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Suggested solution it is essential to draw a

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Unformatted text preview: d about this – any similar diagram is fine. Insert diagram from chapter 13 problem 7 in old solutions manual 4 Solutions Manual to accompany Organic Chemistry 2e PROBLEM 4 What would be the effect of solvent changes on these reactions? Would the reactions be accelerated or retarded by a change from a polar to a non- polar solvent? Ph3P O Br2 solvent Ph3P Br O O R heat solvent CO2 + NMe3 O NH3 OMe NMe3 solvent R NH2 Purpose of the problem Practice at assessing the likely effect of solvent polarity in terms of the mechanism of the reaction. Suggested solution It is essential to draw a mechanism for each reaction and to identify the rate- determining step in each case. The first two reactions are one- step processes so that makes life easier. O Ph3P Br Br Ph3P Br + Br O CO2 + NMe3 NMe3 Now we need to draw the transition state for each reaction so that we can assess whether it is more or less polar than the starting materials. The way to do this is described on p. 251 of the textbook. O Ph3P Br Br (+) Ph3P O (–) Br Br O NMe3 (–) (+) O NMe3 In the first reaction uncharged starting materials form a partly charged transition state. A polar solvent will stabilise the transition state and accelerate the reaction. In the second case a charged (zwitterionic) starting material gives a partly charged transition state. A polar solvent will stabilise both starting material and transition state but it will stabilise the starting mate...
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