13.E: Polyfunctional Compounds, Alkadienes, and Approaches to Organic Synthesis (Exercises)

Exercise 13-1 1,4-Pentadiene is different from propene in some of its chemical properties; for example, removal of the hydrogens at the 3-position by attack of radicals is much easier than the removal of those on the methyl group of propene. Explain why this should be so. (The rules of Section 6-5B will be helpful in this connection.)

Exercise 13-2 Draw an energy diagram, analogous to Figure 10-10 representing the reaction of Equation 13-1 for the addition of $$\ce{HCl}$$ to 1,3-butadiene, that reflects the fact that the 1,4 adduct is more stable, but is formed less rapidly than the 1,2 adduct. (Notice that of the two ways a proton from $$\ce{HCl}$$ can add to 1,3-butadiene, only one gives a carbocation that is delocalized.)

Exercise 13-3 Write the structures of the intermediates and the addition products expected in each of the following reactions (you may wish to review Chapters 10 and 11):

a. $$\ce{CH_2=C(CH_3)-CH=CH_2} \underset{20^\text{o}}{\overset{\ce{Cl_2}, \: \ce{CHCl_3}}{\longrightarrow}}$$

b. 2,3-dimethyl-1,3-butadiene $$\overset{\ce{B_2H_6}}{\longrightarrow} \: \overset{\ce{H_2O_2}, \: \ce{NaOH}}{\longrightarrow}$$ (product has formula $$\ce{C_6H_{14}O_2}$$)

c. butadiene $$\underset{\text{peroxide}}{\overset{\ce{C_6H_5SH}}{\longrightarrow}}$$

d. 4-methyl-1,3-pentadiene $$\overset{\ce{Ni}, \: \ce{H_2} \: \text{(1 mole)}}{\longrightarrow}$$

Exercise 13-4 What products would you expect from the Diels-Alder addition of tetracyanoethane to cis,trans-2,4-hexadiene and cis,cis-2,4-hexadiene? Explain.

Exercise 13-5 Draw the two possible orientations of diene to dienophile for the addition of cis-butanedioic anhydride to trans,trans-2,4-hexadiene. Which adduct would you expect to form preferentially? Explain.

Exercise 13-6 Predict the [4 + 2] addition products of the following reactions; show your reasoning:

a. 1,3-cyclopentadiene + trans-$$\ce{C_6H_5CH=CHCO_2H}$$

b.

c.

e. 1,3-cyclopentadiene + $$\ce{CH_2=CHCN}$$

f. trans-1,3-pentadiene plus propenal (two possible structural isomers)

Exercise 13-7 The following hydrocarbon degrades on heating by a reverse Diels-Alder reaction. What product(s) does it give?

Exercise 13-8* Formation of the addition product of $$\ce{SO_2}$$ and 1,3-butadiene has $$\Delta H^0 = -16.5 \: \text{kcal mol}^{-1}$$ for the vapor phase. Assuming the equilibrium constant $$K$$ is unity at $$00^\text{o} \text{C}$$, calculate $$\Delta S^0$$ for the reaction. Compare this value with the $$\Delta S^0$$ that you can calculate for addition of ethene to 1,3-butadiene, which has $$\Delta G^0 = -27 \ : \text{kcal}$$ and $$\Delta H^0 = -47 \: \text{kcal}$$. Estimate the temperature in $$^\text{o} \text{C}$$ that would be required for the equilibrium between ethene and 1,3-butadiene to have $$K = 1$$. (You may be interested to know that an early route for preparation of 1,3-butadiene involved passing cyclohexane through a tube containing a red-hot wire spiral, $$\sim 900^\text{o} \text{C}$$.)

Exercise 13-9* Diethyl phthalate is formed by heating thiopene-1,1-dioxide and diethyl butynedioate:

Show the steps involved in this reaction, with the knowledge that the dioxide by itself does not decompose at the reaction temperature.

Exercise 13-10* Draw structures for the possible [4 + 2] addition or decomposition products in the following reactions:

a. dimerization of 2-methylpropenal (methacrolein), $$\ce{CH_2=C(CH_3)CHO}$$ (two possible structural isomers)

b. 1-ethenylcyclohexene + $$\ce{SO_2} \overset{100^\text{o}}{\longrightarrow}$$

c.

