Substitution vs. Elimination

[alanan84]

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If given a secondary halide, how do you know whether it's substitution or elimination. I know that a strong base or nucleophile will go Sn2 or E2 and a weak base or nuc will go Sn1 or E1 but where do you go from there?

 

[Shingo123]

If given a secondary halide, how do you know whether it's substitution or elimination. I know that a strong base or nucleophile will go Sn2 or E2 and a weak base or nuc will go Sn1 or E1 but where do you go from there?

The only other things I know of are:

High temperature favors elimination.

Hindrance favors elimination, be it on the base or the substrate.

Like a secondary halide with a neopentyl group on it will likely be elimination.

Or a base with three phenyl groups on it like (C6H5)3Cl will favor elimination on a secondary since it can't do backside attack.


I hope that helps.

 

[Astros1fan]

A few clues that I've found thanks to MCAT Gold videos are that if an alkene is produced you know it was E1 or E2, if it was inverted it was SN2, a racemic mixture produced then SN1. Also big bases prefer to replace primiary Hydrogens so it will be E2

 

[elowdds]

I often refer to this link I found here on SDN:

http://www.brynmawr.edu/Acads/Chem/mnerzsto/SN2vsE2_flowchart.htm

Hope this is helpful for your question.🙂

 

[nze82]

If given a secondary halide, how do you know whether it's substitution or elimination. I know that a strong base or nucleophile will go Sn2 or E2 and a weak base or nuc will go Sn1 or E1 but where do you go from there?

If you see the following, the reaction will proceed via E2:

1-Heat
2-Bulky base: Remember that the rate of SN2 reactions is affected by steric hindrance, because the nucleophile must be able to approach the carbon from the back side and attack it. However, steric hindrance will NOT affect the rate of E2 reactions, because in case of elimination reactions the base is abstracting the small proton. So, using a bulky base limits SN2 reactions, while having no effect on E2.

Finally, notice that SN2 and E2 are competing with one another, under the circumstances you've described. Therefore, you can get both an elimination product and a substituted product.

Good Luck👍