Question

2. The following alkyl halide has unknown stereochemistry at the position indicated. Upon undergoing an E2 reaction the single product formed is the alkene shown. Indicate the stereochemistry of the starting materials that would allow for this alkene to form as the exclusive product. Draw a Newman projection down the central bond indicating the relationship between the groups to help support your answer. to Lo

Answer 1

For elimination the alpha C-H bond should be in anti-periplanar alignment with the C-leaving group bond or C-Br bond. So, partial Newman projection should be like:

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Now, the beta carbon with respect to Br has substituents priority order: C(Br) > Ph > CH₃ > H. So the configuration should be R (1 to 2 to 3 clockwise while 4 is below). Now, see our Newman projection. C-H(4) is below. So, have to arrange the substituents on the backward carbon such that from top view 1 to 2 to 3 becomes clockwise. Obviously (1) is the front carbon containing Br. So, in the left hand there must be Ph and in the right hand there must be CH₃.

CH₃ 51T

Now, to get a cis product we must have the second Ph ring attached to front carbon, on the same side of the other one(in backward carbon). So, final Newman projection is:

CH3 -

Slightly rotate the projection:

HACH

Now, lowest priority bond C-H is below. The priority order is Br > Ph > C (methyl) > H

So, configuration must be R. So, in the mother structure Br should remain below the plane and H should remain above:

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