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NUCLEOPHILIC SUBSTITUTION. THE GENERAL REACTION AND TERMINOLOGY. R. X. R. Y. Y. X. SUBSTITUTION REACTIONS. One group takes the place of another. +. +. Y takes the place of X ( Substitution ). Y “displaces” X. NUCLEOPHILIC SUBSTITUTION. NUCLEOPHILIC DISPLACEMENT. leaving

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Presentation Transcript
slide1

NUCLEOPHILIC SUBSTITUTION

THE GENERAL REACTION AND TERMINOLOGY

slide2

R

X

R

Y

Y

X

SUBSTITUTION REACTIONS

One group takes the place of another.

+

+

Y takes the place of X ( Substitution )

Y “displaces” X

slide3

NUCLEOPHILIC SUBSTITUTION

NUCLEOPHILIC DISPLACEMENT

leaving

group

substrate

product

nucleophile

The nucleophile “displaces” the leaving group.

This is a “substitution” reaction :

Nu substitutes for X (takes its place).

slide4

IMPORTANT:

This is a reaction at sp3 (tetrahedral) carbon atoms.

sp3

sp2

sp

yes

no

Compounds that have sp2 or sp carbons generally

do not give nucleophilic substitution reactions.

….. of course there are a few special cases that do react,

but we won’t discuss them in this chapter.

slide6

EXAMPLE 1

*

(s)

soluble

insoluble

iodide displaces bromide at carbon

*

NaI is soluble in acetone, NaCl and NaBr are not

slide7

C

H

3

H

C

C

2

C

H

3

E1

competes

EXAMPLE 2

more about

this later

solvolysis of t-butyl chloride in water

water displaces chloride at carbon

*

The extra proton is taken by a second water molecule.

slide8

DISPLACEMENT REACTIONS

NUCLEOPHILIC SUBSTITUTION REACTIONS

( substitution at carbon )

can be compared to …..

ACID-BASE REACTIONS

( substitution at hydrogen )

slide9

COMPARE THESE SUBSTITUTIONS

DISPLACEMENT AT CARBON

nucleophile

substrate

product

leaving

group

DISPLACEMENT AT HYDROGEN

base

acid

conjugate

acid

conjugate

base

slide10

THESE REACTIONS HAVE A

WIDE RANGE OF SUBSTRATES

slide11

COMMON SUBSTRATES

( Leaving group varies )

see Table 10-3

alkyl halides

alcohols

alcohols

require acid

and then

H2O leaves

tosylates

ABBREVIATION

alkyl p-toluenesulfonate

slide12

THERE IS ALSO A

WIDE RANGE OF NUCLEOPHILES

A WIDE VARIETY OF COMPOUNDS CAN BE MADE

slide13

NUCLEOPHILES

see Table 10-4

A WIDE SELECTION OF NUCLEOPHILES MAKES POSSIBLE THE SYNTHESIS

OF MANY TYPES OF ORGANIC COMPOUNDS:

R-Y + Nu R-Nu + Y

Nucleophile

Product

Class

alkyl halides

R

X

alcohols

R

O

H

ethers

R

O

R

nitriles

R

C

N

O

esters

R

'

C

O

R

alkynes

R

'

C

C

R

thiols

S

H

R

slide14

THE NUCLEOPHILE DOES NOT HAVE TO BE CHARGED

+

-

H

O

H

O

R

+

R

B

r

+

B

r

H

H

O

H

Under some circumstances

water will react.

H

-

+

H

O

R

+

+

B

r

H

O

3

Nucleophile

Product

Class

alcohols

R

O

H

ethers

R

O

R

amines

R

N

H

2

amines

R

'

N

H

R

slide15

“LOOK-ALIKE” REACTIONS

RBr + NaOH ROH + NaBr

Two reactions follow....

Both appear to be identical and follow

the general pattern shown above.

However, they are different !

slide16

rate = k2 [RBr] [NaOH]

rate = k1 [RBr]

TWO “LOOK-ALIKE” REACTIONS

RBr + NaOH ROH + NaBr

1)

55oC

high conc. NaOH

2)

55oC

( + some alkene by E1,E2 )

low conc. NaOH

slide17

Notice that substrate in reaction 1 has

no b-hydrogens ….. why not?

slide18

REACTION 1

THE SN2 REACTION

slide19

55oC

rate = k2 [RBr] [NaOH]

k2 = 0.022 liter/mole-sec

-

bimolecular

concerted

SN2

substitution

nucleophilic

bimolecular

slide20

transition

state TS

Concerted (one step) reaction

Concerted (one step) reaction

SN2

E

N

E

Ea

R

G

Y

starting

D

H

material

product

slide21

REACTION 2

THE SN1 REACTION

slide22

55oC

rate = k1 [RBr]

k1 = 0.010 liter/mole-sec

unimolecular

two steps

slow

SN1

fast

substitution

also

nucleophilic

alkene (via E1)

unimolecular

slide23

TWO STEP REACTION

carbocation

SN1

intermediate

TS

1

E

TS

2

N

E

Ea

2

R

G

Ea

1

Y

step 1

step 2

starting

D

H

material

product

slide24

carbocation

intermediate

TS

1

TS

2

starting

material

TWO STEP REACTION

SN1

E

N

E

R

G

Ea

Y

1

Ea

2

DH

step 1

step 2

product