[MasterTardigrade003]

This is really confusing… Even though the conditions are fit for an SN2 to take place why is an E2 undertaken ??…. Like after applying NBS on Benzyl cyclohexane and then applying ethoxide ion I get an elimination product as an answer on in my book……. But thats not the only problem at some places the substitution product is also favored….

[phth]

Is an SN2 mechanism favored for a tertiary alkyl halide?

[MasterTardigrade003]

No it isn't..

[phth]

Where does NBS add to a molecule containing aliphatic carbons, and benzylic carbons?

[MasterTardigrade003]

I drew the result in the attachment...

[MasterTardigrade003]

But I am not troubled about this. I am worried what happens at a large in such reactions where I face Sn2 on secondary carbons....
But is this a conditional case where elimination is observed...??
Confusing..

[Babcock_Hall]

As a general principle E2 and S_N2 are competing processes.  Is ethoxide a strong base?  Is ethoxide a strong nucleophile?

[MasterTardigrade003]

Well ethoxide sure is a stronger base than OH^- but it is also a good nucleophile but still, considering the fact that E2 requires bulky bases like tertiary butoxides,etc... This just gets more confusing..

[Babcock_Hall]

E2 reactions require a strong base, but the base can be large or small.  Test-butoxide may give a different product from methoxide, owing to sterics.

Ethoxide and hydroxide are roughly equal in strength when the solvent is water.  In ethanol, the pK_a values are perturbed.

[MasterTardigrade003]

I agree that they require a strong base but but for the case of ethoxide, methoxide and butoxide shouldn't we be comparing them on the basis of their size..
Anyways, this confusion is cleared when we look at the degree of the carbon (except the one here where I got confused) but what about the specific condition I am talking about..??

[Babcock_Hall]

Do you mean specifically with respect to 2° carbons?  One organic textbook that I have indicated that a particular strong base/strong nucleophile, namely the acetylide ion, gives primarily substitution against primary alkyl bromides but E2 elimination against secondary alkyl bromides. 

[MasterTardigrade003]

Yes... I mean specifically about 2° carbons....And also specifically about the cases involving ethoxide ions.. In the case that I mentioned do the structures around that carbon where bromine free radical attacks are also involved in creating steric effects?? If this is the case then in conditions where there are sterics around a carbon we should get more eliminations... Is my hypothesis correct...
Thank you by the way...!!!!

[Babcock_Hall]

I don't understand your question.  If a base is promoting an E2 elimination of an alkyl halide, there is no bromine free radical produced.  Bromide ion (bearing a negative charge) is produced.  With respect to the size of a base, the size may influence which proton is removed, but in the E2 elimination, the proton must be alpha to the leaving group, meaning that it must be on an adjacent carbon.  It might help to draw out the reaction in which you are interested.

[MasterTardigrade003]

Okay just tell me what happens when there is a secondary carbon halide and an ethoxide ion as the reagent and the media is in one condition polar and in another non-polar....