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Chemistry 303bfall, 1999

SECOND EXAMINATION

7:30 PM, NOVEMBER 22TH, 1999

Duration: 2.5 hr

Name____________________________________________________________

Lab TA__________________________________________________________(if you do not know his/her name, give day of lab section. NOT Hooley nor Chlenov)

This is an "open book" examination; you may use anything which is not alive.

Note: if you do not know the complete or specific answer, give a partial or general answer--

WRITE SOMETHING

Write only in the space provided for each question.

Score:

1___________/16

2___________/18

3___________/27

4___________/18

5___________/21

Total: /100

There are 10 pages in this exam, followed by three pages of spectra. All 13C NMR spectra are "proton-decoupled", as usual. Please check now to be sure you have a complete set.

The last three pages of the exam are Spectra and Spectral data for three molecules, X , Y, and Z. You may detachthese sheets and leave them separate from the exam.

If you are using a resonance argument in your answer, draw the relevant resonance structures.

If you are asked to analyze a structure and you have no idea what it is, do a general analysis of the data for partialcredit.

Please be aware that a small number of students will be taking the exam at different times up until the evening onTuesday. It would be well not to discuss the exam until after that time.

PLEDGE:_________________________________________________________________

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I. (16 pts) Consider the isomers, F, G, H . Think about how mass spec, UV, IR, or NMR could differen-  tiate them. Respond to the questions below, and give the single most clear-cut feature for each differentiation.

OH   OH O

F   G  H

 A. (2 pts)  Would IR allow you to differentiate F  from G,  assuming you do not have comparison spectra for each compound?

_________________________________________________________________________________________ B. (2 pts) Would IR allow you to differentiate G  from H, assuming you do not have comparison spectra for 

each compound?

_________________________________________________________________________________________C. (2 pts)Would Mass Spec allow you to differentiate F  from G?

_________________________________________________________________________________________ D. (3 pts) Would 1 H NMR allow you to differentiate F  from G?

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  continued next page E. (3 pts) Would 1 H NMR allow you to differentiate G from H ?

_________________________________________________________________________________________ F.   (2 pts) Would 13C NMR allow you to differentiate  F  from H.

_________________________________________________________________________________________G.   (2 pts) Would 13C NMR allow you to differentiate G from H.

II. (18 pts). Consider the 1H NMR and 13C NMR spectra for dichlorotetrafluoroacetone, Z (p 12). To analyze  the 13C NMR spectrum, it may be useful to look at the structure and generate a possible spectrum and then  look at the actual spectrum to guide a final correlation. Recall the following nuclear spins: 1H, 13C, 19F = 1/2  12C, 16O = 0. [35Cl and 37Cl are special but behave as though they have zero spin with regard to coupling.]A. (2 pts)  Is the 1 H NMR spectrum consistent with the structure? Explain.

B . (3 pts) Would you predict that the C=O stretching frequency in the IR spectrum of Z to be higher, lower,or the same frequency as that for acetone?  Explain carefully.

C. (1 pt) How many non-equivalent carbons are there in this structure? 1 2 3 4 other(circle single correct answer)

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D. (2 pts)  How do you explain the peak at 0 ppm in the 13C NMR spectrum?

  continued next pageE. (2 pts)  How do you explain the family of peaks at 78 ppm in the 13C NMR spectrum?

F. (4 pts)  How do you account for the peaks between 110 and 125 ppm in the 13C NMR spectrum? Assign to one or more carbons and explain the pattern (relative intensities and peak separations).

G. (4 pts)  How do you account for the peaks between 170 and 180 ppm? Assign to one or more carbonsand explain the pattern (relative intensities and peak separations).

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III.  (27 pts) Consider the spectral data for molecule X on p 13.A. (7 pts)  From the 4000-1500 cm-1 region of the infrared spectrum alone, which of the following

  functional groups are ruled out? (circle those which are ruled out, and explain in one sentence how you  came to that conclusion.):

carbonyl group (C=O)

_____________________________________________________________________

alcohol (-OH)

_____________________________________________________________________

amino (-NH2)

_____________________________________________________________________

cyano (-CN)

_____________________________________________________________________

aldehyde (RCHO)

_____________________________________________________________________

simple alkenes (consider each case separately)

 

H

H R

H

H

H R

R

R

H R

H

R

H R

R

R

R R

R

  B. (4 pts). From the mass spectral data, what is the molecular weight of the compound? ______

  Give an estimate of the number of carbon atoms. ______

  What common elements are ruled out by the mass spec data alone (cross out your choices): and explain

in one sentence: H? N? O? F? Cl? Br? I?

