Midterm exam 1 March, answers PDF

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CHEM 2220 Midterm March 1 2018

Name: __________________________ Student Number: _____________________ University of Manitoba - Department of Chemistry

CHEM 2220 - Introductory Organic Chemistry II – MIDTERM TEST MIDTERM TEST, Thursday March 1 2018, 6 – 8 pm This is a 2-hour test, marked out of 50 points total. Part marks are available on all questions for reasonable responses. Put all answers in the spaces provided. If more space is required you may use the backs of the exam pages but be sure to indicate that you have done so. A spectroscopic data sheet is attached at the end of the exam.

QUESTION

MARKS

1. Mechanism

4 Marks

2. Mechanism

6 Marks

3. Mechanism

6 Marks

4. Reactions and Products

28 Marks

5. Spectra and Structures

6 Marks

TOTAL

50 Marks

CHEM 2220 Midterm ANSWER KEY March 1 2018

Page 2 of 8

1. (4 MARKS) In class, we have seen that strained rings may be quite stable in isolation, but they can undergo interesting reactions when they are activated by protonation. Suggest a plausible stepwise mechanism for the following reaction, which occurs extremely rapidly. Note that there is no water present in this process.

CHEM 2220 Midterm ANSWER KEY March 1 2018

Page 3 of 8

2. (6 MARKS TOTAL) Alkenes do not usually undergo E/Z isomerization very easily, but 1,3butadienes such as A readily isomerize to form B with only moderate heating. It is significant that even when this reaction is heated for many days, only A and B are ever observed; isomer C is not formed at all.

A cyclic intermediate compound is formed during the mechanism for this isomerization. a. (2 Marks) Using the simplified “shorthand method” shown in your textbook, draw the HOMO for a 1,3-diene such as A or B, showing the phase of each orbital lobe by shading or labelling with “+” and “-”. Identify any nodes in the HOMO.

b. (1 Mark) Based on orbital considerations, what type of rotation must be involved in this process? 4 pi electrons, thermal, therefore CONROTATORY

c. (2 Marks) Draw the structure of the cyclic intermediate that forms in the isomerization, showing its stereochemistry.

d. (1 Marks)

What is the name given to this specific class of reaction?

Electrocyclic

CHEM 2220 Midterm ANSWER KEY March 1 2018

Page 4 of 8

3. (6 MARKS) The following reaction mimics the process by which many natural products of the terpene class are biosynthesized. Write out a stepwise mechanism for this reaction. You do not have to consider any stereochemistry for this problem.

This is Groutas problem #105. As Groutas shows, the reaction is actually concerted once water is lost to form the starting carbocation. HOWEVER: although the Groutas answer gives the general sense of what happens, it is actually wrong because he drew the initial structure INCORRECTLY! A methyl is in the wrong place, and this messes up the entire sequence of steps. You can write the mechanism as a single concerted cascade (provided it is done correctly) or stepwise as shown here. Note however that we want to avoid vinylic cations by linking the attack of water with the attack of the triple bond on the cation....