216 S09-Exam #2 - Page 2. Name_____Key______________

I. (9 points) The goal of recrystallization is to obtain purified material with a maximized recovery. For each of the following cases, explain using less than 25 words why ONE of the two goals, (A) maximum recovery and (B) purity, would be adversely affected by the recrystallization method indicated.

(1) After dissolving a solid in a solvent at high temperature, the solution is not filtered. (B) would be adversely affected. Crystals obtained would be contaminated with insoluble impurities. (2) Crystallization is accelerated by immediately placing the flask of hot solution in an ice-water bath. (B) would be adversely affected. Impurities dissolved in the solvent would be trapped in the crystalline lattices and these would not be removed by suction filtration. (3) The crystals obtained after recrystallization are collected by gravity filtration. (B) would be adversely affected. Crystals would be wet with the impurity-containing solvent and upon drying such crystals, impurities would adhere to the crystals. II. (14 points) For the following reaction sequence, draw the correct structures in the boxes provided. In addition, provide in the box below a step-by-step mechanism for the entire transformation. Make sure to draw the structure of the intermediate for the last, intramolecular Wittig process.

note: Ph = C6H5

ylide intermediate

O

H

O

Na

PhPPhPh

Br+

C9H8O+ Ph3P=O+ NaBr

3 3O

O

HP

PhPhPh

O

O

HPPhPhPh

orO

Mechanism:

O

H

O

Na

PhPPhPh

Br

8

O

O

H

P

PhPh

Ph

O

O

O

H

P

PhPh

Ph

O

OH P

PhPh

PhH

+ Ph3P=O

arrows: 2 pts

arrows: 2 pts

arrows: 2 pts

structure: 2 pts

216 S09-Exam #2 - Page 3. Name_____Key_________________

III. (10 points) (1) (6 points) The Grignard reaction has to be carried out under anhydrous conditions. Explain what would happen to the Grignard reagent formed from bromoethene (BrCH=CH2) and Mg if the solvent ether used for the reaction is contaminated with a small amount of water (i.e., more than one mol equivalent to the BrCH=CH2). Show the structure of the Grignard reagent to be produced from BrCH=CH2 and Mg and provide a chemical mechanism using the curved arrow convention as to what effect the water can have on the Grignard reagent.

Mg Br

Grignard reagentfrom BrCH=CH2

HO

HHH

H Mg Br

HH

H

H

HH

HHO-MgBr+

(2) Draw in the box provided the structure of the expected major organic product.

4

1. Mg/ether2. aq NH4Cl

BrO

CH3

HO

CH3

H

IV. (13 points) Draw in the box the structure of the expected product from the reaction shown below and provide in the box a reasonable step-by-step mechanism using the curved arrow convention that accounts for its formation. You need not show the mechanism for the protonation step.

KOH/H2Oreflux

3

HCl/H2O(protonation)

O

O

Draw the structure of one of the two expected diastereomers.

OH

OOH

10

Mechanism:

O

O

HO

O

O

OHO

O

O HOH

O

O

O

arrows: 1 ptarrows: 2 pts

arrows: 1 pt

structure: 2 pts

structure: 2 pts

structure: 2 pts

O

OOH

Alternatively, could start with

216 S09-Exam #2 - Page 4. Name_______Key______________

V. (4 points) The proton NMR spectrum of a mixture of acetonitrile (CH3C≡N; δ 2.10 ppm), methylene chloride (CH2Cl2; δ 5.20 ppm), and benzene (C6H6; δ 7.26 ppm) is integrated to give values of 12, 6, and 18 for their signals, respectively. In what mole ratios are these three substances present? No explanation required. No partial credit will be given to this problem.

acetonitrile : methylene chloride : benzene = : :

12/3 : 6/2 : 18/64 3 3

VI. (12 points) For the following two Diels-Alder reactions, draw in the boxes provided the structure of the expected product [for question (1)] and the structures of the two appropriate starting reactants [for question (2)].

methylene chloride(solvent)

+ O

O

Oendo product

(1)

(2)

+

OCH3

+ enantiomer

OCH3

OCH3

O

O

4

44

CH3O O

O

O

H

H

H

O OCH3

OCH3O

VII. (14 points) Show how many peaks you would expect to observe in the proton-decoupled 13C NMR spectra of each of the following compounds. Indicate your answers in the box provided. Do not count the 13C NMR peaks of the solvent and TMS.

(2) (4)

(5) (6)

(1)

(7)

(3)-OOC

NH3+

O

O CH3

CH3H3C

H3C

(room temperature)

O

O CH3

CH3H3C

H3C

-OOC

NH3+

H

OO

O

H3C CH3

OO

ON

O

6 5 7 9

5 7 9

216 S09-Exam #2 - Page 7. Name _______Key____________

IX. (continued).

(1) (1 points) What is (are) the unit(s) of unsaturation of this compound? __one______ (2) (2 points) What does the IR spectrum tell you in terms of the presence of an O-containing functional group in this molecule? ___C=O_____ (3) (2 points) What does the 13C NMR spectrum tell you in terms of the presence of an O-containing functional group in this molecule? ___ester_____ (4) (4 points) What is the structure of this compound? (no partial)

4

O

O

(5) (9 points) Assign all 1H and 13C chemical shifts to the corresponding H and C atoms in the proposed structure (partial credit possible).

9

CC

OC

C CH

O

H

H H

HH

H

2.0 ppm

3.85 ppm

CHH

H

HH

0.93 ppm

1.92 ppm170 ppm

20 ppm*

18 ppm*

27 ppm

70 ppm

*These could be reversed.