Substitution and Elimination

Substitution and Elimination Substitution and Elimination - Start

2015-11-26 54K 54 0 0

Description

. Competing Reactions. Alkyl halides can react with Lewis bases by . nucleophilic substitution and/or elimination.. C. C. H. X. +. Y. :. –. C. C. Y. H. X. :. –. +. C. C. +. H. Y. X. :. –. +. . ID: 206349 Download Presentation

Embed code:
Download Presentation

Substitution and Elimination




Download Presentation - The PPT/PDF document "Substitution and Elimination" is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.



Presentations text content in Substitution and Elimination

Slide1

Substitution and Elimination

Competing Reactions

Slide2

Alkyl halides can react with Lewis bases by nucleophilic substitution and/or elimination.

C

C

H

X

+

Y

:

C

C

Y

H

X

:

+

C

C

+

H

Y

X

:

+

-elimination

nucleophilic substitution

Two

Competing Reactions

Slide3

How can a prediction be made which reaction pathwayIs followed for a particular alkyl halide?

C

C

H

X

+

Y

:

C

C

Y

H

X

:

+

C

C

+

H

Y

X

:

+

-elimination

nucleophilic substitution

Two Competing Reactions

Slide4

Comparison

Slide5

A systematic approach is to choose as a referencepoint the reaction followed by a typical alkyl halide(secondary) with a typical Lewis base (an alkoxideion).

The major reaction of a

secondary alkyl halidewith an alkoxide ion (a strong base similar to hydroxide) is elimination by the E2 mechanism.

Elimination versus Substitution

Slide6

CH

3CHCH3

Br

Na

OCH2CH3ethanol, 55°C

CH

3

CHCH3

O

CH2CH3

CH3CH=CH2

+

(87%)

(13%)

Example

Slide7

Br

E2

Figure

CH

3

CH2

O

••

••

Slide8

Br

S

N2

Figure

CH

3CH2

O

••

••

Slide9

Given that the major reaction of a secondaryalkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with:

1

) decreased crowding at the carbon that bears the leaving group

When is Substitution Favored?

Slide10

Question

Which one of the following alkyl halides would be expected to give the highest ratio of

substitution to elimination on treatment with sodium ethoxide in ethanol (50°C)?

A) 1-bromopentane

B) 2-bromopentane

C) 3-bromopentane

D) 2-bromo-3-methylbutane

Slide11

Decreased crowding at carbon that bears the leaving group increases substitution relative to elimination.

primary alkyl halide

CH

3CH2CH2Br

Na

OCH2CH3ethanol, 55°C

CH

3CH=CH2

+

CH3CH2CH2OCH2CH3

(9%)

(91%)

Uncrowded Alkyl Halides

Slide12

primary alkyl halide + bulky base

CH

3(CH2)15CH2CH2Br

K

OC(CH3)3

tert-butyl alcohol, 40°C

+

CH

3(CH2)15CH2CH2OC(CH3)3

CH3(CH2)15CH=CH2

(87%)

(13%)

But a Crowded Alkoxide Base Can Favor Elimination Even with a Primary Alkyl Halide

Slide13

Given that the major reaction of a secondaryalkyl halide with an alkoxide ion is elimination by the E2 mechanism, we can expect the proportion of substitution to increase with:

1) decreased crowding at the carbon

that bears the leaving group 2) decreased basicity of the nucleophile

When is Substitution Favored?

Slide14

Question

Which one of the following statements is

true

?

A) CH

3

CH

2

-S

-

is both a stronger base and more nucleophilic than CH3CH

2-

O

-

B) CH

3

CH

2

-S

-

is a stronger base but less nucleophilic than CH

3

CH

2-

O

-

C) CH

3

CH

2-

S

-

is a weaker base but is more nucleophilic than CH

3

CH

2-

O

-

D) CH

3

CH

2-

S

-

is both a weaker base and less nucleophilic than CH

3

CH

2-

O

-

Slide15

Weakly basic nucleophile increases substitution relative to elimination

(70%)

CH

3CH(CH2)5CH3

CN

KCN

CH

3

CH(CH

2)5CH3

Cl

p

Ka (HCN) = 9.1

DMSO

secondary alkyl halide +

weakly basic nucleophile

Weakly Basic Nucleophile

Slide16

Weakly basic nucleophile increases substitution relative to elimination

secondary alkyl halide + weakly basic nucleophile

Weakly Basic Nucleophile

Na

N3

pKa (HN3) = 4.6

I

(75%)

N

3

Slide17

Question

Which one of the following compounds gives the highest subtitution-to-elimination ratio

(most substitution least elimination) on reaction with 2-bromobutane?

A) NaOCH

3

B) NaNH

2

C) NaC

º

N

D) NaC

º

CH

Slide18

Tertiary alkyl halides are so sterically hinderedthat elimination is the major reaction with allanionic nucleophiles. Only in solvolysis reactionsdoes substitution predominate over eliminationwith tertiary alkyl halides.

Tertiary Alkyl Halides

Slide19

Question

Which one of the following statements is

true

concerning substitution and elimination in

tert

-butyl bromide?

A) the mechanism generally believed to be available to (CH

3

)

3

CBr are S

N

1 and E1

B) the mechanism generally believed to be available to (CH

3

)

3

CBr are S

N

1, S

N

2 and E1

C) the mechanism generally believed to be available to (CH

3

)

3

CBr are S

N

1, S

N

2 and E2

D) the mechanism generally believed to be available to (CH

3

)

3

CBr are S

N

1, E1 and E2

Slide20

(CH

3)2CCH2CH3

Br

+

CH

3

CCH

2CH3

O

CH2CH3

CH

3

CH

2=CCH2CH3

CH

3

CH

3C=CHCH3

CH

3

+

ethanol, 25°C

64

%

36%

2

M sodium ethoxide in ethanol, 25°C

1%

99%

Example

Slide21

Question

Reactions proceeding through this mechanism give a racemic mixture:

A)

S

N

1

B) S

N

2

C) E1

D) E2

Slide22

Question

Methyl bromide reacts with sodium

ethoxide

in ethanol by this mechanism:

A)

S

N

1

B) S

N

2

C) E1

D) E2

Slide23

Question

3-bromo-3-methylpentane reacts with sodium ethoxide in ethanol by this mechanism:

A)

S

N

1

B) S

N

2

C) E1

D) E2


About DocSlides
DocSlides allows users to easily upload and share presentations, PDF documents, and images.Share your documents with the world , watch,share and upload any time you want. How can you benefit from using DocSlides? DocSlides consists documents from individuals and organizations on topics ranging from technology and business to travel, health, and education. Find and search for what interests you, and learn from people and more. You can also download DocSlides to read or reference later.