11.E: Alkenes and Alkynes II (Exercises)

Exercise 11-1 For each of the following reactions determine the oxidation state of the carbons in the reactants and products and decide whether the overall changes involve oxidation, reduction, or neither.

a. $$\ce{CH_4} + \ce{Cl_2} \rightarrow \ce{CH_3Cl} + \ce{HCl}$$

b. $$\ce{CH_3CH=CH_2} + \ce{HCl} \rightarrow \ce{CH_3CH(Cl)CH_3}$$

c. $$\ce{CH_3CH=CH_2} + \ce{HOCl} \rightarrow \ce{CH_3CH(OH)CH_2Cl}$$

d. $$\ce{(CH_3)_2C=CH_2} + \ce{(CH_3)_3CH} \rightarrow \ce{(CH_3)_2CHCH_2C(CH_3)_3}$$

e. $$n \ce{(CH_2=CHCN)} \rightarrow \ce{-(CH_2CH(CN))}_n-$$

f. $$\ce{CH_3OH} \rightarrow \ce{CH_2=O} + \ce{H_2}$$

Exercise 11-2 Balance each of the following equations. You may need to add $$\ce{H_2O}$$ to one side or the other of the equations.

a. $$\overset{\oplus}{\ce{K}} \overset{\ominus}{\ce{Mn}} \ce{O_4} + \ce{RCH=CH_2} \rightarrow \ce{RC} \overset{\ominus}{\ce{O_2}} \overset{\oplus}{\ce{K}} + \ce{CH_2=O} + \ce{MnO_2}$$

b. $$\ce{CrO_3} + \ce{C_6H_5CH_2CH_3} \rightarrow \ce{C_6H_5CH_2CO_2H} + \ce{Cr}^{3 \oplus}$$

Exercise 11-3 Draw structures for the products expected from the following reactions. Show configurations where significant.

a.

b. cis-2-butene $$\underset{25^\text{o}}{\overset{\ce{D_2}, \: \ce{Pt}}{\longrightarrow}}$$

c. $$\ce{CH_2=CHCOCH_3} \underset{25^\text{o}}{\overset{\ce{H_2}, \: \ce{Pt}}{\longrightarrow}}$$

d. 1-penten-3-yne $$\overset{\ce{H_2}, \: \ce{Pd-Pb}}{\longrightarrow}$$

e. $$\ce{C_6H_5C \equiv CC_6H_5} \overset{\ce{H_2}, \: \ce{Pd-Pb}}{\longrightarrow}$$

f. 1,3-dimethylcyclopentene $$\underset{25^\text{o}}{\overset{\ce{H_2}, \: \ce{Pt}}{\longrightarrow}}$$

Exercise 11-4* The conditions of catalytic hydrogenation sometimes lead to rearrangement of a double bond from one location to another. Using 1-butene as an example, show how operation of the equilibria shown in the mechanism of Figure 11-2 could lead to rearrangement of 1-butene to 2-butene over a hydrogenation catalyst in the presence of $$\ce{H_2}$$. If $$\ce{D_2}$$ were used for reduction of 1-butene under these circumstances, suggest where and how much deuterium might be introduced into the butane formed.

Exercise 11-5 Use bond energies to explain the following facts:

a. Ethyne is more easily hydrogenated catalytically than nitrogen.

b. Ethyne is more easily hydrogenated catalytically than ethene.

c. Ethene is more easily hydrogenated catalytically than methanal $$\left( \ce{CH_2O} \right)$$.

d. In the hydrogenation of nitrogen, ammonia is formed; in contrast, the hydrogenation of ethyne leads to ethane, not methane.

Exercise 11-6

a. Would you expect a carbon-nitrogen triple bond to be hydrogenated more, or less, easily than a carbon-carbon triple bond?

b. Why is it difficult to hydrogenate a tetrasubstituted alkene such as 2,3-dimethyl-2-butene?

Exercise 11-7* Accurate $$\Delta H^0$$ and $$\Delta G^0$$ values in $$\text{kcal mol}^{-1}$$ for hydrogen addition to 1-butene, cis- and trans-2-butene in the gas phase at $$25^\text{o}$$ follow:

a. From the data (and after reviewing Section 4-4B), calculate $$\Delta S^0$$ for each of these reactions at $$25^\text{o}$$ $$\left( 298 \: \text{K} \right)$$. Why is $$\Delta S^0$$ so large for these reactions?

b. Calculate $$\Delta H^0$$, $$\Delta G^0$$, and $$\Delta S^0$$ for $$\ce{CH_2=CHCH_2CH_3} \rightarrow \ce{CH_3CH=CHCH_3}$$ (trans) and for $$\ce{CH_3CH=CHCH_3}$$ (cis) $$\rightarrow \ce{CH_3CH=CHCH_3}$$ (trans). Are the $$\Delta S^0$$ values in accord with your expectations? What can you conclude as to how good a qualitative measure heats of hydrogenation are of relative alkene stabilities?

