8 • Additional Problems
8 • Additional Problems
The following alkene undergoes hydroboration–oxidation to yield a single product rather than a mixture. Explain the result, and draw the product showing its stereochemistry.
From what alkene was the following 1,2-diol made, and what method was used, epoxide hydrolysis or OsO4?
Which of the reactions below would result in a product mixture that would rotate plane-polarized light?
Iodine azide, IN3, adds to alkenes by an electrophilic mechanism similar to that of bromine. If a monosubstituted alkene such as 1-butene is used, only one product results:
10-Bromo-α-chamigrene, a compound isolated from marine algae, is thought to be biosynthesized from γ-bisabolene by the following route:
Draw the structures of the intermediate bromonium and cyclic carbocation, and propose mechanisms for all three steps.
Isolated from marine algae, prelaureatin is thought to be biosynthesized from laurediol by the following route. Propose a mechanism.
Dichlorocarbene can be generated by heating sodium trichloroacetate. Propose a mechanism for the reaction, and use curved arrows to indicate the movement of electrons in each step. What relationship does your mechanism bear to the base-induced elimination of HCl from chloroform?
Reaction of cyclohexene with mercury(II) acetate in CH3OH rather than H2O, followed by treatment with NaBH4, yields cyclohexyl methyl ether rather than cyclohexanol. Suggest a mechanism.
Use your general knowledge of alkene chemistry to suggest a mechanism for the following reaction.
Treatment of 4-penten-1-ol with aqueous Br2 yields a cyclic bromo ether rather than the expected bromohydrin. Suggest a mechanism, using curved arrows to show electron movement.
Hydroboration of 2-methyl-2-pentene at 25 °C, followed by oxidation with alkaline H2O2, yields 2-methyl-3-pentanol, but hydroboration at 160 °C followed by oxidation yields 4-methyl-1-pentanol. Suggest a mechanism.
Reactions of Alkenes
Predict the products of the following reactions (the aromatic ring is unreactive in all cases). Indicate regiochemistry when relevant.
The cis and trans isomers of 2-butene give different cyclopropane products in the Simmons–Smith reaction. Show the structures of both, and explain the difference.
Predict the products of the following reactions. Don’t worry about the size of the molecule; concentrate on the functional groups.
Addition of HCl to 1-methoxycyclohexene yields 1-chloro-1-methoxycyclohexane as a sole product. Use resonance structures of the carbocation intermediate to explain why none of the alternate regioisomer is formed.
Synthesis Using Alkenes
Which of the following alcohols could not be made selectively by hydroboration–oxidation of an alkene? Explain.
(a) (b) (c) (d)
Plexiglas, a clear plastic used to make many molded articles, is made by polymerization of methyl methacrylate. Draw a representative segment of Plexiglas.
Poly(vinyl pyrrolidone), prepared from N-vinylpyrrolidone, is used both in cosmetics and as a component of a synthetic substitute for blood. Draw a representative segment of the polymer.
When a single alkene monomer, such as ethylene, is polymerized, the product is a homopolymer. If a mixture of two alkene monomers is polymerized, however, a copolymer often results. The following structure represents a segment of a copolymer called Saran. What two monomers were copolymerized to make Saran?
Using an oxidative cleavage reaction, explain how you would distinguish between the following two isomeric dienes:
Compound A, C10H18O, undergoes reaction with dilute H2SO4 at 50 °C to yield a mixture of two alkenes, C10H16. The major alkene product, B, gives only cyclopentanone after ozone treatment followed by reduction with zinc in acetic acid. Identify A and B, and write the reactions.
Draw the structure of a hydrocarbon that absorbs 2 molar equivalents of H2 on catalytic hydrogenation and gives only butanedial on ozonolysis.
α-Terpinene, C10H16, is a pleasant-smelling hydrocarbon that has been isolated from oil of marjoram. On hydrogenation over a palladium catalyst, α-terpinene reacts with 2 molar equivalents of H2 to yield a hydrocarbon, C10H20. On ozonolysis, followed by reduction with zinc and acetic acid, α-terpinene yields two products, glyoxal and 6-methyl-2,5-heptanedione.
Evidence that cleavage of 1,2-diols by HIO4 occurs through a five-membered cyclic periodate intermediate is based on the measurement of reaction rates. When diols A and B were prepared and the rates of their reaction with HIO4 were measured, it was found that diol A cleaved approximately 1 million times faster than diol B. Make molecular models of A and B and of potential cyclic periodate intermediates, and then explain the results.
Reaction of HBr with 3-methylcyclohexene yields a mixture of four products: cis- and trans-1-bromo-3-methylcyclohexane and cis- and trans-1-bromo-2-methylcyclohexane. The analogous reaction of HBr with 3-bromocyclohexene yields trans-1,2-dibromocyclohexane as the sole product. Draw structures of the possible intermediates, and then explain why only a single product is formed in this reaction.
We’ll see in the next chapter that alkynes undergo many of the same reactions that alkenes do. What product might you expect from each of the following reactions?