Sn2 and E2 question on chad's quiz help please

[SN1]

Ok I stumbled across a question on chad's quiz

The correct answer is C because it would be an Sn2 reaction BUT let say I had a weak nucleophile but a strong base and I would be doing E2

so the solultion says both answer A and B would be correct IF I was doing E2 but which would be the major product?

I know for E2 you are supposed to grab a beta hydrogen that is anti-periplanar(180 degrees opposite) to the leaving group but in this case don't I have TWO beta carbons that have hydrogens that are antiperiplanar to the leaving group? So which would be the major product? Would the major product be produced from taking a beta hydrogen from the MOST substituted carbon(ie; least number of H; zaitsev's rule)?

so for E2 am I supposed to follow the anti-periplanar rule first AND then Zaitsev's or the other way around?

and also I just have to worry about the anti-periplanar thing if I have a chiral center correct?

THANKS>

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[SN1]

NOT one reply. Thank you SDN

 

[diene]

"The outcome of E2 reactions is controlled by the stereochemical requirements described above. Where there is a choice, the more stable alkene will be the major product."

http://www.mhhe.com/physsci/chemist...s/carey04oc/ref/ch05eliminationreactions.html

So I am assuming that the one following Zaitsev's rule would be the major product

 

[sagarkanodiya]

Assuming you are reacting this secondary halide with strong base which is weak nucleophile.


When we form alkene, we eliminate hydrogen or other leaving group.Here we have two beta carbons. beta carbon on left has there hydrogen attached one on dash, one on wedge and one in plane. beta carbon on right has two hydrogens one on dash and one on wedge. So both beta carbons have hydrogens in dash positions( anti-periplanar to bromine wedge position). So we could form both products given in A and B. But major product would be Zaitsev product. I don't know why are you saying we don't have anti-periplar hydrogen. But yes I would say you look for anti-periplanar hydrogen first then Zaitsev rule. If two products are formed then you know that major one would be Zaitsev product. So if you don't have anti-periplanar hydrogen located on adjacent carbon which is more substituted then I really don't know what happens. But I assume the following input might help you. Following input is from chad's site.

some input from chad,
"So for a tertiary halide, adding a bulky base will always yield the Hofmann product as the major product.

But with a secondary halide, it depends on the reaction conditions as to which product you'll get. Unfortunately some professors/textbooks teach that you "always" get the Hofmann product when a bulky base is used and this is how it's presented in the Destroyer problem 133. But the truth is that it's not so clearcut with the secondary halide. For secondary halides I tend to lean toward the Zaitsev product still being the major product but at low temperatures I bet I could get the Hofmann product to be the major product. My point is NOT that secondary halides ALWAYS give the Zaitsev product as they may or may not. The point is that when it's presented that a bulky base ALWAYS results in the Hofmann product it's an oversimplification that's not necessarily true. My hope is that you wouldn't see a secondary halide on the atual DAT due to the discrepancies that are out there on this issue"

 

[SN1]

Cool thanks.