11.13: Alkenes and Alkynes I (Problems)

Exercise 11-1 Deduce possible structures of the substances whose infrared spectra are shown in Figure 11-2. Assign as many of the bands as you can to specific stretching and bending vibrations by comparison with Figure 10-1. Be sure your structural assignments fit with the spectrum - that is, think twice about assigning a structure than has \(\ce{-CH_2}-\) groups if there are no \(\ce{-CH_2}-\) bands in the spectrum, or that has no \(\ce{-CH_3}\) groups when there appear to be strong \(\ce{-CH_3}\) absorptions.

Figure 11-2: Infrared spectra for Exercise 11-1.

Exercise 11-2 Deduce the structures of the substances whose proton nmr spectra are shown in Figure 11-3. Analyze the spectra in as much detail as you can in terms of chemical shifts and spin-spin splitting.

Figure 11-3: Proton nmr spectra at \(60 \: \text{MHz}\) with TMS as the standard at \(0 \: \text{ppm}\). See Exercise 11-2.

Exercise 11-3 Sketch the principal features you would expect for the infrared and proton nmr spectra of each of the following substances. (It will be helpful to review Sections 9-7 and 9-10.)

a. \(\ce{CH_3C \equiv CCH_3}\)
b. \(\ce{CH_3C \equiv CH}\) (expect a four-bond coupling of about \(3 \: \text{Hz}\))
c. \(\ce{CH_3CH_2C \equiv CCH_2CH_3}\)
d. \(\ce{HC \equiv C-CH=C-C \equiv CH}\) (cis and trans)

Exercise 11-4 Deduce the structure of a compound with the following spectral properties:

No electronic absorption was evident at wavelengths longer than \(200 \: \text{nm}\).

Exercise 11-5 Use the bond energies (Table 4-3) to calculate \(\Delta H^0\) for the addition of \(\ce{Br_2}\), \(\ce{Cl_2}\), \(\ce{I_2}\), \(\ce{HOCl}\), \(\ce{HCl}\), \(\ce{HBr}\), \(\ce{HI}\), and \(\ce{H_2O}\) to ethene in the gas phase. The addition of \(\ce{HCl}\), \(\ce{HBr}\), and \(\ce{HI}\) is energetically unfavorable in dilute water solution. Why should this be so?

Exercise 11-6 Addition of chlorine to trans-2-butene in ethanoic acid (acetic acid, \(\ce{CH_3CO_2H}\) as solvent gives \(74\%\) meso-2,3-dichlorobutane, \(1\), \(24\%\) 2-chloro-1-methylpropyl ethanoate, \(2\), and \(2\%\) 3-chloro-1-butene, \(3\). (Note: \(2\) is formed as a \(D\),\(L\) pair, although only one enantiomer is shown here.)

Write a mechanism to show how all three products could be obtained from a common chloronium ion intermediate. Draw structures for the products expected from addition of chlorine to cis-2-butene in ethanoic acid. Show the configurations. If necessary, review Section 5-5 before working this problem.

Exercise 11-7 In the formation of ethenebromonium ion from 1-bromo-2-fluoroethane and \(\ce{SbF_5}\) is \(\ce{SO_2}\), is the \(\ce{SbF_5}\) playing the role of an acid, a base, an electrophile, or a nucleophile? How strong a nucleophile do you judge \(\ce{SbF_6^-}\) to be? Explain.

Exercise 11-8 Show the steps involved in the formation of ethyl hydrogen sulfate from ethene and sulfuric acid. Show how diethyl sulfate, \(\ce{(CH_3CH_2O)_2SO_2}\), could be formed from the same reagents.

Exercise 11-9* Suppose we were gradually to add water to a solution of \(\ce{C_3H_2^+-SbF_6^-}\) and excess \(\ce{HSbF_6}\) in \(\ce{SO_2}\). What changes would you expect to take place? Write equations for the reactions you expect to occur.

Exercise 11-10* Assess the possibility of adding ammonia, \(\ce{NH_3}\), to 2-methylpropene with the aid of sulfuric acid as a catalyst.

