Chapter 25

Write a condensed structural formula for each of the following: (a) an acid with the formula ${\mathrm{C}}_4{\mathrm{H}}_8{\mathrm{O}}_2$, (b) a cyclic ketone with the formula ${\mathrm{C}}_5{\mathrm{H}}_8\mathrm{O},(\mathrm{c})$ a dihydroxy $\mathrm{com}-$ pound with formula ${\mathrm{C}}_3{\mathrm{H}}_8{\mathrm{O}}_2$, (d) a cyclic ester with formula ${\mathrm{C}}_5{\mathrm{H}}_8{\mathrm{O}}_2$

Although carboxylic acids and alcohols both contain an \- OH group, one is acidic in water and the other is not. Explain the difference.

Indole smells rather terrible in high concentrations but has a pleasant floral-like odor when highly diluted. It has the following structure: Indole is a planar molecule. The nitrogen is a very weak base, with a $K_b$ of $2\times {10}^{-12}$. Explain how this information indicates that the indole molecule is aromatic in character.

Which of the following peptides have a net positive charge at pH 7? (a) Gly- Ser-Lys, (b) Pro-Leu-Ile, (c) PheTyr-Asp.

Glutathione is a tripeptide found in most living cells. Partial hydrolysis yields Cys-Gly and Glu-Cys. What structures are possible for glutathione?

Starch, glycogen, and cellulose are all polymers of glucose. What are the structural differences among them?

Monosaccharides can be categorized in terms of the number of carbon atoms (pentoses have five carbons and hexoses have six carbons) and according to whether they contain an aldehyde (aldo- prefix, as in aldopentose) or ketone group (keto- prefix, as in ketopentose). Classify glucose and fructose in this way.

Can a DNA strand bind to a complementary RNAstrand? Explain.

Explain why the boiling point of ethanol $\left({78}^{\circ }\mathrm{C}\right)$ is much higher than that of its isomer, dimethyl ether $(-{25}^{\circ }\mathrm{C})$ and why the boiling point of ${\mathrm{CH}}_2{\mathrm{F}}_2(-52{}^{\circ }\mathrm{C})$ is far above that of ${\mathrm{CH}}_4(-{128}^{\circ }\mathrm{C})$

An unknown organic compound is found on elemental analysis to contain $68.1\mathrm{\%}$ carbon, $13.7\mathrm{\%}$ hydrogen, and $18.2\mathrm{\%}$ oxygen by mass. It is slightly soluble in water. Upon careful oxidation it is converted into a compound that behaves chemically like a ketone and contains $69.7\mathrm{\%}$ carbon, $11.7\mathrm{\%}$ hydrogen, and $18.6\mathrm{\%}$ oxygen by mass. Indicate two or more reasonable structures for the unknown.