Chemistry 303fall 1995

THIRD EXAMINATION

7:30 pm, December 5th, 1995

Duration: 2 hr

Name________________________________________________

Lab TA.______________________________________________(if you do not know his/her name, give time 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 with each question.

Writing the "mechanism" means showing the electron flow with the arrow formalism relating all intermediates. It does not mean drawing the transition states, unless such is specifically called for.

Score:

1._______/20

2._______/20

3._______/15

4._______/17

5________/12

6._______/16

total: _____/100

There are 9 pages in this exam; please check now to be sure you have a complete set.

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

Pledge:_________________________________________________________________________

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1. (20 pts). Consider substitution reacitons on the structurally related compounds, U and V.

Br

OBr

Na+ -SMeMeOH

Na+ -SMeMeOH

Ag+

Ag+

MeOH

MeOH

SMeOMe

OSMe

OOMe

W

X

Y

Z

U

V

A. (5 pts) Write the most likely mechanism for the formation of Z, including a representation of thetransition state for the rate-determining step, showing all partial charges and partial bonds. Be sureto distinguish between transition states and intermediates (if any). On the chart below, draw a freeenergy diagram representing the progress of the reaction. Show clearly the activation energy for therate-determining step.

E

reaction progress

B . (4 pts) Use your mechanism and transition state picture for (A) above to explain why U and V bothreact much faster than ethyl bromide under these conditions.

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C. (6 pts) Write the most likely mechanism for the formation of W, including a representation of thetransition state for the rate-determining step, showing all partial charges and partial bonds. Be sureto distinguish between transition states and intermediates (if any). On the chart below, draw a freeenergy diagram representing the progress of the reaction. Show the activation energy for the rate-determining step.

E

reaction progress

D . (5 pts) Specify whether the formation of X or of W is slower, and explain carefully using the freeenergy diagram.

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2. (20 pts) Consider the following substitution reaction:

H ClH3C

OH CCH3

HPh

OH no stereochemistry intended

M [α] +22oN

H2O

The pure enantiomers of N show [α] = +16o, and [α] = -16o.

A. (2 pts) What is the absolute configuration of M? R or S (circle best answer)

B. (5 pts) Nothing is perfect. Using 0.1 M NaOH in water, the product N is isolated by simple chromatography and shown to have [α] +12o.

Write the mechanism(s) for the formation of N , including a careful drawing of the stereoisomer likelyto predominate under these conditions. Specify the configuration for this isomer as R or S

C. (3 pts) Why does the product show [ less than that for a pure enantiomer of N?

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2. Continued: Re-drawn:

H ClH3C

OH CCH3

HPh

OH no stereochemistry intended

M [α] +22oN

H2O

D. (5 pts) Using 0.001 M NaOH in water with M, the reaction proceeds slower and, after purification by chromatography, the product N has [α] = +4o.

Explain carefully the most likely reason for the change in rate and rotation for the product.

MeS

Me

OSuppose the water is replaced by DMSO as solvent, with 0.1 M NaOH again.

DMSOE. (5 pts)

i. Please analyze the effect of the new solvent on the reaction rate compared to that for part (B)above?

ii. Would you expect [ ] to increase or decrease compared to that in part (B) above? Explain.

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3. (15 pts) Consider the following reactions:

Cl H2Oheat

[X] + [Y] + [Z]C5H10O C5H10O C5H8

Selectred Spectral data possibly useful in confirming the structures. You need not interpret the spectral data,but if you do not know the structure, interpretation can get partial credit.

[X ] 1H NMR: δ 1.82 (s, 3H), 1.94 (s, 3H); 2.52 (br s, lH), 4.5 (d, J=7, 2H), 5.3 (t, J=7, 1H)UV: no absorption above 200 nm

[Y] 1H NMR: δ 1.42 (s, 6H), 2.43 (br s, 1H); overlapping multiplets 5-6 ppm, 3HUV: no absorption above 200 nm

[Z] 1H NMR: δ 1.85 (s, 3H), overlapping multiplets 5-6 ppm, 5HUV: λmax 224 nm (ε 18,000)

A (10 pts) Write structures for X and for Yconsistent with the spectral data and a mechanism fortheir formation under these conditions. What is the name of this mechanism? Would you expect X or Yto be favored?

B (5 pts) Write a structure for Z consistent with the spectral data and a mechanism for its formationunder these conditions. What is the name of this mechanism?

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4. (17 pts) The reaction of 1-methylcyclohexene with a mixture of bromine and water gives a single product,A. The product can undergo reaction with NaH in ether solution to give a new molecule, B by loss of theelements of HBr.

CH3 Br2, H2OA

NaHether

OCH3

BC7H13BrO

A (6 pts) The following structures have the correct molecular formula for A. Pick the one which ismost likely for A and draw the mechanism which explains its selective formation. Draw allintermediates and be sure your mechanism reveals the regio- and stereo-selectivity.

Br

H

Me

OHBr

H

OH

MeHO

H

Me

BrHO

H

Br

Me

Me

BrH

OH

B. ( 2 pts) The following structure is identical to which of the chair drawings? (circle correct answer)HO

Br

H

Me

C. (2 pts) In structure ( ), what is the absolute configuration of the carbon bearing the OH group? R S (circle)

D. (2 pts) Which is the least stable molecule? (circle single best answer)Explain in one sentence:

E. (5 pts) Draw the alternate (less stable) chair arrangment for A. Which chair form of A is the best precursorof B? Show by writing a mechanism for formation of B from A.

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5. (12 pts) Compound B can be converted into two new isomers, under the following conditions:

a. Li NMe2 CH3

OHb. neutralize

CH2

OHCH3

OH

not observed

+

C D

EO

CH3

B

A. (2 pts) Is the LiNMe2 behaving as a strong base or nucleophile in this process?

B . (6 pts) Write a mechanism for the formation of C ; Name this mechanism. [It may be helpful to draw a chair representation of B at the outset]

C. (4 pts) Explain carefully why E is relatively disfavored following a similar mechanism.

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6. (16 pts). There is a wonderful leaving group: N2! It can be arranged by treating an amino group with HONO (nitrous acid) to form a diazonium ion, which then ionizes with loss of N2 or can be nudged out by an electron pair.

R-NH2 + HONO R N R + N2NH2O

diazonium ion

The nudging can be internal, as reflected in the following examples. The interesting point here is that theproduct structure depends on the stereo-arrangement of the groups on the cyclohexane ring.

N2+

OH

HO

N2+

H

OH

O

H

OHN2

+

H H

H

O H

A B.

C.

H

H

H

H

H

Write mechanisms for each case, A-C. The key idea in each example is "what nudges out the N2+?" Be sureyour description makes clear why the particular product is favored in each case.Write clearly.

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