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Chemistry 303

Second Examination

November 14th, 1994

7:00 pm-9:00 pm

Name:_____________________________________________________________

Lab TA name___________________________________________________________

This is an open book test. You may use anything that is not alive.

Write all of your answers in the space beneath each question. If you mess up the assigned space, you maychoose another space of equivalent size (e.g., on the back of a page) but it is your obligation to make clear wherethe answer is you wish graded.

Turn in this exam and no other material at 9:00 pm.

READ EACH QUESTION CAREFULLY AND COMPLETELY. ANSWER ONLY WHAT IS ASKED!

THIS EXAM HAS SEVEN PAGES; Please check immediately to be sure you have a complete copy.

THERE IS ALSO A SET OF "DATA SHEETS", primarily spectral data for your reference (5 PAGES).DO NOT TURN IN THE DATA SHEETS

As always, there is opportunity for substantial partial credit without a complete answer--write something!

If you do not have an answer to an early part of the question, just proceed and answer the rest in general termsas best you can.

Good luck!Grade:

1__________

2__________

3__________

4__________

5__________

6._________

total score:

Pledge:_________________________________________________________________________________

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I. ( 25 pts). Using the empirical formula indicated at each part, propose one (perhaps different) structurewhich best fits the description for each of the following statements. No formal charge nor unpaired electronsallowed. In each case, explain your thinking in words and pictures (mechanism).

A. C6H12 alkene most reactive toward Br2.

B. C6H12 alkene with weakest (lowest intensity) C=C stretching frequency in the IR.

C. C5HnO (n = any integer) carbonyl compound with C=O stretch at lowest wavenumber.

D. C5HnO (n = any integer) most deshielded H (indicate which H in your structure).

E. C5HnO (n = any integer) !"!# absorption at longest wavelength.

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II. (32 pts). Consider the Data Sheets, pp. 2-5. These concern a laboratory problem in which a bottle of compound A was found without a label. The Orgo detective did a little chemistry and spectroscopy. She foundthat compound A reacted with ozone (reductive processing) to give two compounds (B and C) which wereseparated and their structures were determined by IR, NMR, and Mass spectra. CompoundA reacted withHCl in ether solution to give one compound, D.

D HCl B + C

b. reduction

a. O3A

C

A. Consider the spectral data and write the best structure for C.

1. Are the parent ion mass and relative intensity of theP+1 consistent with your structure? Explain.

2. Specify one feature of the IR spectrum for C which is especially consistent with the assignedstructure.

3. Analyze the NMR spectrum (data sheets, p. 4) relative to your structure for C:

i. assign $ values and note relative areas for each peak or group of peaks and calculate the

corresponding $ based on your structure. Show your calculations.

ii. describe the pattern of each group of peaks, state the J values (approx), and correlate the

splitting pattern with your structure, pointing out numbers of equivalent H nearby.

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B. Draw the structure of B and correlate it with the spectral data; Point out one feature of the IR spectrumwhich is consistent with the structure. Then explain how the NMR spectrum is consistent with the structure;you need not show chemical shift calculations

IR:

NMR:

B

C. Draw the structure of D and correlate it with the spectral data: Explain how the high mass region of themass spectrum is consistent with the structure. Then explain how the NMR spectrum is consistent with thestructure; you need not show chemical shift calculations.

MS:

NMR:

D

D. Write a structure for A which best fits the chemical and spectroscopic data. Point outone feature of theIR spectrum which is consistent with your structure. Then explain how the NMR spectrum is consistent withthe structure; you need not show chemical shift calculations. Specify what pattern each set of equivalent Hswould give in a simple "first order" situation. NOTE: there are at least two (closely related) structures whichare indistinguishable based on the data given. For a 2-pt bonus, draw the alternative structure.

A

IR:

NMR:

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  cont on next pageE. Write a careful mechanism for the reaction of A with HCl with the usual arrow formalism and discuss

briefly any questions of regioselectivity.

III. (12 pts).  Compounds K and L are structurally related. As usual for ethane derivatives, there severalfavorable (minimum eclipsing) conformational isomers for each. Using the figures below, draw in the Newmanprojection of two staggered conformers (anti, gauche) for each. Perhaps surprising at first, that isomer whichis of lower energy is anti for one and gauche for the other. Indicate clearly your reasoning as to whichconformer of each pair is more stable. Note that Cl and OH are similar in size, and there are polar bondsinvolved.

 

CH2H2C

O

OH

K

 

CH2H2C

O

Cl

L

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IV. (16 pts).  As expected, the reaction of alkene E with HBr takes a different course depending on thereaction conditions. The reaction is simple using HBr with peroxide, and gives a major product,F. In the caseof E under polar conditions, a multitude of products is observed including the "expected" product G, along withstructures H and I.

 

Br

Br

+G

F

+

H   I

E

HBr, ROOR

heat

HBr

polar

A. Write a mechanism for the formation of F, including the structure of F.

B. Draw the structure of G and write a careful mechanism which rationalizes the formation

  of G, H, and I from E.

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V. (8 pts). In the reaction of bromine with 2-methylpropene, there is a strong solvent effect on the rate.The reaction goes almost 1000 times faster in a polar solvent (e.g., acetic acid) compared to non-polar (CCl4).The products and the rate determining step are the same in each solvent. Write the mechanism for additionbromine to 2-methylpropene and explain why a more polar solvent leads to a strong increase in rate. Areaction coordinate diagram for each process would be useful.

E

extent of reaction

E

extent of reaction

_______________________________________________________________________________________ VI. (7 pts). Write the best mechanism which rationalizes the following reaction.

 

Br

HBr

polar

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