ORGANIC CHEMISTRY I – PRACTICE EXERCISE Elimination Reactions


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ORGANIC CHEMISTRY I – PRACTICE EXERCISE Elimination Reactions and Alkene Synthesis
1) One of the products that results when 1-bromo-2,2-dimethylcyclopentane is heated in ethanol is shown below. Give a mechanism by which it is formed and give the name of this mechanism.
CH3
CH3

2) Provide the structure of the major organic product in the following reaction.

CH3
Br HD

NaOCH3 CH3OH

3) Provide the structure of the major organic product from following reaction.

H

CH3 H
Br

NaOCH3 CH3OH

4) Which diastereomer of 1-bromo-4-t-butylcyclohexane, the cis or the trans, undergoes elimination more rapidly when treated with sodium ethoxide? Explain your answer.

5) Provide the structure of the major organic product from the following reaction.
Br
KI

H3C

Br

6) When 1-iodo-1-methylcyclohexane is treated with NaOCH2CH3 as the base, the more highly substituted alkene product predominates. When KOC(CH3)3 is used as the base, the less highly substituted alkene predominates. Give the structures of the two products and offer an explanation.

7) Which of the following statements apply to E1 reactions of alkyl halides? Choose as many as necessary.

I.

Rate = k[base]

II. Rate = k[base][RX]

III. Rate = k[RX]

IV. The reactions occur in two or more distinct steps.

V.

Rearrangements are sometimes seen.

8) What is Saytzeff's rule?

9) What major product results when 2-bromo-2-methylbutane is treated with sodium ethoxide.
10) How many distinct alkenes can result from E2 elimination of the compound below? Give their structures and IUPAC names.

I CH3

11) Give the major product and the mechanism of the following reaction.

H3C Ph

H Ph
Br

NaOCH3 CH3OH

12) Predict the most likely mechanism and the product for the reaction below.

H3C H3C Cl

NaOCH3 CH3OH

13) Predict the most likely mechanism and the product from the reaction between 2-chloro-2-methylpentane and sodium ethoxide in ethanol.

14) The major product which results when 2-chloro-2-methylpentane is heated in ethanol is an ether. Show and name the mechanism by which this ether forms.

15) Which of the following mechanisms feature carbocation intermediates?

A) SN1 only B) SN2 only C) E1 only D) E2 only E) both SN1 and E1

16) Which mechanism(s) give(s) alkenes as the major products, Sn1, Sn2, E1, or E2?

17) Which compound produces only one alkene when treated with sodium methoxide?

A) 2-chloro-2-methylpentane B) 3-chloro-3-ethylpentane C) 3-chloro-2-methylpentane D) 2-chloro-4-methylpentane E) 2-chloro-3-ethylpentane

18) When 3-iodo-3-ethylpentane is treated with sodium methoxide in methanol, the major organic product is an ______ that is generated through an ______ mechanism.
A) ether, SN1 B) ether, SN2 C) ether, E1 D) alkene, E2 E) alkene, E1

19) Provide the structure of the major alkene product of the reaction below.

H3C

CH3 Br

NaOH

20) Based on Saytzeff's rule, select the most stable alkene.

A) 1-methylcyclohexene B) 3-methylcyclohexene C) 4-methylcyclohexene D) They are all of equal stability

21) Based on Saytzeff's rule, select the most stable alkene.

A) 1,2-dimethylcyclohexene B) 1,6-dimethylcyclohexene C) cis-3,4-dimethylcyclohexene D) They are all of equal stability

FOR QS. 22-24, DRAW ALL ALKENE PRODUCTS AND CIRCLE THE PREDOMINANT ONE.

22)

Br CH3

NaOH acetone

23)
Br CH3
(CH3)3CO- K+
(CH3)3COH

24) Br Zn CH3COOH
Br

25) Propose a detailed, step-by-step mechanism for the reaction shown below.

CH3

CH3

CH3

H2SO4

OH

D

CH3

26) Draw all likely products of the following reaction and circle the product you expect to

predominate.

OH
H2SO4

D

27) Draw all likely products of the following reaction and circle the product you expect to
predominate.
CH3

OH

H2SO4

D

28) Which base, ammonia (NH3) or triethylamine [(CH3CH2)3N], would be more effective to use for the following conversion?
Base
Cl

29) Which compound would undergoe dehydrohalogenation with strong base to give the alkene shown below as the only alkene product?

CH3 CH2

C H

A) 1-chloropentane

B) 2-chloropentane

C) 3-chloropentane

D) 1-chloro-2-methylbutane

E) 1-chloro-3-methylbutane

CH CH3

FOR SYNTHESES # 30-32 GIVE THE MISSING REAGENTS AND STRUCTURES.

30)

A

C

B

31)

A

C

B

32)

A

C

B

33) Which of the following statements applies to the E2 mechanism?
A) It occurs with inversion of stereochemistry. B) It occurs with racemization of stereochemistry. C) It proceeds through the more stable carbocation intermediate. D) The C-H and C-X bonds that break must be anti. E) Use of a bulky base gives the more highly substituted alkene product.