Exercise 11-11* Show by projection formulas the stereochemical course you would expect for the acid-catalyzed addition of \(\ce{D_2O}\) to fumaric acid to give the deuterated malic acid \(4\). Be sure you consider the stereochemistry of the \(\ce{C-D}\) bond relative to the \(\ce{C-O}\) bond. Indicate your reasoning.

Exercise 11-12* The hydration of fumaric acid catalyzed by fumarase in \(\ce{D_2O}\) leads to malic acid with only one \(\ce{C-D}\) bonds, which is selectively removed when malic acid is enzymatically reconverted to fumaric acid. The configuration of deuteriomalic acid prepared in this way has been shown to correspond to the following projection formula:

Deduce the stereochemistry of both the forward and backward reactions (hydration and dehydration) from this information.

Exercise 11-13 Explain how Markownikoff's rule for orientation in electrophilic additions can be accounted for in terms of the modern view of how these reactions occur, using the reaction of \(\ce{HCl}\) with 1-methylcyclohexene as an example.

Exercise 11-14 Predict the major product(s) of each of the following electrophilic addition reactions (under conditions of kinetic control):

a. 1-butene with concentrated \(\ce{H_2SO_4}\)
b. 2-methylpropene in \(10\%\) aqueous \(\ce{H_2SO_4}\)
c. 2-methyl-2-butene with \(\ce{Br_2}\) in methanol as solvent.

Exercise 11-15 Arrange ethene, propene, and 2-methylpropene in order of expected ease of hydration with aqueous acid. Show your reasoning.

Exercise 11-16 Use the electronegativity chart of Figure 10-11 and Table 10-2 to predict the products of the following additions under conditions of kinetic control. Indicate the configuration of the products where possible.

a. 2-methyl-2-butene with \(\ce{ICl}\), and with \(\ce{INO_2}\)
b. a carbon-nitrogen double bond with water
c. \(\ce{N}\)-bromosuccinimide and cyclohexene in aqueous medium
d. 2-methylpropene with \(\ce{HOF}\)
e. 2-methylpropene with \(\ce{N}\)-bromosuccinimide and \(70\%\) hydrogen fluoride in pyridine (Notice that the pyridine serves only to moderate the activity of the hydrogen fluoride.)

Exercise 11-17 Make atomic-orbital models of the 1- and 2-fluoroethyl carbocations (\(\ce{CH_3CHF^+}\) and \(\ce{FCH_2CH_2^+}\)). Predict which should be formed more rapidly by the addition of \(\ce{H^+}\) to fluoroethene. Give your reasoning.

Exercise 11-18 Predict the predominant product from addition of hydrogen chloride to each of the following alkenes. Give your reasoning.

a. \(\ce{CH_2=CCl_2}\)
b. \(\ce{(CH_3)_2C=CCl_2}\)
c. \(\ce{CF_3-CH=CH-CH_3}\)
d. \(\ce{CH_3OCH=CHF}\)

Exercise 11-19 The ethenyl carbocation, \(\ce{CH_2-CH}^\oplus\), apparently is formed much more easily by addition of a proton from \(\ce{HCl}\) to ethyne than it is by \(S_\text{N}1\) reactions of ethenyl chloride. Deduce from bond energies why this should be so.

Exercise 11-20 Show how the rearrangement of ethenol to ethanal could take place in aqueous solution with water behaving as both a proton acceptor (base) and a proton donor (acid).

Exercise 11-21 Predict the predominant products in each of the following reactions. Show the expected configurations of the intermediates and products.

a. 2-butyne with mercuric ethanoate, \(\ce{Hg(OCOCH_3)_2}\), in ethanoic acid
b. ethenylbenzene with aqueous sulfuric acid containing mercuric sulfate, \(\ce{HgSO_4}\)
c. ethyne with mercuric chloride in methanol

(Note: A mercuric salt of structure \(\ce{HgX_2}\) is a potential source of an electrophile, \(\ce{HgX}^\oplus\). Mercuric sulfate probably is a source of the electrophilic cation, \(\ce{HgOSO_3H}^\oplus\), in aqueous sulfuric acid.)

