9. Alkenes and Alkynes

Which species in each pair is more stable?

f.

Which reagent or test could be used to distinguish between $3-\mathrm{pentyne}$ and $1-\mathrm{pentyne}$?

In 1935, J. Bredt, a German chemist, proposed that a bicycloalkene could not have a double bond at a bridgehead carbon unless one of the rings contains at least eight carbons. This is known as Bredt's rule. Explain why there cannot be a double bond at this position.

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?

Which of the following statements correctly identifies the allylic carbons in 1-butene (${\mathrm{CH}}_2={\mathrm{CH}}_2\mathrm{CH}{2}_{}\mathrm{CH}{3}_{}$)?

Which of the following alkenes is classified as an $E$ (entgegen) alkene?

When compound A, 2-butanol (${\mathrm{CH}}_3\mathrm{CH}\left(\mathrm{OH}\right)\mathrm{CH}2{\mathrm{CH}}_3$), is dehydrated with concentrated $H_2\mathrm{SO}4$, what is the major alkene product?

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

Given the following alkenes: (A) $\mathrm{CH}={\mathrm{CH}}_2$ (ethylene), (B) ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_3$ (2-butene), (C) ${\mathrm{CH}}_{3}C={\mathrm{CH}}_3$ (2-methyl-2-butene), and (D) ${\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2$ (propene), rank them from most stable to least stable.

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

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)

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.

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

(h)

(i)

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?

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