Chapter 4

Write detailed structures and predict which compound in each pair would have (1) the lower boiling point and (2) the higher water solubility. a. \(\mathrm{H}_{2} \mathrm{NCH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}, \mathrm{H}_{3} \mathrm{CCH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3}\) b. \(\mathrm{CH}_{3} \mathrm{OCH}_{3}, \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{OH}\) ?. \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{OH},\left(\mathrm{CH}_{3}\right)_{2} \mathrm{COH}\) d. \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}, \mathrm{HCO}_{2} \mathrm{CH}_{3}\) e. \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{CO}_{2} \mathrm{H}, \mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{7} \mathrm{CO}_{2} \mathrm{H}\)

The heat of combustion of 1 mole of liquid decane to give carbon dioxide and liquid water is \(1620.1\) kcal The heat of vaporization of decane at \(25^{\circ}\) is \(11.7\) kcal \(\mathrm{mol}^{-1}\). Calculate the heat of combustion that would be observed for all the participants in the vapor phase.

a. Calculate \(\Delta H^{0}\) from bond energies for the conversion of 1 -hexene to cyclohexane at \(25^{\circ}\) and from this, with \(\Delta S^{0}\) as \(-20.7\) e.u. \(\mathrm{mol}^{-1}\), calculate the equilibrium constant \(K_{\text {eq from Equation } 4-2 . \text { For comparison, calculate the equilibrium }}\) constant that would be expected if the degrees of disorder of the reactants and the products were equal (i.e., \(\Delta S^{0}=0\) ). b. How large can \(\Delta S^{0}\) be at \(25^{\circ}\) for a reaction before our \(\pm 15\) kcal rule starts to give incorrect answers?

Knowing that the equilibrium constant \(K_{\text {eq for formation of nonane from solid carbon and hydrogen gas is }}\) \(4.7 \times 10^{-5}\), explain why liquid nonane does not forthwith decompose into its elements.

a. N-Bromosuccinimide (NBS) is an excellent brominating reagent and is used widely to prepare bromoalkenes from alkenes (Wohl-Ziegler reaction): The reaction is initiated with chemical initiators (peroxides) and is as selective as bromination with molecular bromine. Write plausible propagation steps (three of them) for this reaction, given the fact that the actual brominating agent appears to be molecular bromine that is generated from NBS by HBr. b. What products would you expect to be formed on bromination of 2 -methylbutane with N-bromosuccinimide?

A \(C-F\) bond energy can be computed from thermochemical studies of the vapor- phase reaction $$ \mathrm{CH}_{4}+4 \mathrm{~F}_{2} \rightarrow \mathrm{CF}_{4}+4 \mathrm{HF} \quad \Delta H^{0}=-460 \mathrm{kcal} $$ Show how the \(\Delta H^{0}\) value for this reaction may be used to calculate the energy of the \(\mathrm{C}-\mathrm{F}\) bond if all the other bond energies are known.

The entropy change \(\Delta S^{0}\) for the formation of chloroethane by chlorination of ethane is \(+0.5 \mathrm{e} . \mathrm{u}\), and for the formation of chloroethane by combination of hydrogen chloride with ethene \(\Delta S^{0}\) is \(-31\) e.u. Explain. $$ \begin{array}{ll} \mathrm{CH}_{3}-\mathrm{CH}_{3}+\mathrm{Cl}_{2} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl}+\mathrm{HCl} & \Delta S^{0}=+0.5 \mathrm{e.u} \\\ \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{HCl} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Cl} & \Delta S^{0}=-31 \mathrm{e.u} \end{array} $$

The heat of combustion of cyclopropane, \(\left(\mathrm{CH}_{2}\right)_{3}\), to give carbon dioxide and liquid water is \(499.8 \mathrm{kcal} \mathrm{mol}^{-1}\). Show how this value, assuming normal \(\mathrm{C}-\mathrm{H}\) bond strengths, can be used to calculate the average \(\mathrm{C}-\mathrm{C}\) bond energy of cyclopropane.

Explain why the product distribution in the chlorination of propane by sulfuryl chloride is expected to differ according to whether the hydrogen- abstraction step is accomplished by \(\mathrm{Cl} \cdot\) or \(\cdot \mathrm{SO}_{2} \mathrm{Cl}\)

tert-Butyl hypobromite is a radical brominating agent that is similar to tert- butyl hypochlorite (Exercise 4 \(18^{*}\) ), but is less easily prepared than the hypochlorite. A good substitute, provided radical bromination is possible, is a mixture of \(\mathrm{BrCCl}_{3}\) and \(\mathrm{ce}\left(\mathrm{CH}_{3}\right)_{3} \mathrm{COCl}\) Thus, bromination of cyclohexene results if a high ratio of bromotrichloromethane to hypochlorite is used. O=C(O)CCC1CCCCC1Br Suggest how this reaction is initiated and propagated, and explain why it is necessary to have an excess of bromotrichloromethane.