Exercise 11-8 Consider that it is necessary to synthesize pure samples of $$D$$,$$L$$-hexane-3,4-$$\ce{D_2}$$ and meso-hexane-3,4-$$\ce{D_2}$$. Show how this might be done both with diimide and catalytic-type reductions, assuming that any necessary deuterium-labeled reagents and six-carbon organic compounds are available.

Exercise 11-9

a. Show how $$1$$ is formed by hydroboration of 1,5-cyclooctadiene.

b. What product would you expect from the hydroboration of 2,4-dimethyl-1,4-pentadiene with $$\ce{BH_3}$$?

c. Explain why diborane adds to methylcyclohexene to give tris-(trans-2-methylcyclohexyl)borane in preference to the cis isomer.

Exercise 11-10 What products would you expect from hydroboration of the following alkenes with a dialkylborane, $$\ce{R_2BH}$$, followed by isomerization at $$160^\text{o}$$?

a. $$\ce{CH_3CH=CHCH_3}$$

b.

c.

d.

Exercise 11-11 Predict the products in each step of the following reaction sequences:

a.

b. $$\ce{(CH_3)_2C=CHCH_3} \overset{\ce{B_2H_6}}{\longrightarrow} \: \overset{\ce{C_2H_5C \equiv CC_2H_5}}{\longrightarrow} \: \overset{\ce{H_2O_2}, \: ^\ominus \ce{OH}}{\longrightarrow}$$

c. $$\ce{(CH_3)_2C=CHCH_3} \overset{\ce{B_2H_6}}{\longrightarrow} \: \overset{\ce{C_2H_5C \equiv CC_2H_5}}{\longrightarrow} \: \overset{\ce{CH_3CH_2CO_2D}}{\longrightarrow}$$

Exercise 11-12

a. Draw the structure and configuration of the product expected of the reaction between 1-bromo-1-hexyne and diethylborane, $$\ce{(C_2H_5)_2BH}$$.

b. When the product is treated with sodium methoxide, $$\ce{NaOCH_3}$$, then with propanoic acid, trans-3-octene is formed. Show the steps involved in forming this trans-alkene.

Exercise 11-13 A hydrocarbon of formula $$\ce{C_{11}H_{18}}$$ on reaction with ozone in dichloromethane gave, after the addition of water and finely divided zinc, three products in equimolar amounts that were identified as 2-butanone $$\left( \ce{CH_3COCH_2CH_3} \right)$$, methanal $$\left( \ce{CH_2O} \right)$$, and cyclohexane-1,4-dione . Draw the structure of the hydrocarbon $$\ce{C_{11}H_{18}}$$.

Exercise 11-14* When 2-butene reacts with ozone in the presence of 2-propanone, two structurally different ozonides are obtained, as well as ethanal:

Suggest how these products can be formed from the molozonide(s) of 2-butene.

Exercise 11-15 Alkynes react more slowly than alkenes with permanganate and usually give dicarbonyl compounds. An example follows:

a. What is the change in oxidation state of carbon in this reaction?

b. If $$\ce{Mn}$$(VII) is reduced to $$\ce{Mn}$$(IV), how many moles of permanganate are required per mole of alkyne?

Exercise 11-16 Starting with cyclohexene, show how you could prepare each of the following compounds:

a. the epoxide of cyclohexene
b. cis-cyclohexane-1,2-diol
c. trans-cyclohexane-1,2-diol

Exercise 11-17 Suppose you were given four unlabeled bottles, each of which is known to contain one of the following compounds: pentane, 1-pentene, 2-pentyne, or 1-pentyne. Explain how you could use simple chemical tests (preferably test-tube reactions) to identify the contents of each bottle. (Notice that all four compounds are low-boiling liquids.)

Exercise 11-18* Show how the (\Delta H\) values of the following processes can be combined to calculate the heat of solution of $$\ce{Na}^\oplus \left( g \right) + \ce{Cl}^\ominus \left( g \right)$$ at $$298^\text{o} \text{K}$$.

