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152 Organic Chemistry Solutions Manual nucleophilic than the diene. This is so because the HOMO reacts. The HOMO of an alkene is just the populated π orbital. The HOMO of a diene is resulting from the antibonding addition of the two separate π orbitals and is less stable than either π orbital. R danger of allyl cation formation in acid solution D H OH HOMO of the diene The other question is why work at low temperature and buffer the solution? The low temperature favours the kinetic product and discourages the slower second epoxidation. A by-product from epoxidation is the acid H and there is a danger that this may catalyse opening of the monoepoxide to give the allyl cation in the frame. A buffer prevents the solution from becoming too acidic. Problem 5 The synthesis of a tranquillizer uses this step. Give mechanisms for the reactions. HN HN CF₃ 1. HBr CF₃ 2. CI CI Purpose of the problem An electrophilic addition followed by a substitution (revision of Chapter 17) as part of a useful application. Suggested solution HBr adds to the unsymmetrical alkene to give the tertiary benzylic cation and then the tertiary bromide. HN H Br HN Br HN Br CF₃ CF₃ CF₃ CI CI CI The bromide hydrolyses by an SN1 mechanism to the alcohol. The same cation is an intermediate in the addition of HBr to the alkene and in the SN 1 substitution. HN HN HN Br CF₃ CF₃ CF₃ CI CI CI Problem 6 Explain this result. 1. Br₂ 2. KCN, base CN