key - chemistry.msu.edu · midterm #2 1 fall 2019 cem 251: organic chemistry i sections 25-36...

CEM251, Fall 2019 Midterm #2

1

FALL 2019 CEM 251: Organic Chemistry I

Sections 25-36 TUE-THU 8:00 – 9:20 AM

October 24, 2019

This is a closed book and note examination. If boxes are provided for your answers, only what is written in the boxes will be graded. You have 80 min to complete the test.

Section 25 Thu 10:20 – 11:10 AM Jose Section 26 Thu 10:20 – 11:10 AM Sophie Section 27 Mon 12:40 – 1:30 PM Emily Section 28 Mon 9:10 – 10:00 AM Emily Section 29 Tue 10:20 – 11:10 AM Emily Section 30 Thu 1:50 – 2:40 PM Rahele Section 31 Thu 4:10 – 5:00 PM Jose Section 32 Tue 3:00 – 3:50 PM Rahele Section 33 Tue 4:10 – 5:00 PM Jose Section 34 Thu 4:10 – 5:00 PM Sophie Section 35 Thu 3:00 – 3:50 PM Rahele Section 36 Thu 1:50 – 2:40 PM Sophie

Name:______________________________ PID: _______________________________

Section:____________ TA: _______________

Question Points Score

1 6 2 8 3 10 4 20 5 12 6 9 7 15 8 20

Bonus 10

Total 100

Green

KEY


2

1. Provide a name or a structure for the following compounds. (6 pts, 3 pts each box) (Make sure to include stereochemistry (R) or (S)) a. (3S,4S)-4-cyano-3-methyl-octane b. 2. How many stereogenic centers (chiral carbons) are there in each of the following compounds (put your answer in the box and mark the stereocenters with an asterisk *) (8 pts, 2 pts each).

Cl

BrBr

OH

CN

man•

.

(15,31211 chloro ,3' Isopropylr"

cyclopentanea

4 !"

g'' '

x.x

G xx x x xx Of


3

3. Label the following Reaction Energy Diagram with the reaction information missing. You must clearly indicate/label the position of reactants, products, transition state(s), change in enthalpy and Gibbs energy, activation energies and possible intermediate(s)) (6 pts)

- Is this reaction Endothermic or Exothermic? (2 pts) - Describe all the steps of the reaction that the diagram is based on using classic reaction arrows (i.e Aà B à …) and clearly indicate which is the rds (rate determining step). (2 pts)

A

B

C

DEner

gy

Reaction Progress

TSETSE

A TSE f\ Edd '

Eas

Ea ,Intan

.

2

,

- ..

!.

.

t.name?ts9so

SM

O

A- ⇒ B → C - > D


4

4. Draw the following molecules in the chair conformation and their ring-flipped form. You will be graded on correctly drawing the axial and equatorial bonds and correctly putting the groups where they should be (Hint: make sure you keep track of your Cs; you do not have to show C-H bonds) (12 pts, 2 pts each).

i. Which isomer (A-C) is, overall, the most stable (lowest energy)? (2 pts) ii. Explain your choice above and indicate the evidence behind your reasoning. (To

fully answer the question keep asking yourself “WHY” until you have explained your observations with Energy-based arguments). (2 pts)

C

B

A

ring

flipped

ring

flipped

ring

flipped

3 at 3044 i \

I4 /

Zl

y y 21Z

l

l

YEE lZou

4a \ y

l leg

r,

I 4

21 } '

I

led

aWH ' \ ✓ 4 4%4

" f I 4 '

l

A

Comp .

A → 3 ego . large groups . →

lowest E. When large groups are equatorial

they point away from the ring ,minimizing

star cod interaction .


5

iii. Why we care about chair conformations? Here is a real life example:

D-Glucose, a simple sugar we are all familiar with as the energy source in living organisms and a component of many complex carbohydrates, exists in 2 anomeric forms, a (alpha) and b (beta) anomer. The structures of a and b D-Glucose are shown below.

