Chapter 10

Addition of chlorine to trans-2-butene in ethanoic acid (acetic acid, \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) 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.)

In the formation of ethenebromonium ion from 1 -bromo- 2 -fluoroethane and \(\mathrm{SbF}_{5}\) is \(\mathrm{SO}_{2}\), is the \(\mathrm{SbF}_{5}\) playing the role of an acid, a base, an electrophile, or a nucleophile? How strong a nucleophile do you judge \(\mathrm{SbF}_{6}^{-}\) to be? Explain.

Show the steps involved in the formation of ethyl hydrogen sulfate from ethene and sulfuric acid. Show how diethyl sulfate, \(\left(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{O}\right)_{2} \mathrm{SO}_{2}\), could be formed from the same reagents.

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 \(\mathrm{HCl}\) with 1 -methylcyclohexene as an example.

Predict the major product(s) of each of the following electrophilic addition reactions (under conditions of kinetic control): a. 1-butene with concentrated \(\mathrm{H}_{2} \mathrm{SO}_{4}\) b. 2-methylpropene in \(10 \%\) aqueous \(\mathrm{H}_{2} \mathrm{SO}_{4}\) c. 2 -methyl-2-butene with \(\mathrm{Br}_{2}\) in methanol as solvent.

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

Make atomic-orbital models of the 1 - and 2 -fluoroethyl carbocations \(\left(\mathrm{CH}_{3} \mathrm{CHF}^{+}\right.\) and \(\mathrm{FCH}_{2} \mathrm{CH}_{2}^{+}\) ). Predict which should be formed more rapidly by the addition of \(\mathrm{H}^{+}\) to fluoroethene. Give your reasoning.

The ethenyl carbocation, \(\mathrm{CH}_{2}-\mathrm{CH}^{\oplus}\), apparently is formed much more easily by addition of a proton from \(\mathrm{HCl}\) to ethyne than it is by \(S_{\mathrm{N}} 1\) reactions of ethenyl chloride. Deduce from bond energies why this should be so.

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).

A radical of structure \(\mathrm{CH}_{3} \dot{\mathrm{C}}=\mathrm{CHBr}\) is involved in the light-induced addition of \(\mathrm{HBr}\) 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.

Propenenitrile (acrylonitrile, \(\mathrm{CH}_{2}=\mathrm{CHCN}\) ) will polymerize readily at \(-50^{\circ}\) in a polar solvent [e.g., dimethylmethanamide, \(\left.\mathrm{HCON}\left(\mathrm{CH}_{3}\right)_{2}\right]\) under the influence of sodium cyanide, \(\mathrm{NaCN}\). 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?

It has been reported that a mixture of 2 -methylpropane-2-D, and 2 -methylpropane- \(1-{ }^{13} \mathrm{C}\), \(\left({ }^{13} \mathrm{C}\right.\) 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- \({ }^{13} \mathrm{C}-2-\mathrm{D}\). The reaction is speeded up greatly by addition of small amounts of 2-methylpropene. Explain. Would you expect any significant formation of \(\mathrm{D}_{2} \mathrm{SO}_{4}\) when the reaction is carried out in the presence of 2 -methylpropene? Why?

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

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

2-Methylpropane (containing traces of 2-methylpropene) is converted by a large excess of deuteriosulfuric acid \(\left(\mathrm{D}_{2} \mathrm{SO}_{4}\right)\) 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-D \(_{9}\) can be formed more rapidly than 2 -methylpropane- \(\mathrm{D}_{n}\) with \(n<9\) in the early stages of the reaction.