9. Alkenes and Alkynes

Which of the following alkenes would you expect to have the lowest heat of hydrogenation?

Given the following alkenes: (A) $\mathrm{CH}{2}_{=}\mathrm{CH}{2}_{}$ (ethylene), (B) $\mathrm{CH}{2}_{=}\mathrm{CHCH}$₃ (propene), (C) $\mathrm{CH}{3}_{}\mathrm{CH}=\mathrm{CHCH}$₃ (cis-2-butene), and (D) $\mathrm{CH}{3}_{}\mathrm{CH}=\mathrm{CHCH}$₃ (trans-2-butene), which of the alkenes above is the least stable (highest in energy)?

Which of the following is the most stable alkene?

Which of the following molecules contains a triple bond?

When $3$-bromo-$3$-methylheptane is treated with a strong base, how many different alkenes (constitutional isomers) can be formed as major products via E$2$ elimination?

Which of the following correctly identifies an appropriate alkyne starting material (a) and the intermediate alkene product (b) in the partial hydrogenation of ${\mathrm{CH}}_3-C_2\equiv C_2-{\mathrm{CH}}_3$ using Lindlar's catalyst?

Which of the following is the correct structure for $3-\mathrm{hex}\mathrm{yne}$?

Which of the following alkenes is the major product formed when 2-bromobutane is treated with alcoholic $K_{O}H$ (potassium hydroxide)?

Consider the Cope rearrangement, a reaction we describe in Chapter 20.

(a) Using the knowledge we have gained here in Chapter 9, suggest a one-step, concerted mechanism that explains the formation of B from A.

(b) Which side of the reaction would you expect to be favored? Justify your answer.

(c) Which product, A or B, would you expect to be hydrogenated with the more exothermic heat of hydrogenation?

Of the compounds you named in Problem 45:

b. Which is the least stable?

a.

b.

c.

d.

e.

f.

Which species in each pair is more stable?

c.

d.

a. Which is the most stable: 3,4-dimethyl-2-hexene, 2,3-dimethyl-2-hexene, or 4,5-dimethyl-2-hexene?

b. Which compound has the largest heat of hydrogenation?

c. Which compound has the smallest heat of hydrogenation?

Of the compounds you named in Problem 45:

a. Which is the most stable?

a.

b.

c.

d.

e.

f.

When $2-\mathrm{bromo}\mathrm{octane}$ is treated with a strong base, how many different alkenes (constitutional isomers) can be formed as major products via $E2$ elimination?

Which of the following is the most stable alkene?