HW12Key - Assume that the absolute energy associated with...

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Graded Homework Problem #12 – Answer Key Deadline : 3:00 p.m., Thursday, 3/29/07 LATE WORK WILL NOT BE ACCEPTED OR GRADED!!! This problem is worth a total of 20 raw points. When methyl bromide (CH 3 Br) is exposed to sodium hydroxide (NaOH), a reaction occurs which produces methanol (CH 3 OH) and sodium bromide (NaBr). The mechanism of this transformation involves a single step, which is shown below. Please note that the sodium cation is not shown in the mechanism, since it does not play a role in the reaction! HO C Br H H H C HO H H H Br Use the bond dissociation energies in Appendix 3 of your textbook to estimate Δ H for this reaction. Please look at the numbers in parentheses, which give the bond dissociation energies in units of kcal/mol . Write your answer in the box below. Next to the box, indicate whether this reaction is endothermic or exothermic by circling the appropriate choice. Δ H = kcal/mol ENDOTHERMIC EXOTHERMIC Sketch a reaction energy diagram for this transformation, using the template below.
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Unformatted text preview: Assume that the absolute energy associated with the reactants is 0 kcal/mol. In your reaction energy diagram, label the positions of the reactants (as R ), the products (as P ), and the transition state ( ‡ ). Also label Δ H in your diagram. Three possible structures of the transition state are shown to the right of the reaction energy diagram template. Circle the structure which best represents the transition state, based on your estimate of Δ H and Hammond’s postulate. A dotted line represents a partial bond. Energy (kcal/mol) Reaction Progress 0 +20 – 20 – 40 HO C Br H H H H C H H Br HO C HO H H H Br Bond broken = H 3 C–Br (BDE = 72 kcal/mol) Bond formed = H 3 C–OH (BDE = 92 kcal/mol) –20 R P ‡ Δ H According to Hammond’s postulate, the transition state for an exothermic reaction step resembles reactants more than products....
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