Exercise 11-22 Show the steps involved in the base-initiated addition of methanol to 2-hexen-4-yne (review Section 6-6).

Exercise 11-23 Sodium chloride in the presence of \(\ce{OH}^\ominus\) with 2-hexen-4-yne does not yield \(\ce{CH_3CCl=CH-CH=CHCH_3}\). Explain.

Exercise 11-24 Write two different radical mechanisms for peroxide-initiated addition of hydrogen chloride to alkenes and consider the energetic feasibility for each.

Exercise 11-25 Calculate the \(\Delta H^0\) values for initiation and chain-propagation steps of radical addition of hydrogen fluoride, hydrogen chloride, and hydrogen iodide to an alkene. Would you expect these reagents to add easily to double bonds by such a mechanism?

Exercise 11-26

a. Bromine adds to diethyl fumarate (diethyl trans-butenedioate) to give the meso adduct \(7\), and to diethyl maleate (diethyl cis-butenedioate) to give the \(D\),\(L\) adduct \(8\), provided that the reaction mixtures are kept at \(25^\text{o}\) or less and are carefully protected from light. Deduce whether the stereochemistry of the reaction is suprafacial or antarafacial under these conditions.

b.* Diethyl maleate rearranges rapidly to diethyl fumarate on irradiation with ultraviolet light, provided that a trace of bromine is present. Irradiation of equimolar amounts of bromine and diethyl maleate leads to a mixture of \(D\),\(L\) and meso adducts. Under these conditions the fumarate ester gives only the meso adduct. Show the steps involved in these transformations and explain clearly why the light-induced addition to the cis ester is not stereospecific.

Exercise 11-27 A radical of structure \(\ce{CH_3} \overset{\cdot}{\ce{C}} \ce{=CHBr}\) is involved in the light-induced addition of \(\ce{HBr}\) to propyne. What geometry would you expect it to have? Draw an atomic-orbital picture of the radical with particular attention to the hybridization of orbitals at the radical center.

Exercise 11-28 Bromotrichloromethane, \(\ce{BrCCl_3}\), adds to 1-octene by a radical-chain mechanism on heating in the presence of a peroxide catalyst. Use the bond-energy tables to devise a feasible mechanism for this reaction and work out the most likely structure for the product. Show your reasoning. Show the most likely product of addition of \(\ce{BrCCl_3}\) to 1-octyne. [Note: Radical-chain reactions involve abstraction of atoms, not abstraction of groups.]

Exercise 11-29 Propenenitrile (acrylonitrile, \(\ce{CH_2=CHCN}\)) will polymerize readily at \(-50^\text{o}\) in a polar solvent [e.g., dimethylmethanamide, \(\ce{HCON(CH_3)_2}\)] under the influence of sodium cyanide, \(\ce{NaCN}\). Show the initiation and propagation steps of this reaction, and predict the structure of the polymer. Why is a polar solvent necessary? Why does this polymerization proceed but not that of propene under the same conditions?

Exercise 11-30 Write a reasonable mechanism for termination of ethene polymerization by disproportionation. Calculate \(\Delta H^0\) values for termination of the chain reaction by combination and disproportionation. Which is the more favorable process?

Exercise 11-31 It has been reported that a mixture of 2-methylpropane-2-\(\ce{D}\), , and 2-methylpropane-1-\(\ce{^{13}C}\), , (\(\ce{^{13}C}\) is the stable carbon isotope of mass 13) is converted only very slowly by sulfuric acid to a mixture containing the two starting materials, ordinary 2-methylpropane and 2-methylpropane-1-\(\ce{^{13}C}\)-2-\(\ce{D}\).

The reaction is speeded up greatly by addition of small amounts of 2-methylpropene. Explain. Would you expect any significant formation of \(\ce{D_2SO_4}\) when the reaction is carried out in the presence of 2-methylpropene? Why?

Exercise 11-32 For each of the following groups of substances designate which is the strongest electrophile and which is the strongest nucleophile. Give your reasoning.

a. \(\ce{H_2O}\) \(\ce{H_3O}^\oplus\) \(\ce{HO}^\ominus\)

b. \(\ce{I_2}\) \(\ce{I}^\ominus\) \(\ce{H_2} \overset{\oplus}{\ce{O}} \ce{I}\)

c.

d. \(\ce{CH_3} \overset{\cdot \cdot}{\ce{N}} \ce{H_2}\) \(\ce{CH_3} \underset{\cdot \cdot}{\overset{\cdot \cdot}{\ce{N}}} \ce{H}^\ominus\) \(\ce{CH_3} \overset{\oplus}{\ce{N}} \ce{H_3}\)

e. \(\ce{HO-NO_2}\) \(\ce{H_2} \overset{\oplus}{\ce{O}} \ce{-NO_2}\) \(^\oplus \ce{NO_2}\) \(^\ominus \ce{O-NO_2}\)

f. \(\ce{HF}\) \(\ce{HF} + \ce{SbF_5}\) \(\ce{SbF_6^-}\) \(\ce{F^-}\)

g. \(\ce{FSO_3H}\) \(\ce{ClSO_3H}\) \(\ce{HOSO_3^-}\)

Exercise 11-33 Identify the nucleophile and the electrophile in each of the following reactions:

a.

b. \(\ce{H_2C=CH_2} + \ce{Br_2} \rightleftharpoons \ce{H_2C(Br)} \overset{\oplus}{\ce{C}} \ce{H_2} + \ce{Br}^\ominus\)

c.

d. \(\ce{CH_2=CHCN} + \ce{NH_2^-} \rightleftharpoons \ce{NH_2-CH_2}- \overset{\ominus}{\ce{C}} \ce{HCN}\)

Exercise 11-34 Indicate what reagents and conditions would convert cyclohexane to the following derivatives:

Exercise 11-35 Use the electronegativity chart (Figure 10-11) to predict how the indicated bond would be expected to be polarized for each of the following compounds:

a. \(\ce{H_3C-H}\)
b. \(\ce{H_3Si-H}\)
c. \(\ce{CH_3-Li}\)
d. \(\ce{CH_3-MgCH_3}\)
e. \(\ce{CH_3S-Cl}\)
f. \(\ce{H_2N-SCH_3}\)
g. \(\ce{H_2N-OH}\)
h. \(\ce{H_2N-Br}\)
i. \(\ce{H_2P-Cl}\)
j. \(\ce{(CH_3)_3Si-Cl}\)

Exercise 11-36 Draw structures for the major products of each of the following reactions. Indicate the stereochemistry of the product, where possible (\(\ce{D}\) is deuterium, the hydrogen isotope of mass 2.)

a.

b. \(\ce{CH_3CH=CH_2} + \ce{HF} \rightarrow\)

c. cis-1-phenylpropene \(+ \ce{DBr} \overset{\ce{CH_2Cl_2}}{\longrightarrow}\)

d. \(\ce{C_6H_5C \equiv CCH_3} + \ce{HCl} + \ce{CH_3COOH}\) (solvent) \(\rightarrow\)

e. \(\ce{CH_3C \equiv CH} \overset{\text{1. } \ce{HgCl_2}}{\longrightarrow} \overset{\text{2. } \ce{DCl}}{\longrightarrow}\)

f. \(\ce{C_6H_5CH=CH_2} + \ce{CH_3SCl} \rightarrow\)

g.

Exercise 11-37 Why is molecular fluorine generally unsatisfactory as a reagent to convert alkenes to 1,2-difluoroalkanes?

Exercise 11-38 Why does the addition of chlorine to 2-pentene in methanol give a mixture of the following products?
2,3-dichloropentane \(\left( 16\% \right)\)
2-chloro-3-methoxypentane \(\left( 35\% \right)\)
3-chloro-2-methoxypetane \(\left( 49\% \right)\)

Exercise 11-39 Suggest a mechanism to account for the following reaction:

Exercise 11-40 In Section 1-1I, the addition of bromine to tetrachloroethene was reported to be catalyzed by aluminum bromide. What is the function of aluminum bromide in this addition?

