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The details of the elimination reactions Elimination vs substitution

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CHAPTER 9 CHAPTER 9

ELIMINATION REACTIONS

Contents

The details of the elimination reactions Elimination vs substitution

1

Elimination vs. substitution

(2)

9.1 The General Reaction

⇒ This reaction is termed as 1,2-elimination or β-elimination

(3)

9.2 Bimolecular elimination; E2 reaction

⇒ common reaction between alkyl halides and strong bases

⇒ The strong bases are also nucleophiles, the E2 competes with SN2

Concerted reaction involves breaking and forming

several bonds several bonds simultaneously

3

(4)

The bond dissociation Energy of C-D bond is about 1.2 Kcal/mol larger than that for C-H bond

Kcal/mol larger than that for C H bond

C-D bond is being broken during the RDS !

Almost same rate

rate

C-D bond is not being broken

being broken

(5)

9.3 Stereochemistry of the E2 reaction

St l t i i t f E2

Stereoelectronic requirements of E2

Two sigma bonds (C-H and C-L) should lie in the

Two sigma bonds (C H and C L) should lie in the same plane, because these two bonds overlap to form a pi bond

form a pi bond.

More stable conformation is desirable

5

(6)

Two possible conformations

anti-coplaner conformation is more stable, therefore anti elimination is prefered in the E2 reaction. p

Syn elimination is rare, while possible when –H & -L are held syn-coplaner (eclipsed or nearly eclipsed)

held syn coplaner (eclipsed or nearly eclipsed)

(7)

Example; elimination reactions of the diastereomers of 1- bromo-1,2- diphenylpropane

only (Z)-1,2-diphenyl-1-propene is produced

E ti f thi t t l d (Z) lk Enantiomer of this reactant also produce (Z)- alkene

only (E)-1,2-diphenyl-1-propene is produced E ti f thi t t l d (E)

7

Enantiomer of this reactant also produce (E)- alkene

(8)

Trans-diaxial elimination Trans diaxial elimination

For the anti elimination of cyclohexane

⇒ The leaving group and the hydrogen must be trans

They also must be in the axial y position p

(9)

examples

two alkyl groups + one chloride are in the y g p axial position; less stable conformation

two alkyl groups are in the equtorial position; more stable conformation

elimination of neomenthyl chloride is 40 times faster

9

⇒ elimination of neomenthyl chloride is 40 times faster

(10)

However syn elimination does occur in rigid molecules when the living group and the hydrogen are held in an when the living group and the hydrogen are held in an eclipsed conformation (Fig. 9.3)

(11)

9.4 Direction of Elimination; Zaitsev’s rule

11

(12)

Zaitsev’s rule is an empirical rule based on experiments P d t d l t t l

⇒ Product development control

More stable alkene is a major product

(13)

Hoffmann elimination

A ti t Z it l

⇒ An exception to Zaitsev’s rule

ex) Elimination of 4° ammonium salt accords to )

Hoffmann’s rule; Less substituted product dominates.

13

(14)

Why Hoffmann elimination for the compounds with quaternary nitrogen?

with quaternary nitrogen?

⇒ The reaction involves a poor leaving group (trialkyl amine group) and a strong base. Then the breaking of C-H bond is more advanced in the Hoffmann elimination than in the E2

in the Hoffmann elimination than in the E2.

⇒ Then acidity of the hydrogen determines the reactivity CH

N CH2-CH-CH3 CH3

H2C-H2C Alkyl groups push electrons

making hydrogen less acidic

CH3

H H

making hydrogen less acidic

OH

More acidic hydrogen because less alkyl group More acidic hydrogen because less alkyl group

(15)

Steric effect of base

Hydrogens on less substituted C are more accessible i ll f l b

especially for large base

Zaitsev’s rule

Hoffmann’s rule 15

(16)

Conjugation effect

⇒ Conjugated product more stable

⇒ Conjugated product more stable

(17)

9.5 Unimolecular elimination; E1

17

(18)

Reaction mechanisms of S

N

1 and E1

1. RDS; the formation of carbocation

2. The transition state of the substitution is lower in energy;

(19)

Investigating mechanisms

E1 or S

N

1

Leaving groups do not affect the

product ratio

first step is RDS

19

(20)

9.6 Regiochemistry and Stereochemistry of E1

1. Stereochemistry of the leaving group and the

hydrogen is not important in E1; need not coplanar 2. Product distribution follows

Zaitsev’s rule

(21)

9.7 Competition between Elimination & Substitution

S

N

2 and E2

1 S 2/E2 is favored over S 1/E1 with good base or nucleophile;

1. SN2/E2 is favored over SN1/E1 with good base or nucleophile;

the strength of base ↑, the rate of both SN2 and E2 ↑ ex) using ethoxide SN2/E2 not SN1/E1

ex) using ethoxide, SN2/E2 not SN1/E1

2. Steric hindrance favors E2 over SNN2 ex)

21

(22)

3. Among the bases, stronger base (Nu:) favors E2 over SN2, and weaker base favors SN2.

base favors SN2.

⇒ In elimination, hydrogen is easily abstracted by strong base

4

High temperature favors E2 over SN2

4.

High temperature favors E2 over SN2

⇒ ΔG = ΔH - TΔS; Temp ↑, ΔS contribution ↑.

(23)

S

N

1 and E1

RDS of SN1 and E1 involves the formation of carbocation.

1. SN1 and E1 most commonly occur with tertiary (best) or secondary substrate in polar solvent and in absence of strong base (or

l hil ) nucleophile).

2. Difficult to control the ratio of E1 and SN1

23

(24)

Competition on different substrate

Primary substrates; RCH

2

L (1°)

⇒ Compounds with low steric hindrance 1. SN2 with any nucleophile

2 E2 ith b lk t b hi h i t d l hil h 2. E2 with bulky strong base which is not a good nucleophile such

as tert-butoxide

3 SN1 or E1 reaction does not occur because primary carbocation 3. SN1 or E1 reaction does not occur because primary carbocation is very very unstable, while allylic and benzylic substrate SN1 or E1 is possible.

(25)

Secondary substrates; R

2

CHL (2°) Secondary substrates; R

2

CHL (2 )

F h i ll ibl

Four mechanisms are all possible

1. S

N

2 with nucleophile that are not too basic 2. E2 with strong base g such HO - or RO -

3. S

N

1 (and/or E1) with weak nucleophile such as ROH;

such reaction is called as solvolysis (alcohols are also y ( solvents.

25

(26)

Tertiary substrates; R

3

CL (3°)

1 S 2 i i ibl b th t hi d d 1. S

N

2 is impossible because they are too hindered 2. S

N

1 (E1) with polar solvents and in the absence of

t b

strong base

3. E2 with strong base such as HO

-

or RO

-

(27)

Nucleophilicity vs Basicity

‰ Nucleophilicity: ability to cause substitution rxn

‰ Nucleophilicity: ability to cause substitution rxn H

2

O < ROH < Cl- < Br- < OH- < OR- < I- < CN- H

2

O < ROH < Cl < Br < OH < OR < I < CN

‰ Basicity: ability to accept proton in acid-base rxn y y p p I- < Br- < Cl- < ROH < H

22

O < CN- < OH- < OR-

27

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