9. Alkenes and Alkynes

Which species in each pair is more stable?

a.

Conjugated dienes, molecules containing two alkenes separated by one single bond, are discussed in detail in Chapter 21.

(a) Considering the observed ∆H° of hydrogenation, is hexa-1,3-diene (conjugated) or hexa-1,4-diene (unconjugated) more stable?

Conjugated dienes, molecules containing two alkenes separated by one single bond, are discussed in detail in Chapter 21.

(b) How might you account for this difference in stability?

Bromination of buta-1,3-diene with a single equivalent of Br₂ can give either of two products. (a) Which of these products (A or B) would you predict to be more stable? Justify your answer.

Rank each group of compounds in order of increasing heat of hydrogenation.

(b)

Rank each group of compounds in order of increasing heat of hydrogenation.

(a) hexa-1,2-diene; hexa-1,3,5-triene; hexa-1,3-diene; hexa-1,4-diene; hexa-1,5-diene; hexa-2,4-diene

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?

Rank the following alkenes in order of stability (1 = most stable; 5 = least stable). Explain your order.                                                                                                                                                                                   

The same alkane is obtained from the catalytic hydrogenation of both alkene A and alkene B. The heat of hydrogenation of alkene A is 29.8 kcal/mol, and the heat of hydrogenation of alkene B is 31.4 kcal/mol. Which alkene is more stable?

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.

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.

Use the data in Table 7-2 to predict the energy difference between 2,3-dimethylbut-1-ene and 2,3-dimethylbut-2-ene. Which of these double-bond isomers is more stable?

Explain why each of the following alkenes is stable or unstable.

(h)

(i)