reaction mechanism 2

7/30/2019 Reaction Mechanism 2

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CHE101 (B.Tech. Chemistry Course)

Lecture27.28

Substitution ReactionsSN1 and SN2


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TWO SUBSTITUTION REACTIONS

SN1 / SN2


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RBr + NaOH ROH + NaBr

In this reaction:

CH3 Br + CH3 OH + Br-

20% waterNaOH

80% ethanol

1)

rate = k2 [RBr] [NaOH]

55oC

SN2

primary

Two similar reactions,

different kinetics.

First Case


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CH3C

CH3

CH3

OH+CH3C

CH3

CH3

Br20% water

Br-

+NaOH80% ethanol

rate = k1 [RBr]

55oC

SN1

tertiary

2) Second Case


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Is this difference significant, or is it just a

mathematical illusion of some kind?

SN1 SN2

rate = k1 [RBr] rate = k2 [RBr] [NaOH]

QUESTION


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THE WORK OFHUGHES AND INGOLD


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It was found experimentally that :

Most primary compounds undergo SN2 reactions

with second order kinetics.

Most tertiary compounds undergo SN1 reactionswith first order kinetics.

Most nucleophilic substitution reactions fit one

or the other of these two patterns.

HUGHES AND INGOLD ca. 1940

Is this significant, or is it just a mathematical

illusion of some kind?


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2-BROMOOCTANE

A STUDY IN STEREOCHEMISTRY

Lets use an optically-active compound.


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CH3CH2CH2CH2CH2CH2 CH CH3

Br

2-BROMOOCTANE

*

R CH CH3

Brcan react bothSN1 and SN2

chiral

secondary

abbreviation


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R C

Br

H

CH3

R C

OH

H

CH3

R C

H

CH3+

low conc [ OH- ]

high conc [ OH- ]

> 1 M

1) H2O

2) -H+

SN1

SN2

< 0.1 M

DIFFERENT KINETICS AT DIFFERENT BASE CONCENTRATIONS

k2[RBr][OH]

k1[RBr]

The two mechanisms

compete.


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Total Rate = k1 [RBr] + k2 [RBr] [OH-]

SN2

SN1

[ OH- ]

[RBr] constant

Rate

TOTAL

k2 [RBr] [OH-]

k1 [RBr]

dominates at

high [OH-]

dominates at

low [OH-]

SN1 SN2


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2-Bromooctane is chiral .....

Is the stereohemical result the same in

each case :

What if we start with an enantiomerically pure

compound (one enantiomer only)?

STEREOCHEMICAL RESULTS

CH3CH2CH2CH2CH2CH2 CH CH3

Br

*

SN1 and SN2 ?


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R

H

CH3 Br

R

H

HOCH3

R

H

CH3 OH

R

H

HOCH3

SN2 conditions

SN1 conditions

high [OH-]

low [OH-]

enantiomers

racemic mixture

R-(-)-2-bromooctane

S-(+)-2-octanol

complete

inversion

complete

racemization

STEREOCHEMISTRY CHANGES

[ ]D = +10.3o

[ ]D = -36.0o

[ ]D = 0o

+

R

S

R S


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Can we explain the stereochemical results

with our proposed mechanisms for

SN1 and SN2 reactions?

Lets Try .......

THE MECHANISMS


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SN1


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Fastest for tertiary substrates.

Need a good leaving group.

Product will be racemic.

Carbocation intermediate.


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R

H

CH3 Br

C

H

RCH3 O H

OH

H

CH3 R

OH

H

RCH3

+

+

enantiomers

SN1 MECHANISM

planar

carbocation

sp2

(S) (R)

(R)

50%

50%

racemic mixture

RACEMIZATION

-

attacks top

and bottom

equally


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SN2


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Fastest for primary and methyl substrates.

Needs a strong nucleophile.

Leaving group does not need to be primarily good.


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LIKE POOL OR BILLIARDS

1) two balls at restand touching2) forceful shot

directly on axis

3) straight-on

collision

4) momentum

transfer

Nu

Nu

Nu

X

X

X

X

CONCEPTUAL ANALOGY 1

Similar in concept

to an attack from

the back forcingthe nucleophile to

leave.


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C

R

HCH3

Br:

C

R

HCH3

HO :

C

HCH3

R

BrHO

H O:..

..

activated complex

is trigonal planar (sp2 )

(R)-configuration

(S)-configuration

configuration

is inverted

Ea

HO C B

partial bonding

2pTHE INVERSION

PROCESS

sp3

sp3

sp2


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ACTIVATED COMPLEX FOR SN2

C

R

HCH3

BrHO

trigonal planar (sp2)

5 bonds to Carbon

breakingforming

but the bonds to

Br and OH are only

half-formed and

are not full bonds.

NOT A STABLE SPECIES

MIDPOINT OF THE REACTION

reaction mechanism 2

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