9. Alkenes and Alkynes

Rank the following compounds from most stable to least stable:

trans-3-hexene, cis-3-hexene, cis-2,5-dimethyl-3-hexene, (Z)-3,4-dimethyl-3-hexene

The energy difference between cis- and trans-but-2-ene is about 4 kJ/mol; however, the trans isomer of 4,4-dimethylpent-2-ene is nearly 16 kJ/mol more stable than the cis isomer. Explain this large difference.

For each set of isomers, choose the isomer that you expect to be most stable and the isomer you expect to be least stable.

(a)

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

(a) 1,2-dimethylcyclopentene

(b) trans-1,2-dimethylcyclopentene

(c) trans-3,4-dimethylcyclopentene

(d) trans-1,2-dimethylcyclodecene

A double bond in a six-membered ring is usually more stable in an endocyclic position than in an exocyclic position. Hydrogenation data on two pairs of compounds follow. One pair suggests that the energy difference between endocyclic and exocyclic double bonds is about 9 kJ/mol. The other pair suggests an energy difference of about 5 kJ/mol. Which number do you trust as being more representative of the actual energy difference? Explain your answer.

Using Table 7-2 as a guide, predict which member of each pair is more stable, as well as by about how many kJ/mol or kcal/mol.

a. cis,cis-hexa-2,4-diene or trans,trans-hexa-2,4-diene

b. 2-methylbut-1-ene or 3-methylbut-1-ene

c. 2-methylbut-1-ene or 2-methylbut-2-ene

d. cis-4-methylpent-2-ene or 2-methylpent-2-ene

Which species in each pair is more stable?

f.

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?