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A QUICK REVIEW OF TWO SUBSTITUTION REACTIONS. S N 1 / S N 2. RBr + NaOH ROH + NaBr. rate = k 2 [RBr] [NaOH]. S N 2. RECALL FROM LAST LECTURE. Two similar reactions, different kinetics. 1). First Case. 55 o C. primary. S N 1. rate = k 1 [RBr].

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A QUICK REVIEW OF

TWO SUBSTITUTION REACTIONS

SN1 / SN2


RBr + NaOH ROH + NaBr

rate = k2 [RBr] [NaOH]

SN2

RECALL FROM LAST LECTURE

Two similar reactions,

different kinetics.

1)

First Case

55oC

primary


SN1

rate = k1 [RBr]

RECALL FROM LAST LECTURE

2)

Second Case

55oC

tertiary


rate = k1 [RBr]

QUESTION

SN1

SN2

rate = k2 [RBr] [NaOH]

Is this difference significant, or is it just a

“mathematical illusion” of some kind?


THE WORK OF

HUGHES AND INGOLD


HUGHES AND INGOLD ca. 1940

It was found experimentally that :

Most primary compounds undergo SN2 reactions

with second order kinetics.

Most tertiary compounds undergo SN1 reactions

with first order kinetics.

Most nucleophilic substitution reactions fit one

or the other of these two patterns.

Is this significant, or is it just a mathematical

illusion of some kind?


2-BROMOOCTANE

A STUDY IN STEREOCHEMISTRY

Let’s use an optically-active compound.


R = C6H13

chiral

can react both

SN1 and SN2

2-BROMOOCTANE

*

abbreviation

*

R

CH

CH3

Br

secondary


DIFFERENT KINETICS AT DIFFERENT BASE CONCENTRATIONS

k1[RBr]

SN1

low conc [ OH- ]

< 0.1 M

high conc [ OH- ]

> 1 M

SN2

1) H2O

2) -H+

The two mechanisms

compete.

k2[RBr][OH]


SN1

SN2

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

TOTAL

SN2

dominates at

low [OH-]

k2 [RBr] [OH-]

dominates at

high [OH-]

Rate

SN1

k1 [RBr]

[ OH- ]

[RBr] constant


STEREOCHEMICAL RESULTS

*

2-Bromooctane is chiral .....

What if we start with an enantiomerically pure

compound (one enantiomer only)?

Is the stereohemical result the same in

each case :

SN1andSN2 ?


STEREOCHEMISTRY CHANGES

R-(-)-2-bromooctane

[a]D = -36.0o

complete

inversion

SN2 conditions

high [OH-]

S

R

S-(+)-2-octanol

SN1 conditions

[a]D = +10.3o

low [OH-]

R

S

+

complete

racemization

enantiomers

[a]D = 0o

racemic mixture


THE MECHANISMS

Can we explain the stereochemical results

with our proposed mechanisms for

SN1 and SN2 reactions?

Let’s Try .......


SN1


SN1 MECHANISM

50%

sp2

+

-

planar

carbocation

attacks top

and bottom

equally

(R)

50%

RACEMIZATION

(S)

enantiomers

(R)

racemic mixture


SN2


R

..

..

:

H

..

:

:

C

Br

..

CH3

H

SN2 MECHANISM

nucleophilic attack

(R)-config

O

R

attacks

back lobe

..

:

C

H O

..

CH3

INVERSION

(S)-config

H


CONCEPTUAL ANALOGY 1

LIKE POOL OR BILLIARDS

1) two balls at rest

and touching

X

2) forceful shot

directly on axis

Similar in concept

to an attack from

the back forcing

the nucleophile to

leave.

Nu

X

3) straight-on

collision

Nu

X

4) momentum

transfer

Nu

X


CONCEPTUAL ANALOGY 2

INVERSION OF AN UMBRELLA IN THE WIND

Inversion of the

umbrella is

similar in concept

to the inversion of

an SN2 atom.


..

R

:

H

O

R

..

:

C

Br

:

HO

C

CH3

H

CH3

H

R

THE INVERSION

PROCESS

2p

HO C B

sp2

partial bonding

HO

Br

C

activated complex

is trigonal planar (sp2 )

CH3

H

configuration

is inverted

sp3

sp3

Ea

(R)-configuration

(S)-configuration


ACTIVATED COMPLEX FOR SN2

MIDPOINT OF THE REACTION

trigonal planar (sp2)

R

forming

breaking

C

HO

Br

5 bonds to Carbon

CH3

… but the bonds to

Br and OH are only

half-formed and

are not full bonds.

H

NOT A STABLE SPECIES


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