Substitution vs Elimination Reactions: The Key Differences

Substitution vs Elimination

Alkyl halide reactions typically follow one of four pathways: SN1, SN2, E1, or E2. When completing synthesis problems or writing out reaction mechanisms, choosing the right pathway for the reaction can be tricky. That's why it is important that you have the best organic chemistry book available. Otherwise, if you were unfortunate enough to be cursed with a sub par textbook, I will try my best to explain the differences between nucleophilic substitution and beta elimination with this hub.

Pay Attention To Solvents and Reactants

The first, most important step to choosing the right reaction pathway is taking into account both the reactants and the solvents involved.  So check whether your solvent is protic or aprotic and polar or nonpolar.  The type of solvent involved has a very big effect on the reaction, so it's important to check.  If you have any confusion about what type of solvent you're using, check out this solvent list from the University of Maine.

Next you will want to check the properties of your replacement group.  Is it a strong base or a strong nucleophile?  Strong bases have preference towards elimination reactions while strong nucleophiles favor substitution.

Last, you'll want to check the molecules themselves.  Primary and secondary alkyl halides typically favor fast SN2 or E2 reactions, while the more strained tertiary alkyl halides favor E1 or SN1.

 
Primary Alkyl Halide
Secondary Alkyl Halide
Tertiary Alkyl Halide
Aprotic and Poor Nucleophile
No Reaction
SN2/NR
No Reaction
Protic and Poor Nucleophile
No Reaction
SN1/NR
SN1 and E1
Good Nucleophile
SN2
SN2
SN1
Strong Base
SN2
E2
E2
Hindered Strong Base
E2
E2
E2

Summary

So what you need to take home from this table is, that if it's a strong base (such as H2N-, R-O-, or OH-) the reaction will most likely be elimination. If the reactant is a good nucleophile, but a poor base then substitution is a safe bet.

Protic, highly polar solvents favor SN1 and E1, while aprotic, polar solvents will be found in SN2 reactions. Following these simple tips will get you through the vast majority of nucleophilic substitution and beta elimination reactions.

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Comments 2 comments

abebe 3 years ago

thanks very much


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TolovajWordsmith 23 months ago from Ljubljana

Organic chemistry still fascinates me, although many years have passed since I got my BS ... I think everybody can understand substitutions and eliminations as the basics of organic reactions which open very interesting world of creating millions of new compounds year after year (we need to make at least dozen for graduation). Unfortunately majority of them stay locked in archives for ever, because science and industry still don't cooperate as they could to their best mutual benefits.

But I strayed from the subject, didn't I? Thanks for simple and understandable explanation!

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