How to tell if reaction will be SN2 or E2?

[alasra]

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I'm having a lot of trouble understanding when a reaction will be SN2 or E2 when there is a strong base/strong nucleophile (-OH, -OCH3, NaOH, KOH, -OCH2CH3, -OR) involved with a secondary halide. Any help is appreciated!

 

[spdental]

Larger groups form E2 because they want to rip off the hydrogen, small groups are able to get past steric repulsion and will form SN2. Additionally, tertiary can never do sn2 because it's so sterically hindered, that will always be E2.

 

[deleted678432]

Its sometimes ambiguous. Sometimes (like -och3), it can do both due to it being nucleophilic, and quite basic. But some (-otertbutyl) are still pretty nucleophilic and basic, yet it is too big to do any sub rxns. This is why you have major and minor products. I'd look up a chart or a video on khan academy or professor dave explains. Once you get it, it's easy

 

[DAT Destroyer]

I'm having a lot of trouble understanding when a reaction will be SN2 or E2 when there is a strong base/strong nucleophile (-OH, -OCH3, NaOH, KOH, -OCH2CH3, -OR) involved with a secondary halide. Any help is appreciated!

Here is a great way to do this. I teach my students the following....... The secondary halide will Do E2 to get the major product if the following are used.....HO- and heat......CH3O-...C2H5O-....NaNH2....or t-Butoxide.......The first four gives E2 Zaitsev, while the large bulky base t-butoxide or LDA give the Hoffman product whereby the less substituted alkene is produced. If you see RS- or CN-....it certainly supports the SN2 reaction........now for the others.......If the nucleophile or solvent is polar protic such as water or alcohol....do Sn1....If polar aprotic solvents are seen such as DMF, THF, DMSO, HMPT you are likely to do Sn2. I would suggest that you get the text by David Klein PhD, which will reinforce the concepts that I presented to you along with several worked out examples.

I hope this helps

Dr. Jim Romano