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9. Alkenes and Alkynes

Alkene Stability

9. Alkenes and Alkynes

Alkene Stability

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Understanding trends of alkene stability.

Understanding trends of alkene stability.

Johnny Betancourt

Heat of Combustion

Heat of Combustion

Johnny Betancourt

Practice this topic

Textbook Question

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

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Textbook Question

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.

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Textbook Question

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.

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Textbook Question

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

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Textbook Question

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. Endoclyclic

heats of hydrogenation (kJ/mol)

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Textbook Question

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

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Textbook Question

Which species in each pair is more stable? f.

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Textbook Question

Which species in each pair is more stable? a.CH3C−HCH2CH3 or CH3CH2CH2C−H2

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Textbook Question

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?

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Textbook Question

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?

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Textbook Question

(••••) LOOKING AHEAD 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.

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Textbook Question

Rank each group of compounds in order of increasing heat of hydrogenation. (b)

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Textbook Question

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

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Textbook Question

(•••) When using sulfuric acid, but in the absence of other nucleophiles like water or bromide ion, less stable alkenes can be isomerized to their more stable isomer. Provide a mechanism for these acid-catalyzed isomerization reactions. [This is one illustration of the principle of microscopic reversibility.]

(a) <IMAGE>

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Textbook Question

(•••) LOOKING AHEAD 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?

<IMAGE>

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Textbook Question

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

<IMAGE> <IMAGE> <IMAGE> <IMAGE> <IMAGE>

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Textbook Question

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?

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Textbook Question

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?

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Textbook Question

Of the compounds you named in Problem 45:

a. Which is the most stable?

a. <IMAGE>

b. <IMAGE>

c. <IMAGE>

d. <IMAGE>

e. <IMAGE>

f. <IMAGE>

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Textbook Question

Of the compounds you named in Problem 45:

b. Which is the least stable?

a. <IMAGE>

b. <IMAGE>

c. <IMAGE>

d. <IMAGE>

e. <IMAGE>

f. <IMAGE>

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Textbook Question

Which species in each pair is more stable?

c. C−H2CH2CH═CH2 or CH3C−HCH═CH2

d. <IMAGE>

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Textbook Question

Use the data in [TABLE 7-2] <IMAGE> 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?

HINT: Heats of hydrogenation are usually exothermic. A larger amount of heat given off implies a less stable alkene, because the less stable alkene starts from a higher potential energy.

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Textbook Question

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

h. <IMAGE>

i. <IMAGE>

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Textbook Question

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

b. <IMAGE> <IMAGE> <IMAGE>

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Textbook Question

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

c. <IMAGE> <IMAGE> <IMAGE> <IMAGE>

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Textbook Question

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.

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