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Chapter 14 Suggested solutions for Chapter 14 103 To avoid this trap we must use the chemistry from Chapters 6 and 14 - carbonyl chemistry. Reactions of the kind we need were used in Problem 5 where formaldehyde was also a reagent. First, we must make formaldehyde from its bisulfite adduct and then add it to the amino group of dapsone. 0 0 H 0 S Ar 0 + Ar N OH S H HO 0 0 H H formaldehyde sodium bisulfite ArNH₂ = dapsone bisulfite adduct Now we can form an iminium salt and add the bisulfite back into this reactive electrophile to give the final product. This is loss of carbonyl oxygen in an unusual setting as the carbonyl group is not there at the start. 0 Na Ar Ar Ar Ar S N OH N N N so HO 0 H H H H Problem 8 Suggest a detailed mechanism for the acetal exchange used in this chapter to make an acetal of a ketone from an orthoester. OMe OMe 0 MeOH, cat. 0 OMe + + OMe 0 H OMe 20 15 min H OMe 0 trimethyl orthoformate methyl formate Purpose of the problem Reminder of chemistry from this chapter - making acetals from ketones is difficult - and drawing a mechanism related to, but different from, one in Problem 4. Suggested solution The equilibrium between a ketone and an alcohol on the one hand and the dimethyl acetal on the ther is unfavourable and must be driven over by devices such as this. The orthoformate is even more unstable than the acetal (three C-O bonds instead of two at the same carbon atom) and the ester even more stable, because of conjugation, than the ketone. There are various mechanisms you might have drawn, which differ only in details such as the order of the steps. Here is one such. OH + 0 OH OMe OMe 0 0 HOMe 0 0 + H 0 + + H OMe HOMe OMe OMe HOMe 0 OMe H OMe OH₂ OH + HCO₂Me H OMe H H OMe H DMe H OMe H OMe The first line shows the normal mechanism for acetal formation and points out the by-product, a molecule of water. This water is consumed in the hydrolysis of the orthoformate (by an acetal hydrolysis mechanism) to give a stable ordinary ester. The favourable second equilibrium pulls the first across to the right.