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54 Chapter 3 REACTIONS OF ALKANES Bromination graph: Iodination graph: 25 30 Prop step Prop step 2 20 25 15 20 10 15 E (kcal mol⁻¹) + 11.5 5 kcal E (kcal 10 +27.5 kcal 0 5 +7.5 kcal -5 -13.5 0 kcal -10 - -5 Prop step Prop step 2 15 -10 Reaction coordinate Reaction coordinate The main difference between the two halogenations is that the first propagation step for iodination is so much more endothermic that the overall reaction becomes endothermic as well. 32. Although Br is sterically smaller than CH₃, the polar C-Br bond gives rise to regions of partial posi- tive and negative charges in the molecule. As the structures below show, in both the gauche and anti confor- mations, the distances between the positive ends of the two C-Br bond dipoles are similar. In the gauche conformation, however, the negative ends of these dipoles are much closer to each other than is the case in the anti form. The repulsion between these two relatively high concentrations of negative charge located near the Br atoms raises the energy of the gauche form above what it would be on the basis of sterics alone. Therefore, the anti form is favored by more than would be the case otherwise. Br Br H H H Br H H H H Br H 33. (a) CH₄ + 2 O₂ CO₂ + 2 H₂O (b) C₃H₈ + 3 CO₂ + 4 H₂O (c) C₆H₁₂ + 9 O₂ 6 CO₂ + 6 (d) C₂H₆O + 3 O₂ 2 + 3 (e) C₁₂H₂₂O₁₁ + 12 O₂ 12 CO₂ + 11 34. (a) C₃H₆O + 4 O₂ 3 CO2 + 3 (b) The energy difference is the difference between the heats of combustion, 6.2 kcal mol⁻ Acetone, whose combustion releases less heat, must have had the lower energy content to begin with. (c) Acetone, with a lower energy content, is the more stable compound. 35. You need to propose a bond-breaking process for sulfuryl chloride, SO₂Cl₂, which will accomplish the same goal as breaking the bond in Cl₂. There is really only one option: a sulfur-chlorine bond. Initiation: :0: :0: :CI + Cl: :0: :0: