Organic Chemistry

T.W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder

Chapter 7

Alkenes and Alkynes I - all with Video Answers

Educators

Chapter Questions

Problem 1

Using the $(E)-(Z)$ designation [and in parts (e) and ( $\mathbf{f}$ ) the $(R)-(S)$ designation as well] give IUPAC names for each of the following: (FIGURE CANNOT COPY)

Kaitlynn Wade

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Problem 2

Rank the following cycloalkenes in order of increasing stability. (FIGURES CANNOT COPY)

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Problem 3

Heats of hydrogenation of three alkenes are as follows: 2-methyl-1-butene $\left(-119 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ 3-methyl-1-butene $\left(-127 \mathrm{kJ} \mathrm{mol}^{-1}\right)$ 2-methyl-2-butenc $\left(-113 \mathrm{kJ} \mathrm{mol}^{-1}\right)$
(a) Write the structure of each alkene and classify it as to whether its doubly bonded atoms are monosubstituted, disubstituted, trisubstituted, or tetrasubstituted. (b) Write the structure of the product formed when each alkene is hydrogenated. (c) Can heats of hydrogenation be used to relate the relative stabilities of these three alkenes? (d) If so, what is the predicted order of stability? If not, why not? (e) What other alkene isomers are possible for these alkenes? Write their structures. (f) What are the relative stabilities among just these isomers?

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Problem 4

Predict the more stable alkenc of cach pair: (a) 2 -methyl-2-pentenc or 2.3-dimethyl-2-butene; (b) cis-3-hexene or trans-3-hexene; (c) 1-hexene or cis-3-hexene; (d) trans- 2 -hexene or 2 -methyl-2-pentene.

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Problem 5

How many stercoisomers are possible for 4 -methyl-2-hexenc, and how many fractions would you obtain if you distilled the mixture?

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Problem 6

Predict the major product formed when 2 -bromobutane is subjected to dehydrobromination using sodium ethoxide in ethanol at $55^{\circ} \mathrm{C}$

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Problem 7

List the alkencs that would be formed when cach of the following alkyl halides is subjected to dehydrohalogenation with potassium ethoxide in ethanol and use Zaitsev's rule to predict the major product of each reaction: (a) 2 -bromo- 3 -methylbutane and (b) 2 -bromo- 2,3 -dimethylbutane.

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Problem 8

Examine Solved Problem $7.3 .$ Your task is to prepare $\mathbf{A}$ in the highest possible yield by dehydrobromination. Which base would you use?

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Problem 9

Consider a simple molecule such as ethyl bromide and show with Newman projection formulas how the anti coplanar transition state would be favored over the syn coplanar one.

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Problem 10

When cis-1-bromo-4-tert-butylcyclohexane is treated with sodium ethoxide in ethanol, it reacts rapidly; the product is 4 -tert-butylcyclohcxenc. Under the same conditions, trans1-bromo-4-tert-butylcyclohexane reacts very slowly. Write conformational structures and explain the difference in reactivity of these cis-trans isomers.

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Problem 11

(a) When cis-1-bromo-2-methylcyclohexane undergoes an E2 reaction, two products (cycloalkenes) are formed. What are these two cycloalkenes, and which would you expect to be the major product? Write conformational structures showing how each is formed. (b) When trans-1-bromo-2-methylcyclohexane reacts in an E2 reaction, only one cycloalkene is formed. What is this product? Write conformational structures showing why it is the only product.

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Problem 12

Your task is to prepare styrene by one of the following reactions. Which reaction would you choose to obtain the better yicld of styrene? Explain your answer. (FIGURES CANNOT COPY)

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Problem 13

Although cthyl bromide and isobutyl bromide are both primary halides, cthyl bromide undergoes $S_{N} 2$ reactions more than 10 times faster than isobutyl bromide does. When each compound is treated with a strong base/nucleophile (EtO"), isobutyl bromide gives a greater yield of elimination products than substitution products, whereas with ethyl bromide this behavior is reversed. What factor accounts for these results?

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Problem 14

Dehydration of 2 -propanol occurs in $14 \mathrm{M} \mathrm{H}_{2} \mathrm{SO}_{4}$ at $100^{\circ} \mathrm{C}$. (a) Using curved arrows, write all steps in a mechanism for the dehydration. (b) Explain the essential role performed in alcohol dehydrations by the acid catalyst. (Hint: Consider what would have to happen if no acid were present.)

