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Chemistry 303fall, 1996

SECOND EXAMINATION

7:30 PM, NOVEMBER 5TH, 1996

Duration: 2 hr

Name____________________________________________________________

Lab TA___________________________________________________________(if you do not know his/her name, give day of lab section)

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___________/35

2___________/14

3___________/27

4___________/07

5___________/07

6.__________/10

Total: /100

There are 7 pages in this exam; please check now to be sure you have a complete set.

There is also a Data Sheet for Molecule X handed out separately.

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

PLEDGE:_________________________________________________________________

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I. ( 20 pts)  A.  (6 pts) Consider the two molecules below. Predict the NMR chemical shift and splitting pattern foeach proton. Show your calculations. (s = singlet, d = doublet, t = triplet, q = quartet)

Ha   C

Hb

Hc

Cl  Ha   C

Cl

Cl

Cl

chemicalshift (ppm)

pattern(circle)

chemicalshift (ppm)

Ha_____ s,d,t,q,otherHb_____ s,d,t,q,otherHc_____ s,d,t,q,other

Ha_____ s,d,t,q,other

pattern(circle)

Calculations:

B. (3 pts) It should be clear that there is a dependence of the chemical shift position on the number of Clsubstituents. Considering our usual parameters of resonance, inductive, hybridization, and steric effects, what d you think is the most important parameter in determining the change in chemical shift for these compounds Explain.

C. Consider the molecule, cis-1-ethoxypropene, A

 

C C

C O C

C

H

H H

H H

H

H

H

HH

1H NMR data: ! 1.4 ppm, 3H, triplet, J = 7 Hz

  ! 1.9 ppm, 3H, doublet, J = 7 Hz

  ! 3.8 ppm, 2H, quartet, J = 7 Hz

  ! 4.5 ppm, 1H, quintet, J = 7 Hz

! 5.9 ppm, 1H, doublet, J = 7 Hz

13C NMR data: !  9.5, 15.5, 67.0, 101.0, 144.0 ppm A

1. (15 pts) Correlate the structure with the 1 H NMR data:

  a. Show clearly (by labeling as H a on the structure) the H which show(s) up at 1.4, and explain here

why it (they) appear(s) as a triplet.

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  b. Show clearly (by labeling as H b on the structure) the H which show(s) up at 1.9 ppm, and explain

why it (they) appear(s) as a doublet. Use the chemical shift correlation tables to justify your assignment.

  c. Show clearly (by labeling as H c on the structure) the H which show(s) up at 3.8 ppm, and explain

why it (they) appear as a quartet.

  d. Show clearly by (labeling as H d on the structure) the H which show(s) up at 4.5 ppm, and explain

why it (they) appear(s) as a quintet.

  e. Show clearly (by labeling as H e on the structure) the H which show(s) up at 5.9 ppm, and explain

why it (they) appear(s) as a doublet.

2. (4 pts)   Note that a simple alkene, such as cis-2-butene, shows NMR peaks at 1.85 (d, 6H) and 5.4

(q, 2H). Considering our usual parameters of resonance, inductive, hybridization, and steric effects, what d

 you think is the most important parameter in determining the difference in chemical shift between the H a

 4.5 ppm and the H at 5.9 ppm in molecule A? Explain.

3. (4 pts)   Correlate the structure of A with the 13C NMR data: write the chemical shift number next tothe appropriate carbon. It may be helpful to keep in mind your analysis in #2, above. Explain anyambiguity.

OA

 

 4. (3 pts)  How would the 1 H NMR spectrum of the trans isomer  B

differ from that for the cis isomer, A, above?O

B

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III.  Consider the spectral data for molecule X on the Data Sheet separate from the exam.

A. (5 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)

  benzene ring

  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

 alkynes: R H R R

  B. (3 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. (3 pts). From the UV data alone, which of the following functional groups are ruled out? Cross out  your choices and explain in a few words.

R OR RR R C NOR

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  D. (4 pts)   How many non-equivalent carbon atoms are indicated in the C-13 spectrum?

  NOTE: one peak falls off to the left of the normal spectral range; it is indicated on the 13C spectrum on aseparate line (above) and the instructions: "offset: 40 ppm". You must add 40 ppm to the peak position of thispeak to get the correct shift.

  E. (6 pts)  For each set of equivalent protons, give the 1 H   NMR chemical shift, the relative area, anddescribe the splitting pattern including approximate coupling constants. You should estimate the J values abest you can and indicate whether each is in the range 0-3 Hz, 4-10 Hz, 11-16 Hz, or over 16 Hz.

F. (6 pts)  Draw the structure which best fits all the data (review your IR conclusions) and then :

1. Explain in detail how your structure fits the pattern of peaks (splitting pattern) and the couplingconstants.

2. Explain how your structure correlates with the 13C NMR spectral data.3. Explain how your structure fits the IR spectrum. Mention the two most important correlations.

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VI. (10 pts)  Consider the isomers, F, G, H . Think about how UV, IR, or NMR could differentiate them.

Respond to the questions below, and give the single most clear-cut feature for each differentiation.

O   O   O

F G H

 A. Would UV allow you to differentiate F  from G?

 B. Would UV allow you to differentiate G  from H ?

 C. Would IR allow you to differentiate F  from G?

  D. Would IR allow you to differentiate G  from H?

 E. Would 1 H NMR allow you to differentiate G from H ?

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