Addition of hydrogen chloride to unsaturated hydrocarbons can be represented by the…

CHEMISTRY · Multi-part · A Level · Chemistry · Energetics · Energetics

Problem

Addition of hydrogen chloride to unsaturated hydrocarbons can be represented by the following reactions carried out at the same temperature and pressure. Reaction 1: C2H4  +  HCl  ⟶  C2H5Cl\mathrm{C_2H_4\; +\; HCl\; \longrightarrow\; C_2H_5Cl} Reaction 2: C2H2  +  HCl  ⟶  C2H3Cl\mathrm{C_2H_2\; +\; HCl\; \longrightarrow\; C_2H_3Cl} The average bond energies (kJ mol−1^{-1}) relevant to these reactions are listed.
BondEavgE_{\text{avg}}
C–C348
C=C614
C≡C839
C–H413
H–H432
H–Cl431
C–Cl327
Assume that the product of Reaction 2 contains one C=C bond and that no rearrangements occur. (a) Using the data provided, calculate the enthalpy change, ΔH1\Delta H_1, for Reaction 1. (b) Using the data provided, calculate the enthalpy change, ΔH2\Delta H_2, for Reaction 2. (c) Compare ΔH1\Delta H_1 and ΔH2\Delta H_2. Which reaction is thermodynamically more favorable? Support your answer quantitatively. (d) Explain how differences in initial bond order and carbon hybridization in the reactants account for the disparity between ΔH1\Delta H_1 and ΔH2\Delta H_2. Focus your discussion on orbital overlap and percent ss character.

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