Suggested solutions for Chapter 19

Transcription

1 s for Chapter PRBLEM 1 Predict the orientation of addition to these alkenes. Simple examples of addition with regioselectivity. The first and last alkenes have different numbers of substituents at each end of the alkene and will give the more stable, more highly substituted cation on protonation. The middle one has the same number of substituents (one) at each end but they are very different in kind. The secondary benzylic cation is preferred to the non- conjugated alternative. R

2 2 Solutions Manual to accompany rganic Chemistry 2e PRBLEM 2 Suggest mechanism and products for these reactions. 2 2 Checking that you understand the bromination mechanism. The question of what product is formed is easily answered as we know bromine adds trans to alkenes. The products are both racemic, of course, as all reagents are achiral and only the relative stereochemistry is shown. 2 2 The mechanism is bromonium ion formation by electrophilic attack of bromine on the alkene and trans opening of the bromonium ion by bromide ion.

3 Solutions for Chapter 19 Electrophilic Addition to Alkenes 3 PRBLEM 3 What will be the products of the addition of bromine water to these alkenes Checking that you understand the bromonium ion mechanism with an external nucleophile. The bromonium ion is formed again but now water attacks as the nucleophile as it is in large excess as the solvent. If the alkene is unsymmetrical, water attacks the more substituted end of the bromonium ion (p. 441 of the textbook). In any case, it does so with inversion

4 4 Solutions Manual to accompany rganic Chemistry 2e PRBLEM 4 By working at low temperature with one equivalent of buffered solution of a peroxy- acid, it is possible to prepare the monoepoxide of cyclopentadiene. Why are these precautions necessary and why does a second epoxidation not occur under these conditions RC 3, ac 3 low temperature A more complicated electrophilic addition with questions of stability and selectivity to consider. More details on this explanation of the reactivity of dienes can be found on p of the textbook. ne of the alkenes in the diene reacts in the usual way to give, first of all, the monoepoxide. The reaction can be stopped there only if the remaining alkene is less nucleophilic than the alkenes in cyclopentadiene. This is indeed the case because the M of a diene is higher in energy than the M of a simple alkene. The M of the diene (Ψ 2 ) results from antibonding addition of the two separate π- orbitals, making the diene more reactive than an isolated alkene. R M of the diene The other questions concern the low temperature, which favours the kinetic product and encourages epoxide formation on the remaining alkene. A by- product from the reaction is RC 2 which could catalyse the opening of the epoxide to give a stable allyl cation (p. 336 in the textbook). The buffer prevents the mixture becoming too acidic. RC 2 by-products danger of decomposition by allyl cation formation

5 Solutions for Chapter 19 Electrophilic Addition to Alkenes 5 PRBLEM 5 The synthesis of a tranquilizer uses this step. Give mechanisms for the reactions An electrophilic addition followed by a substitution: revision of chapter 17. adds to the alkene to form the tertiary cation that captures bromide ion. 1. The bromide is hydrolysed by water. This must be an S 1 reaction as the bromide is tertiary and the nucleophile is water. The same cation is an intermediate in both reactions polar solvent 2

6 6 Solutions Manual to accompany rganic Chemistry 2e PRBLEM 6 Explain this result: KC, base C An electrophilic addition followed by an elimination (revision of chapter 17) and a substitution (revision of chapter 15). Addition of bromine occurs first to give the trans dibromide in the usual way. Base then eliminates one of the bromides in an E2 reaction using the only available trans hydrogen atom. This gives a reactive allylic bromide (p. 336 in the textbook) that reacts with cyanide ion by a favourable S 2 reaction to give the product. B base C E2 S 2 C PRBLEM 7 Suggest a mechanism for the following reaction. What is the stereochemistry and conformation of the product The product has these signals in its 1 MR spectrum: δ 3.9 (1, ddq, J 12, 4, 7) and δ 4.3 (1, dd, J 11, 3). 2 2 Drawing a mechanism for bromination of an alkene with an internal nucleophile, and revision of MR.

7 Solutions for Chapter 19 Electrophilic Addition to Alkenes 7 The mechanism is formation of the bromonium ion and nucleophilic attack by the group at the more substituted carbon. The MR spectrum shows that the protons next to and are both axial (large J of 11 and 12). The and the methyl group must therefore be equatorial. You meet more reactions like this in chapter =