  C. (2 pts).  From the UV data alone, which of the following functional groups are ruled out? Crossout your choices.

 

R OR RR R C NOR

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D. (2 pts)  How many non-equivalent carbon atoms are indicated in the C-13 spectrum? _______  cont...

E. (6 pts) For each set of equivalent protons, give the 1 H   NMR chemical shift, the relative area, and describe the splitting pattern (singlet, doublet, triplet, quartet, quintet, sextet, etc) including approximatecoupling constants. You should estimate the J values as best you can.

  Chemical Shift Rel. Area Splitting pattern Approx. J .

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

_________________________________________________________________________________

F. (6 pts)   Draw the structure for X which best fits all the data (review your IR conclusions)1. Explain in detail how your structure fits the pattern of peaks (splitting pattern) in the 1 H NMR

spectrum. Label the groups of equivalent H in your structure for reference. You need not show calculationof chemical shift positions, but you might do this to confirm your structure.

2. Explain how your structure correlates with the 13C NMR spectral data. Indicate ambiguousassignments.

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  continued next page3. Explain how your structure fits the IR spectrum, based on the peaks from 4000-1500  cm-1.

IV. (18 pts) Consider the 300 MHz 1H NMR spectrum for molecule Y shown in its entirety on p.11 , and thelist of structures, A-D.   A.

(9 pts)Take each structure in turn (except for the correct structure) and point out the most clear-cut

inconsistency between the structure and the spectrum for each of the "wrong" structures. Identify thecorrect structure, and proceed to part  B , next page. It is in your interest to show your work, such as thecalculation of approx. chemical shifts from the additivity tables, if you use them.

HC CH

Cl

CH2

CH2 OH2 C

CH2

OH2 C

CH2

OH2 C

H2 C S

CH2

O   ClH2 CO

ClH2 CO

O

ClCH2

O O

CH2

O

CH2

CH2   CH2

Structures:

B. C.D.

A

__________________________________________________________________________________________

Structure A:

__________________________________________________________________________________________Structure B:

__________________________________________________________________________________________Structure C:

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  continued next page

Structure D:

___________________________________________________________________________B. (9 pts)  1 . Redraw the correct structure here.  Label each carbon ( a,b,c...).

2. Correlate the 1 H NMR spectrum with the structure.a. Give the chemical shift and area of each group of equivalent protons. Calculate the

 predicted chemical shift for each group by using the Additivity Table for group effects on chemical shifts andshow your work.

b. Give the pattern (singlet, doublet, triplet, quartet, quintet, sextet, etc) and approximatecoupling constant for each splitting pattern.  Explain the splitting patterns.

3. Correlate the 13C NMR spectrum with the structure as far as you can. Not all peaks can beassigned unambiguously.

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V. (21 pts)  A.  (3 pts)  Draw the most stable and the least stable chair conformers of cis-1,3-dichlorocyclohexane onthe  templates here. Be sure to draw the bonds carefully. Label each chlorine substituent as to whether it isaxial or   equatorial. Indicate which is more stable.

Cl

Cl

B .   (3 pts)  Describe two different factors which are responsible for the higher energy of the less stableisomer.

C. (4 pts) In the equilibrium between the chair forms of substituted cyclohexanes, one can calculate the energydifference, ∆G. For cyclohexanol, the ∆G value is solvent dependent.  Explain these data. First discuss theenergy difference in benzene and then compare that with the energy difference in water-MeOH 

OH

OH

Keq   ∆Gwater-MeOH: 1.25 kcal/mol  benzene: 0.85 kcal/mol

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  continued next page

D. (5 pts) For cis-1,3-dihydroxycyclohexane, the ∆G values for the two chair isomers are shown.

OH

OH

∆Gwater-MeOH: 3.85 kcal/mol  benzene: 0.65 kcal/mol

1. Explain why the value in water-MeOH is larger compared to that for cyclohexanol in water- MeOH. Note that the value is more than twice that for cyclohexanol. Use pictures; draw both chair forms, etc.

2. Explain why the value in benzene is smaller compared to that for cyclohexanol in water-MeOH.Use pictures.

E. (5 pts) Analyze the stereochemistry of cis-1-chloro-3-fluorocyclohexane. Draw the most stable chair 

 form here. Label  all stereogenic carbons in your structure as R  or S. Draw the mirror image of your structure.

 Is the mirror image an: enantiomer, diastereomer, superimposable meso cannot tell(circle correct ans)

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 F. (1 pts) What will be the optical rotation for your mirror image structure in part E?

(circle single correct answer) (+) (-) zero cannot tell

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