Chemistry 25a

Practice First Hour Exam

150 points total, 50 minutes duration.

Fill in the answers directly on to the question sheet.

1. (10 points) Give a systematic name for both of the following compounds.

2. (10 points) Draw an MO diagram for the C=N bond in H-C=N, showing where the electrons lie.

3. (20 points) (a) Show all major resonance forms for phenoxide ion:

(b) Draw the structure of the conjugate acid of phenoxide ion.

(c) What functional groups are present in your answer to (b)?

4. (30 points) In the free radical chlorination of 2-methylpropane there are two possible products, as shown:

The energy diagram for the reaction is shown:

(a) Show the structures of the two intermediates, IntA and IntB

NOTE: FOR THE SUMMER 2003 TEST THE REMAINING PARTS WOULD NOT BE ON THE FIRST TEST.

(b) Show the activation energy for each reaction, by drawing it on to the above diagram.

(c) Calculate a numerical value for the overall DH for both reaction A and reaction B, given the following homolytic bond dissociation energies (given in kcal/mol). (CH3)2CCH2-H (primary C-H) 98 < > (CH3)2CCH2-Cl (primary C-Cl) 81
(CH3)3C-H (tertiary C-H) 91 < > (CH3)3C-Cl (tertiary C-Cl) 79
H-Cl 103

4. (contd.) (e) Which is the more stable product? Which product forms more quickly?

5. (20 points) Draw an energy curve for the flipping of cyclohexane from one chair to another. Show which minima and maxima correspond to the chairs, half-chairs, twist boats and boat (you need not draw these, merely label them). Which step in the flip is the slowest step?

6. (30 points) Circle the more stable of each of the following pairs.

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