Exercise 11-41 Evaluate (show your reasoning) the possibility that the following reaction will give the indicated product.

If you do not think the indicated product would be important, write the structure(s) of the product(s) you think would be more likely to be found.

Exercise 11-42 2-Methylpropene reacts with ethene and hydrogen chloride under polar conditions to yield 1-chloro-3,3-dimethylbutane. Show a mechanism for this reaction that is consistent with the reactants, conditions, and product. Give your reasoning.

Exercise 11-43 2-Methylpropane (containing traces of 2-methylpropene) is converted by a large excess of deuteriosulfuric acid \(\left( \ce{D_2SO_4} \right)\) rather rapidly to 2-methylpropane with only nine deuteriums.

a. Write a polar mechanism for this hydrogen-exchange reaction that is in harmony with the known chemical properties of sulfuric acid and that predicts exchange of no more than nine of the ten hydrogens of 2-methylpropane.

b. Explain how 2-methylpropane-\(\ce{D_9}\) can be formed more rapidly than 2-methylpropane-\(\ce{D}_n\) with \(n < 9\) in the early stages of the reaction.

Exercise 11-44 Suggest how each of the following compounds may be prepared. Assume any necessary starting hydrocarbons are available. Specify the reaction conditions as closely as possible and indicate when isomer separations may be necessary.

a.

b.

c. 1,1,1-trichloro-3-bromohexane

d. \(\ce{C_6H_5COCH_2CH_3}\)

e. \(\ce{C_6H_5CH(OH)CH_3}\)

f. \(\ce{Cl_2CHCHCl_2}\)

g. \(\ce{CH_2=CHOCOCH_3}\) (ethenyl ethanoate)

h.

Exercise 11-45 Calculate \(\Delta H^0\) values for the following reactions in the gas phase per mole of the principal reactants, using the bond-energy table (Table 4-3).

a. \(n \times \ce{CH_2=CH_2} \rightarrow \ce{-(CH_2-CH_2)}_n-\)

b.

Exercise 11-46 Use bond energies (Table 4-3) to investigate the possibility of adding water to propene, using peroxide \(\left( \ce{ROOR} \right)\) catalysts. It has been reported that \(\gamma\) rays, such as from a cobalt-60 source, decompose water into \(\ce{H} \cdot\) and \(\ce{HO} \cdot\). Could such radiation initiate a chain-addition reaction of water to propene? How about a nonchain mechanism (i.e., one \(\gamma\)-ray photon per molecule of reacting propene)? In a nonchain mechanism, would you expect to get 1-propanol or 2-propanol? What other products may be obtained in a nonchain reaction? Give your reasoning in detail.

Exercise 11-47 Use bond energies (Table 4-3) to investigate the energetic feasibility of adding ammonia \(\left( \ce{NH_3} \right)\) to an alkene by a radical-chain mechanism with the aid of a peroxide \(\left( \ce{ROOR} \right)\) catalyst. What product would you expect to obtain from propene and ammonia by a radical mechanism of addition?

Exercise 11-48 Draw an energy diagram similar to Figure 10-10 for the progress of a two-step reaction \(\ce{A} + \ce{BC} \rightleftharpoons \ce{A-B-C} \overset{\ce{D}}{\longrightarrow} \ce{A-B} + \ce{C-D}\) in which the first step is a rapidly established equilibrium and the second step is the slow or rate-determining step. Label the diagram to show what part represents the transition state, reaction intermediate, overall energy of activation \(\left( \Delta H' \right)\), and overall standard enthalpy change \(\left( \Delta H^0 \right)\) for the process \(\ce{A} + \ce{BC} + \ce{D} \rightarrow \ce{AB} + \ce{CD}\). Assume that the overall equilibrium constant, \(K_\text{eq} > 1\).

