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Chapter 16 Suggested solutions for Chapter 16 125 0 Problem 10 0 This compound racemizes in base. Why is that? HO H Purpose of the problem To draw your attention to the dangers in nearly symmetrical molecules and revision of ester exchange. Suggested solution Ester exchange in base goes through a symmetrical tetrahedral intermediate with a plane of symmetry. Loss of the right-hand leaving group gives one enantiomer of the ester and loss of the left-hand leaving group gives the other. 0 A: H 0 0 0 0 0 H 0 0 H H H Problem 11 Draw mechanisms for these reactions. Will the products be single stereoisomers? 0 0 0 (a) + 0 SH S 0 H OH (b) 0 OH (c) Purpose of the problem Revision of direct (Chapter 6) conjugate addition (Chapter 10) and some minor stereochemical points. Suggested solution Reaction (a) proceeds by a straightforward conjugate addition of a sulfur nucleophile. One stereogenic centre is formed from two achiral reagents and the product must therefore be racemic. 0 0 0 X H 0 0 0 racemic mixture S S S Example (b) creates a centre that is drawn to look chiral but isn't. The molecule has a plane of symmetry. It consists of a single (E,E-) geometrical isomer, of course, as the alkenes are not affected by the reaction. 0 0 H OH E,E achiral H H3B