EXAM 2

CHEMISTRY 220a

Friday, October 16, 2009

 

NAME (print): ________________________________________________________

 

TA:_________________ Sect. Day:_________________ Sect. Time:_____________

 

 

Take a few moments to look over the exam.  Answer each question on the exam paper.

 

No calculators.  You may use molecular models.  Important clues and structures are in bold.

 

 Do all preliminary drawing or computations on the work sheets at the end of the exam. The work sheets will not be graded.

 

The exam is 55 minutes.

 

STOP writing and hand in your exam when you are asked to do so.

 

Remember: Neatness is to your advantage.

 

 

1. (20 pts) Thermochemistry                           ______

 

 

2.  (20 pts) Optical Activity                             ______

                                               

 

3.  (20 pts) Stereochemistry                            ______

                                               

 

4.  (20 pts) Reactions 1  (Do 3 of 4)               ______

 

 

5.  (20 pts) Reactions 2  (Do 3 of 4)               ______

 

___________________________________________

 

Total (100 pts)

 

1)         Thermochemistry: (20 pts.)  Problem 1 in the alkane module of ORGO (a reading assignment on PS3) dealt with the free radical chlorination of 2,2-dimethylpropane (neopentane) to form 1-chloro-2,2-dimethylpropane.

 

a)  (8 pts.) Provide the two propagation steps and the overall reaction for this process.  Place the reactants and products in the appropriate boxes.   

 

 

 

 

b) (6 pts.)  Determine the heat of reaction for each of the propagation steps and for the overall reaction given the typical BDEŐs in the BDE Table (page 8).  Place the values on the appropriate lines under the boxes and at end of each reaction.  Show any calculations below.

 

 

 

 

 

 

 

c)  ( 6 pts.)  Using HessŐs Law and the overall reaction, determine the heat of formation of 1-chloro-2,2-dimethylpropane given: DHfo (neopentane) = -40 kcal/mol; DHfo (HCl) =

 -22 kcal/mol.  Show work below..

 


 

2) Optical Activity: (20 pts.)   a-Pinene (1) isolated from European pines is principally levorotatory, whereas North American pines afford mainly the dextrorotatory enantiomer.

 

a)     (8 pts.)  An 80/20 mixture of a-pinene from European sources shows [a] = -30o.  What is the rotation of the pure, major enantiomer of European a-pinene?  Show work.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

b)    (8 pts.)  Using the results of 1a, what is the percentage composition of a sample of a-pinene of North American origin that displays [a] = +40o? Show work.

 

 

 

 

 

 

 

 

 

 

 

 

c) (4 pts.)  Is a-pinene (1) dextro- or levorotatory? Explain briefly.


3)    Stereochemistry:  (20 pts.)  The fat-soluble, optically-active anti-oxidant vitamin E (a-tocopherol) is shown below.

 

a)     (8 pts.)  How many stereoisomers of vitamin E are there (including the one shown.  Explain briefly.

 

 

 

 

 

 

 

 

 

b)    (8 pts.)  Provide the R/S-configuration for each of the carbons that require them.  [Enter your configurations on the structure.]

 

 

 

 

 

 

 

 

c) (4 pts.)  Nature Made sells vitamin E (d,l-a-tocopherol) and vitamin E natural (d- a -tocopherol).  What might be the sources of these two a –tocopherols?


 

4)  Reactions 1: (20 pts.; equal weight) Provide the product in 3 of 4 of the following reactions. If you do more than three, cross out the one that you do not want graded.  Pay attention to stereochemistry!

 


5.  Reactions 2:  (20 pts.; equal weight)  Complete 3 of 4 of the following problems.  If you do more than three, cross out the one that you do not want graded.

 

a)  Of the two optically-active bromides shown below, circle the one that will have the higher SN2/E2 ratio when treated with sodium methoxide in methanol.  Give a brief rationale.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

b)  Circle the choride that will undergo E2 elimination at a faster rate.  Give a brief rationale.

 

 

 

 

 

 

 

 

 

 

 

 

 

Écontinued

 

 

c)  Circle the tosylate that will undergo E2 elimination slower with aqueous KOH. Give a brief rationale.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

d)  Is path A or B the more efficient way to prepare cyclohexylmethyl ether? Give a brief rationale.

 

 

 

 

 

 



Work Sheets
Work Sheets
Work Sheets