Solutions_Manual_for_Organic_Chemistry_6th_Ed 171

Solutions_Manual_for_Organic_Chemistry_6th_Ed 171 - 8-15...

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Unformatted text preview: 8 -15 continued C (c) o 8-16 c5 . bottom � empty p orbital in \"'-- :Br: � planar carbocation � CH3 Br The planar carbocation is responsible for non-stereoselectivity. The bromide nuc\eophile can attack from the top or bottom, leading to a mixture of stereoisomers. The addition is therefore a mixture of syn and anti addition. 8-17 During bromine addition to either the cis- or trans-alkene, two new chiral centers are being formed. Neither alkene (nor bromine) is optically active, so the product cannot be optically active. The cis-but-2-ene gives two chiral products, a racemic mixture. However, trans-but-2-ene, because of its symmetry, gives only one meso product which can never be chiral. The "optical inactivity" is built into this symmetric molecule. This can be seen by following what happens to the configuration of the chiral centers from the intermediates to products, below. (The key lies in the symmetry of the intermediate and inversion of configuration when bromide attacks.) IDENTICAL-SYMMETRIC CH3 CH3 --------------------------------------------------______________________________________ u __________ u __ • H Br Br Br H f CH3 '---<' 'H + H'" ", Br + H Br/ �CH3 Br CH3 CH3 2S,3S ENANTIOMERS 2R,3R continued on next page -+ �Br M" H"'�H3 H3 �'H G: a Be nuc\eophile could : r OR :Br , �: ( + �------� � �--------� "\ + H",:\ attack at either carbon CH3 H- . l Br + �··H I ':r-( J H Br H + Br CH3 -+ 164 ...
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This note was uploaded on 02/27/2010 for the course CHEM 140 taught by Professor Wade during the Spring '10 term at Whitman.

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