Exam+2alt+ans%2C+F-13

H br only one cation possible ch2 br 1 ch2 ch2 br s

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Unformatted text preview: at, and thus cannot possibly be R or S, and many also do not understand cis and trans or (Z/E). Sorry to say that, but it is lamentably true. * H 3C R CH 3 H R H CH 3CH2 ( S) ( Z) CH2CH 3 ( R) E) 4. Here are the three little steps necessary for most of the 30 points: (a) Protonate the double bond (b) Add bromide (c) displace (with inversion) by HS– Sad to say, these steps were beyond many of you. Note that only compound 2 can give two products. The symmetry of 1 means that there can be only one addition product, and one SN2 substitution product. H Br only one cation possible CH2 — Br + 1 CH2 CH2 Br S N2 only one product possible SH By contrast, compound 2 is less symmetrical, and permits - demands, actually - two addition products and two SN2 substitution products. H – SH Br still only one cation possible Br _ + _ 2 Br addition of bromide ion from both sides gives two bromides Br SN 2 SH now there must be two products and –SH –SH SH SN 2 Br 5 and 6 3 and 4 5. The problem is the axial- axial steric interaction (bump) in A that increases its energy relative to that o...
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