316 Chem F03
Page 1 of 5
Kantorowski
Exam III
5 November 2003
Key
Name: ___________________________________________
This exam contains 5 pages of questions confirm this once you begin. You will have 50 minutes. No models or calculators are permitted. Read all questions carefully answer the question that is asked! Illegible or indecipherable answers may not receive potential partial credit. Good luck!
1.
(10 pts) Draw or name the following compounds using IUPAC nomenclature as appropriate. b) (S)-3-methoxy-2-methylpentane
a) (S)-2-butanol
OH
OCH 3
c)
1
OH
7 4
(4R,7S)-7-chlorooctan-4-ol
or
(4R,7S)-7-chloro-4-octanol
Cl CH2 CH3 H H CH3 Br H
number the chain so that the alcohol has the lower number
d)
(S)-2-bromopentane
2. (6 pts) Identify, using an asterisk (*), all stereocenters in the following compounds.
OH * O * HO *
OH Br *
(none)
Chem 316 F03
Page 2
Initials________
3. (3 pts) What is the percent enantiomeric excess (% ee) of mixture that is one-fourth (R)-2decanol and three-fourths (S)-2-decanol? 75% (S) 25% (R) = 50% ee of the (S) stereoisomer 4. (12 pts) Answer the questions that follow for the reaction coordinate diagram shown.
= RDS = Ea I Ea DH DH P DH (overall) SM
E
reaction coordinate
a) Accurately indicate on the diagram the following features: starting material (SM), intermediates (I), products (P), transition states (), rate-determining step (RDS), enthalpies (DH), activation energies (Ea). b) Is the reaction exothermic, endothermic, or thermally neutral? __endothermic (DH > 0)_______ c) Is the transition state that leads from intermediate to the product early or late? __early_____ d) ...this implies that the transition state structure closely resembles the _intermediate_(Hammond) 5. (10 pts) Consider the chlorination of n-pentane. Draw all of the possible mono-chlorination products paying attention to stereochemistry when applicable. Unrecognized duplicate structures will detract from full credit.
Cl 2 hn
n-pentane
Cl
Cl
Cl Cl
Only four monochlorination products are possible. The first and last structures are achiral; the middle two structures are enantiomers.
Chem 316 F03
Page 3
Initials________
6. (18 pts) Clearly circle the correct answer for the following questions. There is only one correct answer for each; no credit will be given if more than one answer is circled for each question. a) ______________ of cyclopentane is endothermic. Iodination Chlorination Bromination Fluorination
b) Bond-dissociation energy (BDE) values apply to ___________ bond cleavage. heterolytic hydrolytic homolytic
c) The entropy term in the Gibbs free energy relationship is: measured in debye units (D) usually small at ordinary temperatures always positive
d) Identify the most stable carbanion.
e) Identify the most stable carbocation.
2, no resonance
Resonance shows 2 and 3 character
Resonance shows 2 and 2 character
f) Which of the following not would be a good choice if a selective chlorination were desired?
g) Which of the following steps is ignored when calculating DH for free-radical bromination of methane?
H3 C Br Br Br Br H3 C H Br
h) Carbenes are _________ hybridized and have _______________ geometry. sp , tetrahedral i) Carbocations are: nucleophilic electrophilic Lewis bases
3
sp , tetrahedral
2
sp , trigonal planar
3
sp , trigonal planar
2
Chem 316 F03
Page 4
Initials________
7. (8 pts) When you get scared, your body releases the cardiostimulant hormone, (R)-epinephrine (adrenaline) to prepare your body for action ("fight or flight") regardless of the choice. (a) Draw the (R) stereoisomer. (b) Does the nitrogen meet the criteria to be a chirality center? (c) Briefly explain why N can or cannot be assigned an absolute configuration.
OH HO HO H N OH HO CH3 HO H N CH3
The N qualifies as a potential chirality center as it is bonded to four different groups (-H, -CH3, CH2-R, and a lone pair). However, the rapid inversion of N's lone pair prevents it from maintaining any particular conformation. That is, N is conformationally dynamic and an absolute configuration cannot be assigned. 8. (8 pts) Predict the major mono-bromination product.
Br2 D Br Br Br2 hn
via the resonance-stabilized 2 radical:
9. (10 pts) From the group of compounds below identify a pair of enantiomers, a pair of diastereomers, and a meso compound. Be sure you write your answers in the provided boxes.
H Cl A Cl H H Cl B H Cl H Cl C D Cl H Cl H H Br
Cl H H Cl E H Cl F
Br H
Cl H
Cl H
Cl H H Cl
G
H
Enantiomers: __B__ and __G__ (or B and H) Diastereomers: __A__ and __G__ (or E and G) A meso compound: __C___ Note: A & E are the same compound G & H are the same compound
Chem 316 F03
Page 5
Initials________
10. (5 pts) The following pentacoordinate phosphorous compound has three coplanar substituents (referred to as equitorial) and two substituents colinear (referrred to as axial) with the P atom. Is this compound chiral? Briefly explain your decision.
Cl Cl Br Cl OCH3 OCH3 P This structure (the mirror image) is not superimposable on the first structure, therefore these structures are enantiomers. These compounds are chiral.
axial groups
equitorial groups
Br CH3 O mirror P Cl OCH3
11. (6 pts) What are the three steps involved in the free-radical chain reaction? 1. Initiation 2. Propagation 3. Termination 12. (4 pts) How many stereoisomers are possible for a compound containing 4 stereocenters? What feature in a molecule might reduce this number? 4 stereocenters (n = 2)...so, 2 = 2 = 16 possible stereoisomers If an internal mirror plane (s) is present a meso compound would exist among the set of stereoisomers. This renders (at least) two of the stereoisomers equivalent. Therefore, this will reduce the total number of possible stereoisomers.
n 4
Page 1 2 3 4 5 Total
Score
Possible 16 25 18 26 15 100