$\begin{array}{ll} \ce{Na} \left( s \right) + \frac{1}{2} \ce{Cl_2} \left( g \right) \rightarrow \ce{Na}^\oplus \left( aq \right) + \ce{Cl}^\ominus \left( aq \right) & \Delta H^0 = -97 \: \text{kcal} \\ \ce{Na} \left( g \right) \rightarrow \ce{Na}^\oplus \left( g \right) + \ce{e^-} & \Delta H^0 = +118 \: \text{kcal} \\ \ce{Cl}^\ominus \left( g \right) \rightarrow \ce{Cl} \cdot \left( g \right) + \ce{e-} & \Delta H^0 = +83 \: \text{kcal} \\ \frac{1}{2} \ce{Cl_2} \left( g \right) \rightarrow \ce{Cl} \cdot \left( g \right) & \Delta H^0 = +29 \: \text{29} \\ \ce{Na} \left( s \right) \rightarrow \ce{Na} \left( g \right) & \Delta H^0 = +26 \: \text{kcal} \end{array}$

Exercise 11-19 A serious contamination in butenyne made by dimerization of ethyne with cuprous ion is 1,5-hexadien-3-yne. Show how this substance can be formed.

Exercise 11-20 The following physical properties and analytical data pertain to two isomeric hydrocarbons, $$\ce{A}$$ and $$\ce{B}$$, isolated from a gasoline:

Both $$A$$ and $$B$$ readily decolorize bromine and permanganate solutions and give the same products on ozonization. Suggest possible structures for $$A$$ and $$B$$. What experiments would you consider necessary to further establish the structures and configurations of $$A$$ and $$B$$?

Exercise 11-21 It is possible to synthesize two isomeric cycloalkenes of formula $$\ce{C_8H_{14}}$$. Both of these compounds react with hydrogen in the presence of platinum to give cyclooctane, and each, on ozonization followed by reduction, gives:

a. What are the structures of the two compounds?

b. Would the two substances give the same compound on hydroxylation with potassium permanganate?

Exercise 11-22 When 5-decyne is heated with diborane at $$160^\text{o}$$ and the product is oxidized with hydrogen peroxide in basic solution, 1,10-decanediol is obtained. Write equations to show the several reactions involved in these transformations. You need not show detailed mechanisms.

Exercise 11-23

a. Draw the structures and configurations of the products that will be formed by the following reactions of cis-2-butene-2-$$\ce{D}$$:

b. What is the stereochemical relationship between the products of the two reactions in Part a?

Exercise 11-24* Show the structures of the products expected in each step of the following sequences. Be sure to indicate the stereochemistry of reactions where this is important. Remember that $$\ce{D}$$ is the hydrogen isotope of mass 2.

a. cis-2-butene $$\overset{\ce{C_2NND_2}}{\longrightarrow}$$

b. 1-methylcyclohexene $$\overset{\ce{BH_3}}{\longrightarrow} \: \overset{160^\text{o}}{\longrightarrow} \: \overset{\ce{H_2O_2}, \: ^\ominus \ce{OH}}{\longrightarrow}$$

c.

d. propyne $$\overset{\ce{R_BD}}{\longrightarrow} \: \overset{\ce{CH_3CH_2CO_2H}}{\longrightarrow} \: \overset{\ce{O_3}}{\longrightarrow} \: \overset{\ce{H_2O}, \: \ce{Zn}}{\longrightarrow}$$

e. 2-butyne $$\overset{\ce{R_2BH}}{\longrightarrow} \: \overset{\ce{H_2O_2}, \: ^\ominus \ce{OH}}{\longrightarrow}$$

f. $$\ce{CH_3C \equiv CH} \overset{\ce{R_2BD}}{\longrightarrow} \: \overset{\ce{CH_3CH_2CO_2D}}{\longrightarrow}$$

g. 3-methylcyclopentene $$\overset{\ce{BH_3}}{\longrightarrow} \: \overset{160^\text{o}}{\longrightarrow} \: \underset{\text{(as trapping agent)}}{\overset{\ce{CH_3(CH_2)_7CH=CH_2}}{\longrightarrow}}$$

h. 1-pentene $$\overset{\ce{HBr}, \: \ce{ROOR}}{\longrightarrow} \: \overset{\ce{CH_3C \equiv CNa}}{\longrightarrow} \: \overset{\ce{H_2}, \: \ce{Pd-Pb}}{\longrightarrow} \: \overset{\ce{O_3}}{\longrightarrow} \: \overset{\ce{Na_2SO_3}}{\longrightarrow}$$

i. 1-pentene $$\overset{\ce{BH_3}}{\longrightarrow} \: \overset{\ce{H_2N-OSO_3H}}{\longrightarrow}$$

Exercise 11-25 Two stable compounds of formula $$\ce{C_6H_6}$$ react with bromine and with $$\ce{KMnO_4}$$. On hydrogenation with a platinum catalyst at $$25^\text{o}$$, both absorb two moles of hydrogen and form cyclohexane. Write possible structures for these substances and explain how electronic spectra may be used to tell which compound is which.