Exercise 13-11* Suggest a mechanism to show how the following compound may be formed by irradiation of a solution of cis-butenedioic anhydride (maleic anhydride) in benzene:

Exercise 13-12 Formulate chain initiation, propagation, and termination steps for the polymerization of 1,3-butadiene by a mixture of 1,2 and 1,4 addition using a peroxide catalyst. Consider carefully possible structures for the growing-chain radical. Show the expected structure of the polymer and calculate $$\Delta H^0$$ for the polymerization reaction.

Exercise 13-13* The rearrangement of 1,2-butadiene to 2-butyne shown in Section 13-5B uses ethoxide ion as a basic catalyst. When one mole of 1,2-butadiene is treated with one mole of sodium amide in liquid ammonia, and water is added, the product is 1-butyne. Show the steps involved and explain why the product is different when an equivalent amount of a very strong base is used. (You may wish to review Section 11-8.)

Exercise 13-14 Suppose one treated $$100 \: \text{g}$$ of propene with $$125 \: \text{g}$$ of chlorine in the presence of water and isolated $$25 \: \text{g}$$ of excess propene, $$130 \: \text{g}$$ of 1,2-dichloropropane, $$40 \: \text{g}$$ of 1-chloro-2-propanol, and no chlorine from the reaction mixture. Calculate a percent yield and a percent conversion for the products based (a) on propene and (b) on chlorine.

Exercise 13-15 Syntheses have been carried out with one hundred or more sequential reactions. If the yield in each step is $$99\%$$, what would be the overall yield after one hundred steps? Repeat the calculation for a yield of $$99.9\%$$ in each step. What do you conclude from these calculations about the importance of yield in multistep syntheses? (It is interesting to contemplate how even simple organisms can synthesize molecules by what appear to be sequences of 10,000 or more separate steps with very few, if any, errors, and what means the organisms have to check the accuracy of the sequence after each incorporation of a new subunit.)

Exercise 13-16 Devise a synthesis of 2-chloro-2,4,4-trimethylpentane from organic compounds with four carbons or less and any necessary inorganic reagents. Your synthesis should involve the $$\ce{C-C}$$ bond-forming reactions listed in Table 13-4 and other reactions shown in Table 13-5. The product should be 2-chloro-2,4,4-trimethylpentane and not a mixture of its isomers.

Exercise 13-17 Show how each of the following compounds could be synthesized from the indicated starting material and other appropriate organic or inorganic reagents. Specify the reaction conditions, mention important side reactions, and justify the practicality of any isomer separations.

b. 2-hexyne from propyne
c. from 2-butyne

Exercise 13-18 Indicate how you would synthesize each of the following compounds from ethene, propene, 2-methylpropene, or 2-methylpropane, and appropriate inorganic reagents. Specify reagents and the reaction conditions, and justify the practicality of any isomer separations. If separations are not readily possible, estimate the proportion of the desired compound in the final product.

a.

b.

c.

d.

e.

f.

g.

h. $$\ce{CH_3-CH_2-CH_2OH}$$

i.

j.

Exercise 13-19 Assume that it is necessary to synthesize meso-1,4-diphenyl-2,3-butanediol. How could you do this if the only organic reagents at your disposal are methylbenzene and ethyne? In devising a suitable scheme, use any inorganic reagents you consider necessary and specify the reaction conditions (catalysts, solvent, use of acids or bases, and temperature) as closely as possible.

Exercise 13-20 The following key intermediates can be used in a synthesis of cyclotetradeca-1,3,8,10-tetrayne. Write the reagents and conditions for achieving transformations between the key intermediates. Be as specific as possible. Notice that more than one step many be involved in any given transformation.

Exercise 13-21 Devise a synthesis of 3-hexyn-1,6-diol from two-carbon compounds using the unsaturated cyclic ether, , as a protecting reagent for hydroxyl groups.

Exercise 13-22 Using the proposed mechanism for the Diels-Alder reaction, explain why you would not expect a reactive dienophile to form [4 + 2] cycloaddition products with the following compounds:

b. 3,4-dimethylidenecyclobutene (Refer to Section 6-6.)
c. 3-methylidenecyclohexane

Exercise 13-23 Write the last step in a synthesis of each of the following substances (give approximate reaction conditions):

a. $$\ce{CH_2=CHCHBrCH_3}$$

b.

c.

d.

e.

f.

g. $$\ce{-[CH_2-CF=CH-CH_2]}_n-$$

Exercise 13-24* 1,2-Propadiene adds hydrogen chloride to yield 2-chloropropene. However, the possibility exists that initial attack of a proton might lead to the 2-propenyl cation (Section 6-6), which then would react with chloride ion to form 3-chloropropene. Using the rules for application of the resonance method (Section 6-5B) and the atomic-orbital model for 1,2-propadiene (Figure 13-4), rationalize why a 2-propenyl cation might not be formed easily by addition of a proton to 1,2-propadiene and why 2-chloropropene is the observed product.

