EXAM 2
CHEMISTRY 220
Friday, October 18, 1996

NAME (print): ___________________________________________________

TA:_____________________    Day:_________________ Time:__________

Take a few moments to look over the exam.  Answer each question on the exam paper.  Do all 
preliminary drawing or computation in a blue book.  Blue books will not be collected.  The exam 
is 55 minutes. The points should be an indication of the time required for solution.  STOP writing 
when you are asked to do so.  REMEMBER: Neatness is to your advantage.

1. (15 pts)	______


2.  (24 pts)	______


3.  (12 pts)	______


4.  (12 pts)	______


5.  (24 pts)	______


6.  (13 pts)	______


7.  (10 pts) BONUS		______
________________________________


TOTAL (100 pts)  		______

1.	(15 pts)  In the free radical chain chlorination of ethane the reaction in eq. 1 occurs.  The 
first propagation step has DHo = -3 kcal/mol while the second propagation step has DHo = 
-29 kcal/mol.  Alternatively, one might have imagined the reaction of ethane with chlorine 
giving 2 moles of methyl chloride by a free radical chain chlorination (eq. 2).  However, 
this process does not occur.  The following bond dissociation energies will be of use.    
DHo (kcal/mol): Cl2 = 59; CH3-Cl = 85; CH3-CH3 = 90.

		C2H6  +  Cl2 --->  C2H5Cl  +  HCl			(eq. 1)
		C2H6  +  Cl2 --->  2 CH3Cl				(eq. 2)

a)	(6 pts)  Write the two propagation steps that sum to eq. 2.  [Propagation step 1 
employs a chlorine atom generated in the initiation step.]

	step 1 --->



	step 2 --->

b)	(6 pts) Compute (show work) for the exo/endothermicity [DHo (kcal/mol)] of each 
propagation step and for the overall reaction of part (a).
	

	step 1 --->



	step 2 --->



	reaction --->


c)	(3 pts)  Explain briefly why eq. 1 is favored over eq. 2.

2.	(24 pts)  Citronellal A, an unsaturated aldehyde, (C10H18O; [a]D25 = + 11.5o) is a 
volatile component of lemon grass; it is used as a natural insect repellent.
a)	(3 pts)  How many rings and/or double bonds are in A? [Show calculation]















b)	(6 pts)  The reaction of A with ozone followed by dimethyl sulfide reduction gives 
equal amounts of 2-propanone (acetone) B and the seven carbon dialdehyde C, 
(R)-3-methylhexandial (i.e, ...hexane-di-aldehyde).  Draw the structures of B and 
C?  Draw them accurately and label them B and C.  Show the correct configuration 
for C.

c).	(6 pts)  Draw accurately the two possible structures for A based upon the 
information in part (b).  [Hint:  One of the aldehyde groups was formed from 
ozonolysis; the other one was present in A originally.]













d)	(6 pts)  Hydrogenation of A gives the aldehyde (R)-3,7-dimethyl-octanal D 
(C10H20O, octane aldehyde).  Draw the structure of D.   How does the 
hydrogenation experiment distinguish between the two possible structures of A in 
part (c)?















e)	(3 pts) Circle the correct structure of A in part (c).

3.	(4 x 3 pts = 12 pts)  What is the relationship of the following pairs of compound to one 
another?

	                                                          

4.	(2 x 6 pts = 12 pts)  a-Pinene isolated from European pine trees is principally levorotatory, 
whereas North American pines afford mainly the dextrorotatory enantiomer.
	

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

















b)  Using the result of part (a), what is the percent composition (purity) of a sample of a-
pinene of North American origin that displays [a]D = +40o.  Show work.

5.	(8 x 3 pts = 24 pts)  Place the product of each of the following reactions of 2-ethyl-1-
butene at the end of the appropriate arrow.


				








			                                     

6.	(13 pts)  For the reaction co-ordinate shown below, identify the following:

a)  (3 pts)  An intermediate (by letter).


b)  (3 pts)  The rate limiting step, DG­. (illustrate in the diagram)

 
c)   (3 pts)  DGo for the reaction. (illustrate in the diagram)
]


d)  (2 pts)  Is DGo for the overall reaction positive or negative?



e)  (2 pts)  Is the overall reaction endothermic or exothermic? 



				                                   

7.	(10 pts) BONUS QUESTION:  Remarks overheard being made to a 
customer by a waitperson in a restaurant last Saturday morning in 
Ogunquit, Maine.  "I spent last year studying in Italy.  There is something 
different about the pasta there.  I can actually distinguish one type of pasta 
from another by taste."  Hmm.... I think facetiously.  Perhaps this person 
can distinguish between fusilli and rotelle but cannot distinguish among 
spaghetti, linguine and/or ziti.  Explain.