Exercise 11-26 Suppose one added hydrogen as $$\ce{H_2}$$ to the cyclopropene double bond of 1-methoxy-2-phenyl-3,3-dimethylcyclopropene. Explain how the proton nmr of the product can be used to infer whether the hydrogen added to the double bond in the suprafacial or antarafacial manner. (Review Section 9-10G and 9-10H.)

Exercise 11-27 In the hydrogenation of 1,2-dimethylcyclohexene over a platinum catalyst, the suprafacial addition product is formed. Assuming that the mechanism of this hydrogenation is as shown in Figure 11-2, what conditions must be put on the stereochemistry of each of the postulated steps in order that the overall reaction be suprafacial?

Exercise 11-28* When optically active 3-methylhexane is shaken with a nickel catalyst in the presence of deuterium gas $$\left( \ce{D_2} = \ce{^2H_2} \right)$$ it, like other alkanes, undergoes slow exchange of hydrogen for deuterium at the various carbons. The key observation is that substitution of $$\ce{D}$$ for $$\ce{H}$$ at $$\ce{C_3}$$ causes racemization (i.e., formation of equal amounts of deuterated 3-methylhexane with the two possible configurations at $$\ce{C_3}$$). Assuming that racemization and deuterium exchange occur only by way of the steps shown in Figure 11-2, determine how many steps backward the reaction has to go to produce the racemized, exchanged 3-methylhexane. To work this problem you will need to determine whether, when one starts with optically active 3-methylhexane, the various possible intermediate structures in Figure 11-2 would be chiral or not and how many steps back one would have to go to get to an achiral system. If you are uncertain about chirality we suggest that you review Section 5-1B.

Figure 11-29 Calculate $$\Delta H^0$$ for the reaction $$\ce{CH_3-C \equiv C-H} \rightarrow \ce{CH_2=C=CH_2}$$ from bond energies and also from $$\Delta H^0$$ values for the following reactions:

$\begin{array}{ll} \ce{CH_3C \equiv CH} + 2 \ce{H_2} \rightarrow \ce{CH_3CH_2CH_2} & \Delta H^0 = -69.1 \: \text{kcal} \\ \ce{CH_2=C=CH_2} + 2 \ce{H_2} \rightarrow \ce{CH_3CH_2CH_3} & \Delta H^0 = -71.3 \: \text{kcal} \end{array}$

Explain why the value of $$\Delta H^0$$ calculated form bond energies might be unreliable for the last reaction.

Exercise 11-30

a. Write a mechanism for the sulfuric acid-induced dimerization of 2-methyl-2-butene, indicating the products you expect to be formed. (It will be helpful to review Section 10-8B.)

b. Ozonization of the actual alkene mixture that is formed gives (along with a mixture of aldehydes and ketones) substantial amounts of 2-butanone $$\left( \ce{CH_3COCH_2CH_3} \right)$$. Write a structure and reaction mechanism for formation of a $$\ce{C_{10}}$$-olefin that reasonably might be formed in the dimerization reaction and that, on ozonization, would yield 2-butanone and a $$\ce{C_6}$$-carbonyl compound. (Consider how sulfuric acid might cause the double bond in 2-methyl-2-butene to shift its position.)

Exercise 11-31 How would you distinguish between the components in each of the following pairs using chemical methods (preferably test-tube reactions)?

a. $$\ce{CH_3CH_2C \equiv CH}$$ and $$\ce{CH_3C \equiv CCH_3}$$
b. $$\ce{CH_3CH_2C \equiv CH}$$ and $$\ce{CH_2=CH-CH=CH_2}$$
c. $$\ce{C_6H_5C \equiv CC_6H_5}$$ and $$\ce{C_6H_5CH_2CH_2C_6H_5}$$

Exercise 11-32 How could you distinguish between the compounds in the previous exercise, using spectroscopic methods?

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."