Considering the equilibrium between the two forms, if you buy D-Glucose and you make a solution of it, would you expect it to have 50% a (alpha) and 50% b (beta) anomer, or one of the two isomeric forms will be favored? If one is favored, clearly state which one. (2 pts) Explain your answer using energy-based arguments (2 pts).

D-Glucose

O

OHHO

HOHO O

HOHO

HOOH

HOHO

α - anomer β - anomer

13 - a nomen

- OH change group ) prefers to be

equatorial .


6

5. Label the following functional groups/atoms as Nucleophiles (Nuc) or Electrophiles (E) (Consider lone pairs on Heteroatoms, even if not shown). (12 pts, 2 pts each)

6. Circle the side of the equation that would be favored at the equilibrium (9 pts, 3

pts each)

OH2

ON

Cl

NH2 O

++

+ +

H-Cl O

OHO

O

Cl

OH O

O

O

OH

O

+ +H2N HN

:c : :

O

•


7

7. What is the relationship between the following pairs of molecules? (They are either Identical, Enantiomers or Diastereomers or Constitutional Isomers) (3 pts each, 15 pts).

CH3HBrOHHClH

CH3

CH3HBrOHHCH3Cl

H

vs

ClCl

vs

COOHH OHH OH

HO HCH3

COOHHO HHO HHO H

CH3

vs

COOHH Br

HO HCl H

CH3CH3HBrOHHCH3Cl

COOH

vs

vs

OH OH

Diastereomers

Enantiomers

* a

Identical

Enantiomers

+3 ,not isomers

O°

Contours


8

8. Circle the correct answer. (20 pts, 2 pts each)

a. Which of the following carbanions is the most stable?

b. How many possible stereoisomers are there for the following compound?

c. Which of the following highlighted Hs is the LEAST Acidic?

d. What kind of reaction is the following?

e. Which of the following statements is correct about a “meso” compound? 1. It is not a chiral compound. 2. It has a non-superimposable mirror image.

3. It is not optically active. 4. It contains no chiral carbons.

All of the above 1&2 3&4 1&3 2&4

0

4

6

8

O

O

O

O

FO O

OFHH HH

OOH

Rearrangement

Substitution

Addition

Elimination

Oo .

O

O

O

=O


9

f. Which of the following carbocations is the most stable?

g. Which of the following statements is correct about a “catalyzed reaction”? 1. The catalyst is consumed during the reaction

2. The activation energy of a catalyzed reaction is lower than a non-catalyzed one. 3. The product of a catalyzed reaction is more stable that that of a non-catalyzed one.

4. When catalyzed, the reaction is faster.

All of the above 1&2 1&3 3&4 2&4 h. Which of the following highlighted Hs is the MOST Acidic?

i. What kind of reaction is the following?

j. Which of the following statements is correct about a “racemic mixture”? 1. A racemic mixture is not optically active. 2. A racemic mixture interacts with the circularly polarized light.

3. A racemic mixture is a mixture of achiral compounds. 4. A racemic mixture is a 50:50 mixture of two enantiomers.

All of the above 1&2 2&3 1&4 2&4

OH

OH N

HH

OH

O

BrAddition

Elimination

Rearrangement

Substitution

O

←•

✓

O-


10

Bonus Points (10 pts) a. The OH in structure (A) is LESS acidic than that in structure (B). Using resonance structures, and any other argument you want, briefly but clearly explain your choice. (4 pts) b. Convert the structures on the left into a correct Fisher projection (6 pts)

OH O

H

(A) (B)

OO

Br

Fisher

Projection

Fisher

Projection

OH

o_0

# TT electrons can delocalize"

tho ⇐ Ty, angering to the

19O_O O

cannot reach the

log

Other 0.

-

Ctb 2,3 HEt

Ho s4 H-

X

" RGt3 r ,

H>

4

H RyBt

u


11

Scratch Sheet: Needs to be turned in with the test (if missing, the test will not be graded)


12

key - chemistry.msu.edu · midterm #2 1 fall 2019 cem 251: organic chemistry i sections 25-36...

Documents