iPad 6:48 PM 70% csun.edu www.csun.edu/~hcchm007/333dps5.pdf Chegg.com Chemistry

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iPad 6:48 PM 70% csun.edu www.csun.edu/~hcchm007/333dps5.pdf Chegg.com Chemistry 333 Discussion Name: Problem Set 5 1. Use homolytic bond-dissociation enthalpies (kJ/mol) in Table 4-2 (p. 143 of Wade, Organic Chemistry, 8th edition) to calculate the standard heats of reaction for the following two (2) reactions. A. (H3C)3COH + HBr (H3C) CBr + H2O 2 H2O 2. Draw a labeled reaction-energy diagram (graph of potential energy versus reaction coordinate) fora three-step overall exothermic reaction with the: A. first step being the rate-determining step B. second step being the rate-determining step C. third step being the rate-determining step Label each graph with the following: reactants (r), transition states (ts,), intermediates (int,), products (p), activation energies (Ea, ), and overall standard heat of reaction (AH ). (Note: Each subscript, x, needs to be a number that refers to the appropriate primary mechanistic step. For example, Ea, is the label for the activation energy of the first step.) 0 3. Draw a labeled reaction-energy diagram for a two-step reaction with an early first transition state and a late second transition state. State whether the transition states resemble the starting material, intermediate, or product. Repeat the exercise for a two-step reaction with a late first transition state and an early second transition state, 4. The photochemical reaction of 2,4-dimethylpentane with molecular chlorine affords a mixture of monochlorinated alkyl halides. Draw the structures of these products and predict the molar ratio when the reaction is run at 25 °C. (Note: For these conditions, the relative rates of hydrogen atom abstraction are 5.5:4.5: 1.0 for 3°:2 °:1° alkane carbons.)

Explanation / Answer

Ho(reaction) = H(reactant bonds broken) - H(product bonds formed)

1.

(CH3)C-OH +HBr -à (CH3)C-Br +H2O

Ho(reaction) = H(C-O + H-Br) - H(C-Br + O-H)

Ho(reaction) = H(358 KJ/ mol + 366 KJ/ mol) – H (276 KJ/ mol + 463 KJ/ mol)

Ho(reaction) = -17 KJ/mol

1.

H2O2+H2-à 2H2O

Ho(reaction) = H(O-O + H-H) - H(2* O-H)

Ho(reaction) = H(146 KJ/ mol + 436 KJ/ mol) – H (2* 463 KJ/ mol)

Ho(reaction) = -344 KJ/mol

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