Exercise 13-25 How many stereoisomers would you expect for each of the following compounds? Indicate your reasoning and draw appropriate structural formulas for each one.

b. cyclodecene
c. 1,2,3-trimethylcyclopropane
d. 2,4,6-octatriene
f. 1,4-dichloro-1,2,3-butatriene
g. ethylidene-3-methylcyclohexane

Exercise 13-26 Write structural formulas for the products you would expect from each of the following reactions:

a. 1,2-propadiene and hypochlorous acid (1 mole)
b. 1,3-pentadiene with hydrogen chloride (1 mole)
c. ozonization of 1,3-butadiene followed by reduction with zinc
d. 1,3-butadiene with hypochlorous acid (2 moles)
e. 1,3-butadiene with propenoic acid followed by bromine

Exercise 13-27 Which of the following structures are chiral and which are achiral? (Models will be very helpful.)

a.

b.

c.

d.

e.

f.

g.

h.

Exercise 13-28* Several widely used pesticides are highly chlorinated polycyclic compounds derived from hexachloropentadiene. They include Aldrin, Dieldrin, and Chlordane. Use of these substances is to be curtailed greatly because of undesirable environmental effects.

Suggest a plausible synthesis for each of these compounds from readily available $$\ce{C_2-C_5}$$ compounds, including hexachlorocyclopentadiene. Proceed as in Section 13-7 to see how the carbon framework can be broken down to more familiar smaller fragments and then reconstructed by known reactions.

Exercise 13-29 Suggest reasonable structures for the products of the following reactions:

a.

b.

c. $$\ce{C_6H_5C \equiv CH} + \ce{F_2C=CF_2} \overset{\text{heat}}{\longrightarrow}$$

d.

Exercise 13-30 Draw structures for the products of each of the following reactions, each of which takes place at room temperature or higher. Indicate the stereochemistry expected.

a.

b.

c. $$\ce{CH_2=CH-CH=CH_2} + \ce{CH_2=CH-C \equiv N} \rightarrow$$

d. trans-$$\ce{C_6H_5CH=CH-CH=CH_2} + \ce{CH_2=CHCO_2CH_3} \rightarrow$$

Exercise 13-31* In Section 10-5 we showed that ethyne is much less reactive toward chlorine than is ethene. The same is true for hydrogen chloride. However, when hydrogen chloride adds to 3-butenyne, it adds to the triple bond instead of the double bond, thereby forming 2-chloro-1,3-butadiene instead of 3-chloro-1-butyne. With reference to the discussion in Section 13-2, explain why the order of reactivity of the double and triple bonds of 3-butenyne toward electrophilic reagents may be different from that of ethene and ethyne?

Exercise 13-32 Draw structures for the different ways in which a monomer unit could be added to a growing chain in a radical-chain polymerization of 2-chloro-1,3-butadiene.

Exercise 13-33 Show how you would carry out the following transformations. Notice that each is an example of changing the position or nature of the functional group without affecting the carbon skeleton.

a. $$\ce{CH_3CH_2C \equiv CCH_2CH_3} \rightarrow \ce{CH_3CH_2CH_2CH_2C \equiv CH}$$

b.

c.

d. $$\ce{C_6H_5C \equiv CCH_3} \rightarrow \ce{C_6H_5CH_2COCH_3}$$

e. $$\ce{CH_3C \equiv CCH_3} \rightarrow \ce{CH_3CH=CDCH_3}$$ (cis)

Exercise 13-34 Cycloaddition reactions are valuable for the synthesis of carbocyclic compounds. Each of the following compounds can be formed by either a [4 + 2] or [2 + 2] cycloaddition as a last step in the synthesis. Draw the structures of the reagents you think would undergo cycloaddition to give the compounds shown.

a.

b.

c.

d.

Exercise 13-35 The following reaction occurs in good yield. Show the steps involved in forming the product.

Exercise 13-36* Indicate the steps involved in the following synthesis of bicyclo[4.2.0]-2,4,6-octatriene (benzocyclobutene):

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."