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Problem 15

Rank the following alcohols in order of increasing ease of acid-catalyzed dehydration. (FIGURES CANNOT COPY)

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Problem 16

Acid-catalyzed dehydration of neopentyl alcohol, (CH $_{3}$ ) $_{3} \mathrm{CCH}_{2}$ OH, yields 2 -methyl-2-butene as the major product. Outline a mechanism showing all steps in its formation.

Nicole Krahulik

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Problem 17

Acid-catalyzed dehydration of either 2 -methyl-1-butanol or 3 -methyl-1-butanol gives 2-methyl-2-butene as the major product. Write plausible mechanisms that explain these results.

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Problem 18

When the compound called isoborneol is heatcd with $9 \mathrm{M}$ sulfuric acid, the product of the reaction is the compound called camphene and not bornylene, as one might expect. Using models to assist you, write a step-by-step mechanism showing how camphene is formed. (FIGURE CANNOT COPY)

Lottie Adams

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Problem 19

Predict the products of the following acid-base reactions. If the equilibrium would not result in the formation of appreciable amounts of products, you should so indicate. In each case label the stronger acid, the stronger base, the weaker acid, and the weaker base:
(a) $\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}+\mathrm{NaNH}_{2} \longrightarrow$
(b) $\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{CH}+\mathrm{NaNH}_{2} \rightarrow$
(c) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{3}+\mathrm{NaNH}_{2} \longrightarrow$
(d) $\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{C}:^{-}+\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH} \longrightarrow$
(e) $\mathrm{CH}_{3} \mathrm{C} \equiv \mathrm{C}:^{-}+\mathrm{NH}_{4} \mathrm{Cl} \rightarrow$

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Problem 20

Show how you might synthesize ethynylbenzene from methyl phenyl ketone.

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Problem 21

Outline all steps in a synthesis of propyne from cach of the following:
(a) $\mathrm{CH}_{3} \mathrm{COCH}_{3}$
(c) $\mathrm{CH}_{3} \mathrm{CHBrCH}_{2} \mathrm{Br}$
(b) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHBr}_{2}$
(d) $\mathrm{CH}_{3} \mathrm{CH}=\mathrm{CH}_{2}$

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Problem 22

Your goal is to synthesize 4,4 -dimethyl-2-pentyne. You have a choice of beginning with any of the following reagents: (FIGURE CANNOT COPY)

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Problem 23

Write the structure of compound $\mathbf{A},$ used in this synthesis of the perfume ingredicnt
(Z)-jasmone. (FIGURE CANNOT COPY)

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Problem 24

How would you convert 2 -nonyne into $(E)$ - 2 -nonene?

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Problem 25

Referring to the retrosynthetic analysis for 2 -methylhexane in this section, write reactions for those synthesis routes that are feasible.

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Problem 26

(a) Devise retrosynthetic schemes for all conceivable alkynide anion alkylation syntheses of the insect pheromones undecanc and 2 -methylhcptadecanc (sce "The Chemistry of ... Pheromones" box in Chapter 4 ). (b) Write reactions for two feasible syntheses of each pheromone.

Lottie Adams

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Problem 27

Each of the following names is incorrect. Give the correct name and explain your reasoning.
(a) trans- 3 -Pentene
(b) 1,1 -Dimethylethene
(c) 2 -Methylcyclohexene
(d) 4 -Methylcyclobutene
(e) $(Z)-3$ -Chloro-2-butene
(f) 5,6 -Dichlorocyclohexene

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Problem 28

Write a structural formula for each of the following:
(a) 3 -Methylcyclobutene
(b) 1 -Methylcyclopentene
(c) 2,3 -Dimethyl-2-pentene
(d) $(Z)-3$ -Hexene
(e) $(E)$ - 2 -Pentene
(f) 3,3,3 -Tribromopropene
(g) $(Z, 4 R)-4$ -Methyl-2-hexene
(h) $(E, 4 S)-4$ -Chloro- 2 -pentene
(i) $(Z)$ -1-Cyclopropyl-1-pentene
(j) 5 -Cyclobutyl-1-pentene
(k) $(R)-4$ -Chloro-2-pentyne
(l) $(E)-4$ -Methylhex-4-en-1-yne

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Problem 29

Write three-dimensional formulas for and give names using $(R)-(S)$ and $(E)-(Z)$ designations for the isomers of:
(a) 4 -Bromo-2-hexene
(b) 3 -Chloro- 1,4 -hexadiene
(c) 2,4 -Dichloro- 2 -pentene
(d) 2 -Bromo-4-chlorohex-2-en-5-yne

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Problem 30

Give the IUPAC names for each of the following:
(FIGURES CANNOT COPY)

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Problem 31

Without consulting tables, arrange the following compounds in order of decreasing acidity:
Pentane 1-Pentanol

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Problem 32

Write a three-dimensional representation for the transition state structure leading to formation of 2 -methyl-2-butene from reaction of 2 -bromo- 2 -methylbutane with sodium ethoxide.

