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Chapter 8 Suggested solutions for Chapter 8 53 Problem 13 Explain the various values for these derivatives of the naturally occurring amino acid acid. Say which belongs to which functional group and explain why vary among the different derivatives. H₂NOC CO2H CO₂Et NH₂ NH₂ NH₂ glutamic acid; glutamine; diethyl ester; monoethyl ester; monoethyl ester; 2.19, 4.25, and 9.67 2.17 and 9.13 7.04 2.15 and 9.19 3.85 and 7.84 Purpose of the problem To get you thinking about carbon acids, how their anions are delocalized, and what makes the stable. Notice that at no pH does Suggested solution glutamic acid exist as the form we normally draw, though at pHs The three values for glutamic acid itself must be like this (the remote CO₂H group is normal between 2.2 and 4.2 this form is in the one near the amino group is unusual as we discussed in the answer to Problem 11). equilibrium with the zwitterion. pH 2.19 CO2 pH 4.25 CO2 pH 9.67 1 O₂C NH₃ NH₃ NH₂ glutamic acid In glutamine, the remote CO₂H group is missing and the other two pKₐ values are hardly The dimethyl ester has only the amino group left and its pKₐH value is less because we have replaced the two CO₂ anions with electron-withdrawing ester groups. The two monoesters are most interesting. The first is essentially the same as glutamine - the remote ester has little effect on the two groups. The second has, as we should expect, a remote CO₂H group like that of glutamic cid itself but an amino group like that of the diester showing that the nearer of the two ester groups the diester has the bigger effect. Problem 14 Neither of these methods of making pentan-1,4 diol will work. Explain why not - what will happen instead? Me-MgBr OH OH H Me H 0 OH Mg H Br OH BrMg OH Me H Purpose of the problem help you appreciate the disastrous effects that innocent-looking groups may have because of their weak acidity. Suggested solution The OH group is the Wicked Witch of the West in this problem. Whoever planned these syntheses expected it to lie quietly and do nothing. But, although an OH group is only a weak acid about