  • $S_N 2$ - polar aprotic solvent
  • $S_N 1$ - protic solvent weak base
  • $E2$ - protic solvent strong base
  • $E1$ - protic solvent and heat

Alcohols in the presence of strong acid generally prefer the $S_N 1$ and $E1$ mechanisms.

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