Problem Set 3 Chem 220 Fall 2003

1. Draw each structure and classify each hydrogen as primary, secondary, or tertiary.

(a) methylcyclopentane

(b) 2,3-dimethylbutane

(c) 3-methyloctane

(d) 2,2-dimethylpentane

2. The chlorination of 2-methylbutane gives a mixture of four monochlorinated products.

(a) Draw 2-methylbutane and label the four possible sites for cholrination.

(b) If the combined yield of the two primary halide products 41%, the secondary halide is formed in 36%, and the tertiary halide in 23%, compute the relative reactivity. (Relative reactivity primary:secondary:tertiary)

3. For each of the following compounds, predict (draw) the major product of free-radical bromination.

(a) methylcyclopentane

(b) 2,3-dimethylbutane

(c) 3-methyloctane

(d) 2,2-dimethylpentane

4. Calculate DH⁰ values for the following reactions from bond energies in your book:

(a) H₂ + F₂ ® 2HF

(b) H₂ + I₂ ® 2HI

(c) (CH₃)₃CH + Br₂ ® (CH₃)₃CBr + HBr

(d) (CH₃)₃CH + Cl₂ ® (CH₃)₃CCl + HCl

5. Write a mechanism for the radical bromination of benzene (C₆H₆) (Label the initiation and propagation steps). Calculate DH° values for each step and for the overall reaction. How does this reaction compare thermodynamically with the bromination of other hydrocarbons? (Bond dissociation energies: C₆H₅-H 111 kcal/mol, C₆H₅-Br 81 kcal/mol)

6. Calculate the DG° at 25°C for the following reaction:

CH₃CH₂CH₂CH=CH₂ ® CH₂=CH₂ + CH₃CH=CH₂ DH° = 22.4 kcal/mol DS° = 33.3 e.u.

What is the effect of raising temperature on DG°? What is the temperature at which the reaction becomes favorable? Show units.

7. Draw a reaction profile (potential energy diagram) to represent each of the following situations: Label reactants, products, and activation energy.

(a) an exothermic reaction with a small activation barrier

(b) an exothermic reaction with a large activation barrier

(c) an endothermic reaction with a small activation barrier