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Problem 33

When trans-2-methylcyclohexanol (see the following reaction) is subjected to acid-catalyzed dehydration, the major product is 1-methylcyclohexene:(FIGURE CANNOT COPY)
However, when trans-1-bromo-2-methylcyclohexane is subjected to dehydrohalogenation, the major product is 1 -methylcyclohexene: (FIGURE CANNOT COPY) Account for the different products of these two reactions.

Lottie Adams

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Problem 34

Write structural formulas for all the products that would be obtained when each of the following alky halides is heated with sodium ethoxide in ethanol. When more than one product results, you should indicate which would be the major product and which would be the minor product(s). You may neglect cis-trans isomerism of the products when answering this question.
(FIGURES CANNOT COPY)

Lottie Adams

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Problem 35

When trans-2-methylcyclohexanol (see the following reaction) is subjected to acid-catalyzed dehydration, the major product is 1-methylcyclohexene:(FIGURE CANNOT COPY) However, when trans-1-bromo-2methylcyclohexane is subjected to dehydrohalogenation, the major product is 1 -methylcyclohexene: (FIGURE CANNOT COPY) Account for the different products of these two reactions.
(a) (DIAGRAM CAN'T COPY)
(b) (DIAGRAM CAN'T COPY)
(c) (DIAGRAM CAN'T COPY)
(d) (DIAGRAM CAN'T COPY)
(e) (DIAGRAM CAN'T COPY)

Lottie Adams

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Problem 35

Write structural formulas for all the products that would be obtained when each of the following alkyl halides is heated with potassium tert-butoxide in tert-butyl alcohol. When more than one product results, you should indicate which would be the major product and which would be the minor product(s). You may neglect cis-trans isomerism of the products when answering this question. (FIGURE CANNOT COPY)

Lottie Adams

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Problem 36

Explain the following observations: When tert-buryl bromide is treated with sodium methoxide in a mixture of methanol and water, the rate of formation of tert-butyl alcohol and tert-butyl methyl cther does not change appreciably as the concentration of sodium methoxide is increased. However, increasing the concentration of sodium methoxide causes a marked increase in the rate at which tert-butyl bromide disappears from the mixture.

Lottie Adams

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Problem 37

Which product (or products) would you expect t obtain from each of the following reactions? In each part give the mechanism $\left(\mathrm{S}_{\mathrm{N}} 1, \mathrm{S}_{\mathrm{N}} 2, \mathrm{E}_{1}, \text { or } \mathrm{E} 2\right)$ by which each product is formed and predict the relative amount of each product (i.e., would the product be the only product, the major product, a minor product, etc.?).
(FIGURES CANNOT COPY)

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Problem 38

Arrange the following alcohols in order of their reactivity toward acid-catalyzed dehydration (with the most reactive first): 1-Pentanol.

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Problem 39

Provide a mechanistic explanation for each of the following reactions: (FIGURES CANNOT COPY)

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Problem 40

When the deuterium-labeled compound shown at right is subjected to dehydrohalogenation using sodium cthoxide in cthanol, the only alkene product is 3 -methylcyclohexene. (The product contains no deuterium.) Provide an explanation for this result. (FIGURE CANNOT COPY)

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Problem 41

1-Bromobicyclo[2.2.1]heptane does not undergo elimination (below) when heated with a base. Explain this failure to react. (Construction of molecular models may help.) (FIGURE CANNOT COPY)

Lottie Adams

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Problem 42

(a) Consider the general problem of converting a tertiary alkyl halide to an alkene, for example, the conversion of tert-butyl chloride to 2 -methylpropene. What experimental conditions would you choose to ensure that elimination is favored over substitution?
(b) Consider the opposite problem, that of carrying out a substitution reaction on a tertiary alky / halide. Use as your example the conversion of tert-butyl chloride to tert-buryl ethyl ether. What experimental conditions would you employ to ensure the highest possible yield of the ether?

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Problem 43

Your task is to prepare isopropy i methyl ether by one of the following reactions. Which reaction would give the better yield? Explain your answer. Explain your answer.

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Problem 44

Give the products that would be formed when each of the following alcohols is subjected to acid-catalyzed dehydration. If more than one product would be formed, designate the alkene that would be the major product. (Neglect cis-trans isomerism.) (FIGURE CANNOT COPY)

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Problem 45

Outline a synthesis of propene from each of the following:
(a) Propyl chloride
(b) Isopropyl chloride
(c) Propyl alcohol
(d) Isopropyl alcohol
(e) 1,2 -Dibromopropane
(f) Propyne

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Problem 46

Outline a synthesis of cyclopentene from each of the following:
(a) Bromocyclopentane
(b) Cyclopentanol

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Problem 47

Starting with an appropriate alkyl halide and base, outline syntheses that would yield each of the following alkenes as the major (or only) product:
(FIGURE CANNOT COPY)

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Problem 48

Starting with ethyne, outline syntheses of each of the following. You may use any other needed reagents, and you need not show the synthesis of compounds prepared in earlier parts of this problem.
(a) Propyne
(b) 1 -Butyne
(c) 2 -Butyne
(d) cis-2-Butene
(e) trans- 2 -Butene
(f) 1 -Pentyne
(g) 2 -Hexyne
(h) $(Z)$ - 2 -Hexene
(i) $(E)-2$ -Hexene
(j) 3 -Hexyne
(k) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C}=\mathrm{CD}$
(l) (FIGURE CANNOT COPY)

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Problem 49

What is the index of hydrogen deficiency (IHD, or degree of unsaturation) for each of the following compounds? (FIGURE CANNOT COPY)

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Problem 50

Caryophyllene, a compound found in oil of cloves, has the molecular formula $\mathrm{C}_{15} \mathrm{H}_{24}$ and has no triple bonds. Reaction of caryophyllene with an excess of hydrogen in the presence of a platinum catalyst produces a compound with the formula $\mathrm{C}_{15} \mathrm{H}_{28}$ How many (a) double bonds and ( $\mathbf{b}$ ) rings does a molecule of caryophyllene have?

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Problem 51

Squalene, an important intermediate in the biosynthesis of steroids, has the molecular formula $\mathrm{C}_{30} \mathrm{H}_{50}$ and has no triple bonds. (a) What is the index of hydrogen deficiency of squalene? (b) Squalene undergoes catalytic hydrogenation to yield a compound with the molecular formula $\mathrm{C}_{30} \mathrm{H}_{62}$. How many double bonds does a molecule of squalene have? (c) How many rings?

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Problem 52

Compounds $\mathrm{I}$ and $\mathrm{J}$ both have the molecular formula $\mathrm{C}_{7} \mathrm{H}_{14} .$ Compounds $\mathrm{I}$ and $\mathrm{J}$ are both optically active and both rotate. (FIGURE CANNOT COPY)

Lottie Adams

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Problem 53

Compounds $\mathbf{L}$ and $\mathbf{M}$ have the molecular formula $\mathrm{C}_{7} \mathrm{H}_{14} .$ Compounds $\mathbf{L}$ and $\mathbf{M}$ are optically inactive, are nonresolvable, and are diastereomers of each other. Catalytic hydrogenation of cither $\mathbf{L}$ or $\mathbf{M}$ yields $\mathbf{N}$. Compound $\mathbf{N}$ is optically inactive but can be resolved into separate enantiomers. Propose possible structures for $\mathbf{L}, \mathbf{M},$ and $\mathbf{N}$

Lottie Adams

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Problem 54

Outline a synthesis of phenylethyne from each of the following: (FIGURE CANNOT COPY)

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Problem 55

For each of the following questions, please provide a e that could reasonably be expected to convert the starting material into the final product. In each case, more than one reaction is required, and reactions you have learned in previous chapters may be needed to solve the problem (FIGURES CANNOT COPY)

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Problem 56

Working backwards, deduce the starting material that led to the indicated product through the defined reactions (FIGURE CANNOT COPY)

