Cyanomethane, commonly known as acetonitrile, CH₃CN, is a toxic volatile liquid which is used as a solvent to purify steroids and to extract fatty acids from fish oils. Acetonitrile can be synthesized from methyl isonitrile by the isomerization reaction CH₃NC(g) → CH₃CN(g).

(a) Draw the Lewis structures of methyl isonitrile and cyanomethane; assign a hybridization scheme to each C atom and indicate whether each molecule is polar or nonpolar.

(b) Estimate ΔH° for the isomerization reaction by using the mean bond enthalpies in Table 4E.3. Which isomer has the lower enthalpy of formation?

(c) The isomerization reaction obeys the first-order rate law, Rate = k_r[CH₃NC], in the presence of argon. The activation energy for this reaction is 161 kJ · mol^–1, and the rate constant at 500. K is 6.6 * 10^–4 s^–1. Calculate the rate constant at 300. K and the time (in seconds) needed for the concentration of CH₃NC to decrease to 75% of its initial value at 300. K.

(d) Draw a reaction profile for the isomerization reaction.

(e) Calculate the temperature at which the concentration of CH₃NC will decrease to 75% of its original concentration in 1.0 h.

(f) What purpose does the excess argon serve?

(g) At low concentrations of Ar, the rate is no longer first order in CH₃NC; suggest an explanation.

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TABLE 4E.3 Mean Bond Enthalpies Bond C-H C-C C=C CC* C-N C-F C-Cl C-Br AHB/(kJ.mol-) 412 348 612 518 837 360 743 305 484 338 276 Bond C-I N-H N-N N=N N-O N=O N-F N-Cl O-H 0-0 AHB/(kJ.mol-) 238 388 163 409 210. 630. 270. 200. 463 157