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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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closed as too broad by Mithoron, Todd Minehardt, jerepierre, M.A.R. ಠ_ಠ, airhuff Jun 29 '17 at 19:45

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • $\begingroup$ 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 '15 at 11:22
  • $\begingroup$ @Mith If that were right, then Chem.SE wouldn't exist ;) $\endgroup$ – paracetamol Jun 29 '17 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 '17 at 16:25
  • $\begingroup$ @Mith Point... but there wouldn't be any questions here to begin with :D $\endgroup$ – paracetamol Jun 29 '17 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 '17 at 17:06
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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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