ANSWERS 1)
E1 mechanism with carbocation rearrangement

Br
CH3 ethanol 1) CH3 D

CH3
CH3 + Br

CH3

2)

CH3 rearrangement

CH3

CH3

H3C 3)

H CH3

HOCH2CH3

CH3
CH3 + H2OCH2CH3

2) In questions 2 and 3, only the proton trans to the leaving group can eliminate.
CH3

D
3)
H

CH3

4) Due to the presence of the bulky t-butyl group, the ring is practically locked up in the most stable conformation with the bulky group being equatorial.

Br

H

H

Br

t-Bu H

t-Bu
cis H
t-Bu = t-Butyl group

trans

Of the two isomers, the cis is the only one that fulfills the anticoplanar arrangement for E2, where the leaving group and adjacent proton must be anti to each other and in the same plane.

Br

H

t-Bu H

H H
H
cis

The atoms shown in red fulfill the anticoplanar requirement for E2. Elimination is possible and fast.

Br
H t-Bu
H
H trans
The atoms shown in red cannot fulfill the anticoplanar requirement.
Elimination is slower or not possible

5)

H3C

6) The small, unhindered base ethoxide yields the more stable alkene (Saytzeff’s product, i.e. the more highly substituted alkene). When the bulky t-butoxide base is used, the most accessible hydrogen is removed. This results in the least highly substituted alkene (Hoffman’s product).

I CH3

CH3CH2O

CH3
Saytzeff's product

H3C

CH3 O
CH3

CH2
Hoffman's product

7) III, IV, and V.

8) In elimination reactions, the most highly substituted alkene predominates.

9)

H

CH3

H3C

CH3

10)
1

2

4

6

8

3

5

7

(E)-3-methyloct-2-ene

2

1

3

45 6 7 8

(Z)-3-methyloct-2-ene

2

4

6

8

1

3

5

7

(E)-3-methyloct-3-ene

4

6

8

3

5

7

2 1

(Z)-3-methyloct-3-ene

3

5

7

2

4

6

1
2-Ethylheptene

11) E2. The molecule must rotate around the central cabon-carbon bond to aquire the anticoplanar arrangement required for E2. This is a stereospecific reaction that results in formation of the product where the phenyl groups are cis to each other.

H3C

H

Ph

Ph

12) Strong base and bulky substrate favor E2. Only carbon 6 has protons trans to the leaving group. The pi bond can only form between carbons 1 and 6. Can you name the product by IUPAC rules?

3 45 H3C 2 1 6 H

H3C

H

Cl

NaOCH3 CH3OH

H3C H3C

13) Strong base and bulky substrate favor E2 with preferential formation of Saytzeff’s product.

NaOCH2CH3

Cl

CH3CH2OH

14) Weak nucleophile (ethanol, the solvent) and bulky substrate favor Sn1 as shown below. Can you tell which is the rate-determining step? Can you tell what type of reaction is involved in the last step?

1) Cl

CH3CH2OH D

+ Cl

2)

+ HOCH2CH3

O H

3)

+ HOCH2CH3

O

H

+ CH3CH2OH2
O
ether

15) E

16) E1 and E2

17) B

18) D

19) The reacton is E2. See question 12 for a similar case.

H3C

CH3 Br

NaOH

H3C CH3

20) A

21) A

22)

CH3

CH2

+

23)

CH3

CH2

+

24)

25) First step is protonation of the alcohol by the strong acid to form a potential water molecule as a leaving group. Next is departure of the leaving group with formation of a carbocation. Next is a carbocation rearrangement from secondary to tertiary. The last step is an elimination step where the water abstracts the acidic proton next to the positive charge to form the alkene.

O

1)

+ H O S OH

OH

O

+ HSO4
OH2

2) 3) 4) 26) 27)

OH2
HSO4 or H2O
H

+ H2O + H3O+ or H2SO4

28) Triethylamine. Amines can be nucleophiles or bases. Increasing their steric bulk near the nitrogen atom diminishes their nucleophilicity while retaining the basicity. Since the substrate is sterically accessible to nucleophilic attack, a bulky base is needed to promote elimination.

29) C

30) This is a typical example of a simple, multistep synthesis (in this case only two steps). This

tests your ability to use previously learned reactions (e.g. from ch. 4) to design a synthesis

towards a particular product, in this case cyclopentene. The last step is an elimination reaction.

Any strong base combination will serve the same purpose as NaOH and acetone.
Br

Br2

NaOH

hv

acetone

A

B

C

31) Similar to the previous problem, but this time Hoffman’s product is desired. A bulky base must be used in the last step, such as t-butoxide ion.

Br2

t-BuO-

hn

t-BuOH

Br

A

B

C Hproofdfmucatn's

32) Same as above, but this time Sayteff’s product is desired. A small base must be used in the last step.

33) D

Br2

NaOH

hn

acetone

Br

A

B

C Spraoydtzuecftf's

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ORGANIC CHEMISTRY I – PRACTICE EXERCISE Elimination Reactions