When there is elimination and when there is nucleophilic substitution [closed]








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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.




A [Google search](https://www.google.com/webhp?sourceid=chrome-instant&ion=1&espv=2&ie=UTF-8#q=substitution%20vs%20elimination) 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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