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Unformatted text preview: 7 -22 (a) �H
Br Br NaI acetone o c yclohcxene (b ) c is-hept-3 -ene (d) NaI acetone (5)- 3-bromo- l -ethyl-3-meth y Icyc lohexene CH3 r"fyH CH2CH3
Br rotation The two bromi ne atoms must be anti coplanar in order to e l i minate . Onl y the bromines at a nd C-2 fit that requirement; the bromines at a nd C-3 cannot eliminate . C-l (e) Br <DH �� ·
Br N aI, acetone . 9 H
Br C-2 In the first conformation shown , the bromine atoms are not coplanar and c annot eli minate. Rotati on in this l arge ring can place the two bromines anti-coplanar, generating a trans al kene in the ring. (Use models ! ) t rans-cyclodecene b H 7 -23 The stereochemical requirement of E2 elimination is anti-coplanar; in cyc lohexanes , thi s translates to trans-diaxial . B oth dibromides are trans, but because the t-butyl group must be in an equatorial position , only the left molecule c an h ave the bromines diaxial. The one on the right h as both bromines loc ked into equatori al positions, from which they cannot undergo E2 elimination. (CH3hC #� H
1 44 trans-diaxialcan do E2 � H t rans-diequatorial--cannot do E2 r HI ...
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