Exercise 11-49 Complete the following equations showing the structures of the expected products under the reaction conditions:

a. \(\ce{CH_2=CHCN} + \ce{(C_2H_5)_3SiH} \overset{\ce{ROOR}}{\longrightarrow}\)

b.

c.

d. 1-octene \(+ \ce{Cl_3SiH} \overset{\ce{ROOR}}{\longrightarrow}\)

e. \(\ce{(CH_3)_2C=CH_2} + \ce{CCl_4} \overset{\ce{ROOR}}{\longrightarrow}\)

f. \(\ce{HC \equiv CCH_2OCOCH_3} + \ce{BrCCl_3} \overset{\ce{ROOR}}{\longrightarrow}\)

g. \(\ce{CH_3C \equiv CH} + \ce{CF_3I} \overset{h \nu}{\longrightarrow}\)

h.

i.

j. 1-hexene + \(\ce{CH_3CO_2H} \overset{\ce{ROOR}}{\longrightarrow}\)

Exercise 11-50 This problem illustrates one of the complications that can arise in radical-addition reactions.

Cyclohexene reacts with bromotrichloromethane at \(40^\text{o}\) in the presence of small quantities of peroxides to give a mixture of products: 2-bromo-1-trichloromethylcyclohexane \(\left( 67\% \right)\) and 3-bromocyclohexene \(\left( 33\% \right)\). Account for the formation of both of these products under the reaction conditions.

Exercise 11-51 Describe how you would prepare each of the following compounds from the indicated starting materials. Specify the reagents and reaction conditions as closely as possible.

a. \(\ce{HO_2CCH_2CH_2CH_2CH_2CO_2H}\) (hexanedioic acid) from \(\ce{CH_3CO_2H}\) (ethanoic acid) and \(\ce{HC \equiv CH}\) (ethyne) by a radical-chain addition

b. from methylenecyclopentane

c. \(\ce{CH_3(CH_2)_7COCH_3}\) (2-decanone) from 1-octene

d. 2-decanone from 1-decyne

Exercise 11-52* The molecular weight of a polymer obtained by radical-addition polymerization can be reduced by addition of a thiol, \(\ce{RSH}\). For example, when propenoic acid (acrylic acid) is polymerized in the presence of potassium peroxodisulfate, \(\ce{K_2S_2O_8}\), adding mercaptoethanoic acid, \(\ce{HSCH_2CO_2H}\), causes the average molecular weight of the polymer molecule to become much smaller. Draw the structure of the polymer and explain how the thiol compound functions to reduce the molecular weight. Would an alcohol do as well? Show how the potassium peroxodisulfate could function as an initiator. Would you expect it to decompose more rapidly in alkaline or strongly acidic solution? Explain.

Exercise 11-53

a. 1-Bromocyclopentene adds hydrogen bromide on irradiation with ultraviolet light to give \(94\%\) cis-1,2-dibromocyclopentane and \(6\%\) of the trans isomer. Explain why 1,1-dibromocyclopentane is not obtained and why the cis isomer predominates.

b. From your answer to the questions in Part a, predict the structure and stereochemistry of the major products in the following reactions:

(i) \(\ce{CH_3-C \equiv C-CH_3} + 2 \ce{HBr} \overset{\ce{ROOR}}{\longrightarrow}\)

(ii) \(\ce{CH_3C \equiv CBr} + \ce{HBr} \overset{h \nu}{\longrightarrow}\)

(iii) cis\(\ce{-CH_3CH=CHCH_3} + \ce{DBr} \overset{\ce{ROOR}}{\longrightarrow}\)

(iv) \(\ce{HC \equiv CH} + \ce{DCl} \overset{\ce{HgCl_2}}{\longrightarrow}\) (monoaddition product of \(\ce{DCl}\))

Contributors

John D. Robert and Marjorie C. Caserio (1977) Basic Principles of Organic Chemistry, second edition. W. A. Benjamin, Inc. , Menlo Park, CA. ISBN 0-8053-8329-8. This content is copyrighted under the following conditions, "You are granted permission for individual, educational, research and non-commercial reproduction, distribution, display and performance of this work in any format."