Tutorial topic 8 - In-Class Worksheet PDF

Preview

Summary

In-Class Worksheet...

Description

Topic 8: Alkyl Halides and Elimination Substitution John E. McMurry (2015). Organic Chemistry (9th ed.) USA: Cengage Learning Janice Gorzynski Smith (2010). Organic Chemistry (3rd ed.). New York: McGraw-Hill

Tutorial Question 1 I.

(a)

What alkyl halide forms each of the following alkenes as the only product in an elimination reaction? β α CH2–CH2CH2CH2CH3 CH2=CHCH2CH2CH3 Cl β

(b)

(CH3)2CHCH=CH2

α β

CH2

To give only one product in an elimination reaction, the starting alkyl halide must have only one type of β carbon with H’s.

CH2Cl

(c) β

(d)

α

Cl

β

H3C

H3C

(e)

α

(CH3)2CHCH2–CH2Cl

C(CH3)3

β

Cl

Two β carbons are identical

α β

C(CH3)3

2

Tutorial Question 2 •

Taking into account anti periplanar geometry, predict the major E2 product formed from each starting material.

(a)

Cl

CH3 CH(CH3)2

Cl (CH3)2CH

Cl

choose this conformation. axial Cl

H

H

CH3 CH(CH3)2

H

CH3

H

only product

= CH3 CH(CH3)2

(CH3)2CH H H CH3

3

…Continued I. Taking into account anti periplanar geometry, predict the major E2 product formed from each starting material. Cl (b) Cl

CH3

CH3

β2

(CH3)2CH CH3

Cl

CH(CH3)2

β1

CH(CH3)2

choose this conformation. axial Cl

H

H

H

H

H

(CH3)2CH CH3 H

(loss of β2 H)

CH3 CH(CH3)2

(CH3)2CH CH3

(loss of β1 H) Major product trisubstituted

CH3 CH(CH3)2

4

…Continued II. Explain why (CH3)2CHCH(Br)CH2CH3 reacts faster than (CH3)2CHCH2CH(Br)CH3 in an E2 reaction, even though both alkyl halides are 2º. β2

β1

β2

β1

α

α

α

Major product trisubstituted

A

α α β1

B

β2 β1 Major product disubstituted

disubstituted

α β2 monosubstituted

Since the major and minor products formed from A have more alkyl groups on C=C (making them more stable) than those formed from B, A reacts faster in an elimination reaction. 5

Tutorial Question 3 •

Rank the alkyl halides in each group in order of increasing E2 reactivity. Then do the same for E1 reactivity. A B C

(a)

2º halide

(b)

Br

Br

Br

3º halide

Increasing reactivity in E1 and E2: C < A < B

1º halide

H3C Br

CH3 Cl

3º halide

Cl

2º halide

CH3

Increasing reactivity in E1 and E2: B < A < C

3º halide + better leaving group

The order of reactivity is the same for both E2 and E1: 1º < 2º < 3º

6

Tutorial Question 4 I.

Under certain reaction conditions, 2,3-dibromobutane reacts with two equivalents of base to give three products, each of which contains two new pi bonds. Product A has two sp hybridised carbon atoms, product B has one sp hybridized carbon atom, and product C has none. What are the structures of A, B and C? CH3—CΞC—CH3 CH3

H

H

C

C

Br

Br

CH3

A

sp CH3—CH=C=CH2

B

2,3-dibromobutane sp CH2=CH—CH=CH2

C 7

…Continued II. Which dihalide would you use to synthesize 2-methyl-3-hexyne? A) I B) II C) III D) IV

Tutorial Question 5 I. Draw the organic products formed in each reaction. Br (a) OC(CH ) 3 3

1º halide SN2 or E2

(b)

-

I

1º halide SN2 or E2

E2

Sterically hindered base

OCH2CH3

OCH2CH3

SN2

Strong nucleophile

9

…Continued •

Draw the organic products formed in each reaction. OCH2CH 3

Br

(c)

CH2CH3

CH2CH3

(d)

SN1 product

I CH3CH2OH

E1 product

OCH2CH3 +

+

Weak base 2º halide SN1, SN2, E1, E2

+

+

Weak base 3º halide no SN2

CHCH3

CH2CH3

CH3CH2OH

SN1 product

E1 product

CH3CH=CHCH3

(cis and trans) E1 product 10

…Continued

C(CH3)3

C(CH3)3

II. Br

cis-1-bromo-4-tert-butylcyclohexane

Br

trans-1-bromo-4-tert-butylcyclohexane

(a)

The cis and trans isomers of 1-bromo-4-tert-butylcyclohexane react at different rates with KOC(CH3)3 to afford the same mixture of enantiomers A and B. Draw structures of A and B. (b) Which isomers reacts faster with KOC(CH3)3? Offer an explanation for this difference in reactivity. cis-isomer (a) Two enantiomers: C(CH3)3

A=

Br

C(CH3)3

B=

Trans diaxial (CH3)3C

H (b) Cis-isomer H • Has axial Br atom • Readily reacts trans-isomer (CH3)3C Bulky tert-butyl group • Anchors the cyclohexane ring Br • Occupies the more roomy equatorial position • For dehydrohalogenation to occur, halogen must (CH3)3C be axial for trans diaxial elimination of H and X. trans-isomer • Has equatorial Br atom • Has to ring flip into a highly unstable conformation with both axial C(CH3)3 and Br • Reacts more slowly

H H

Br

11

…Continued C(CH3)3

C(CH3)3

Br

Br

cis-1-bromo-4-tert-butylcyclohexane

(c) (d) (e)

trans-1-bromo-4-tert-butylcyclohexane

Reaction of cis-1-bromo-4-tert-butylcyclohexane with NaOCH3 affords C as the major product, whereas reaction of trans-1-bromo-4-tert-butylcyclohexane affords D as the major product. Draw the structures for C and D. The cis and trans isomers react at different rates with NaOCH3. Which isomer reacts faster? Offer an explanation for the difference in reactivity. Why are different products formed from these alkyl halides when two different alkoxides are used as reagents?

OCH3

(c) Two products: C=

(CH3)3C

OCH3

D= (CH3)3C

12

…Continued

C(CH3)3

C(CH3)3

Br

Br

cis-1-bromo-4-tert-butylcyclohexane

trans-1-bromo-4-tert-butylcyclohexane

(d) • • •

With strong nucleophile –OCH3, backside attack occurs by an SN2 reaction Cis-isomer reacts faster Nucleophile can approach from equatorial position, avoiding 1,3-diaxial interactions cis-isomer

trans-isomer Br

(CH3)3C

H H -

1,3-diaxial interactions -

OCH3 Br

OCH3

Equatorial approach preferred

(CH3)3C

axial approach

(e) • • •

Bulky base –OC(CH3)3 favours E2 mechanism, giving rise to enantiomers A and B Strong nucleophile –OCH3 favours SN2 mechanism Inversion of configuration results from backside attack of the nucleophile13

…Continued III. Which compound below CANNOT undergo β-elimination? a. CH3CH2C(CH3)2CH2Br b. (CH3) 3CCH2Br c. (CH3)3CBr d. CH3CH2CH2Br e. Both a and b IV. Which of the following compounds is UNLIKELY to react by an E1 mechanism? a. I b. II c. III d. all are likely to react

…Continued V. Which is NOT a product expected from the reaction below? a. I b. II c. III d. none of these

VI. Which mechanism would account for the product shown? a. E1 b. E2 c. Reactant CANNOT give the elimination product shown...