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Problem 57

When 1 -cyclobutylethanol (shown below) is treated with concentrated $\mathrm{H}_{2} \mathrm{SO}_{4}$ at $120^{\circ} \mathrm{C}$, one of the products that is formed is methylcyclopentene. Please write a mechanism that can account for the formation of this new product. (FIGURE CANNOT COPY)

Lottie Adams

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Problem 58

cis-4-Bromocyclohexanol (EQUATION CANNOT COPY) racemic $\mathrm{C}_{6} \mathrm{H}_{10} \mathrm{O}$ (compound $\mathbf{C}$ ) Compound C has infrared absorption in the $1620-1680 \mathrm{cm}^{-1}$ and in the $3590-3650 \mathrm{cm}^{-1}$ regions. Draw and label the $(R)$ and $(S)$ enantiomers of product $\mathbf{C}$.

Lottie Adams

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Problem 59

Propose structures for compounds $E-H$. Compound $\mathrm{E}$ has the molecular formula $\mathrm{C}_{5} \mathrm{H}_{8}$ and is optically active. On catalytic hydrogenation $\mathbf{E}$ yields $\mathbf{F}$. Compound $\mathbf{F}$ has the molecular formula $\mathrm{C}_{5} \mathrm{H}_{10},$ is optically inactive, and cannot be resolved into separate enantiomers. Compound $\mathrm{G}$ has the molecular formula $\mathrm{C}_{6} \mathrm{H}_{10}$ and is optically active. Compound $\mathrm{G}$ contains no triple bonds. On catalytic hydrogenation $\mathrm{G}$ yields $\mathrm{H}$. Compound H has the molecular formula $\mathrm{C}_{6} \mathrm{H}_{14},$ is optically inactive, and cannot be resolved into separate enantiomers.

Lottie Adams

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Problem 60

Consider the interconversion of cis- 2 -butene and trans- 2 -butene.
(a) What is the value of $\Delta H^{\circ}$ for the reaction cis- 2 -butene $\longrightarrow$ trans- 2 -butene (see Section $7.3 \mathrm{A}$ )?
(b) Assume $\left.\Delta H^{\circ} \cong \Delta G^{\circ} . \text { What minimum value of } \Delta G^{\ddagger} \text { would you expect for this reaction (see Section } 1.13 \mathrm{A}\right) ?$
(c) Sketch a free-energy diagram for the reaction and label $\Delta G^{\circ}$ and $\Delta G^{\ddagger}$

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Problem 61

(a) Partial dehydrohalogenation of either $(1 R, 2 R)-1,$ 2-dibromo-1,2-diphenylethane or $(1 S, 2 S)-1,$ 2-dibromo-1,2-diphenylethane enantiomers (or a racemate of the two) produces ( $Z$ )-1-bromo-1,2-diphenylethene as the product, whereas (b) partial dehydrohalogenation of $(1 R, 2 S)-1,2$ -dibromo-1, 2-diphenylethane (the meso compound) gives only $(E)$ -1-bromo-1,2-diphenylethene. (c) Treating $(1 R, 2 S)-1,2$ - dibromo-1,2-diphenylethane with sodium iodide in acetone produces only (E) -1,2-diphenylethene. Explain these results.

Lottie Adams

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Problem 62

Using reactions studied in this chapter, show steps by which this alkyne could be converted to the seven-membered ring homolog of the product obtained in Problem $7.39(\mathrm{b}).$
(FIGURE CANNOT COPY)

Lottie Adams

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Problem 63

Predict the structures of compounds $\mathrm{A}, \mathrm{B}$, and C :
$\mathbf{A}$ is an unbranched $\mathrm{C}_8$ allsyne that is also a primary alcohol.
$\mathbf{B}$ is obtained from $\mathbf{A}$ by use of hydrogen and nickel boride catalyst or dissolving metal reduction.
$\mathbf{C}$ is formed from $\mathbf{B}$ on treatment with aqueous acid at room temperature. Compound $\mathbf{C}$ has no infrared absorption in either the 1620-1680 cm-1 or the $3590-3650 \mathrm{~cm}^{-1}$ region. It has an index of hydrogen deficiency of 1 and has one chirality center but forms as the racemate.

Brittany Carnathan

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Problem 64

What is the index of hydrogen deficiency for (a) $\mathrm{C}_7 \mathrm{H}_{10} \mathrm{O}_2$ and (b) $\mathrm{C}_5 \mathrm{H}_4 \mathrm{~N}_4$ ?

Grigoriy Sereda

Grigoriy Sereda

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