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E2. ALKYL HALIDES + STRONG BASE + HEAT. ....... continued. SUMMARY TO DATE. ELIMINATION REACTIONS OF ALKYL HALIDES. STRONG BASE Required. HEAT. Usually required. REGIOSELECTIVE. Follows Zaitsev Rule (Unless Stereochemistry Prevents) - favors most substituted alkene .

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Presentation Transcript
slide1

E2

ALKYL HALIDES + STRONG BASE + HEAT

....... continued

slide2

SUMMARY TO DATE

ELIMINATION REACTIONS OF ALKYL HALIDES

STRONG BASE Required

HEAT

Usually

required

REGIOSELECTIVE

Follows Zaitsev Rule (Unless Stereochemistry Prevents)

- favors most substituted alkene

STEREOSPECIFIC

b-H and X must be ANTI-COPLANAR

- acyclics may have to rotate

- rings may have to invert

slide3

KINETICS

Examination of the rate expression often helps

to understand the mechanism.

The rate expression is determined by experiment.

slide4

RATE EXPRESSIONS

exponents

n

m

K

[A]

[B]

RATE =

concentrations of reactants

A and B in moles / liter

rate “constant”

Actually will change with

temperature and solvent,

the specific molecule, etc.

REACTION ORDER = SUM OF EXPONENTS = n + m

Not all reactants will necessarily show up in the rate expression.

3/2

2

Fractional rate orders are possible :

RATE = K [A] [B]

slide5

CH3CH2

CH

CHCH3

CH3CH2

CH

CHCH3)

This equation explains the

rate behavior :

Rate = K [RBr] [OCH3]

KINETICS

CH3CH2CH2

CH

+

NaOCH3

CH3

35oC

Br

+

CH3OH

Rate = + d (

dt

REACTION RATE DATA

Rate [RBr] [OCH3]

x1 x1 x1

x2 x2 x1

x2 x1 x2

x4 x2 x2

x8 x4 x2

x9 x3 x3

second order rate

slide6

MECHANISM

Elimination

Bimolecular

strong

base

E

2

alkyl

halide

CONCERTED = only one step

All bonds are broken and formed without

the formation of any intermediates.

slide7

Reaction Order

Sum of the exponents of the concentration terms

in the rate expression.

Molecularity

Number of species that come together in the

rate-determining step.

Rate-determining Step

The slowest step in the reaction sequence.

Transition State

An energy highpoint in the energy profile

of a reaction.

Activated Complex

The species that exists at the transition state.

slide8

E2

CONCERTED REACTION

One Step - No Intermediates

slide9

E2 ELIMINATION

d-

d-

activated

complex

mechanism

Concerted : everything

happens at once with-

out any intermediates.

slide10

Concerted (one step) reaction

transition

state TS

E

N

E

R

G

Y

activation

energy Ea

heat of

reaction

DH

starting

material

product

This is what E2 looks like.

slide11

ORBITAL ALIGNMENT

IN MOST CONCERTED REACTIONS THE

ORBITALS BECOME PREALIGNED FOR

A SMOOTH PROGRESSION OF EVENTS

The anti-coplanar arrangement of theb-H and

the halide leaving group X places the orbitals

that undergo change in a perfect alignment.

The coplanar arrangement allows a continuous

movement of electrons from one end of the system

to the other, much like a stack of dominoes each

pushing the next one over.

The two orbitals that will form the pi bond are

already parallel (anti-coplanar) so that the double

can form easily.

slide12

..

When these electrons enter the

back lobe of the adjacent orbital

they “push” the bonding pair out

the other end (along with Br).

CH3

:

O

..

H

..

R

The critical event is

the removal of the b-H.

sp3

H

C

C

H

R

..

sp3

:

:

Br

..

The attack of the base on the

b-hydrogen starts the reaction.

Notice the parallel aligment

of the two sp3 orbitals.

slide13

Note the parallel orbitals

in the pi bond.

..

CH3

O

H

..

.

.

2p

2p

H

R

C

C

R

H

..

:

:

Br

The formation of the double bond

and the loss of bromide finish it.

..

slide14

FRONTIER MO THEORY

The LUMO is present on the b-H only in the

anti -coplanar arrangement.

slide15

ANTI CONFORMATION

LUMO has

density on H

RECALL :

-

B:

Frontier theory

requires a base

or nucleophile

to add to the

LUMO.

LUMO

-

B:

HOMO

LUMO

negative end

of molecule

DENSITY-ELPOT

slide16

SYN CONFORMATION

-

B:

LUMO has

no density

on any H

DENSITY-ELPOT

LUMO

slide17

KINETIC ISOTOPE EFFECT

The reaction slows if the b-H is replaced by D.

This “kinetic isotope effect” shows that breaking

the b C-H bond is a part of the rate-determining

step.

slide18

ISOTOPES OF HYDROGEN

NAME

SYMBOL

MASS

COMPOSITION

H

1

1 proton + 1 electron

PROTIUM

99.985%

D

DEUTERIUM

2

1 proton + 1 neutron

+ 1 electron

0.015%

T

1 proton + 2 neutrons

+ 1 electron

TRITIUM

3

radioactive

( b- ) 12.26 yrs

slide19

ISOTOPE EFFECT

B

B

H

D

C-H

C-D

H

C

H

C

H

C

H

C

B

r

B

r

C-D bond is stronger than C-H

kH

Slows the reaction

if breaking this bond

is part of the

rate-determining step.

approx. 5-8

kD

for an isotope effect

slide20

ORIGIN OF THE ISOTOPE EFFECT

The effect is due to differences

in C-H and C-D bond strengths.

slide21

+

+

vibrational

energy

levels

+

+

zero-point

energy

CH

CD

+

+

average bond

length

BONDING CURVE

(D) 1s

C(sp3)

(H)1s

+

+

r

ENERGY

+ = nucleus

Since D is heavier than H the

C-D bond vibrates slower over

a shorter distance.

o

o

r (bond distance)

slide22

C-D BONDS ARE STRONGER THAN C-H BONDS

bond breaks

here

ENERGY

bond dissociation

energies (CD > CH)

CH

CD

o

o

r (bond distance)

D is heavier than H and the

CD bond vibrates more slowly

over a shorter distance than CH.

slide24

Three types of elimination reactions are conceivable

concerted

E2

just studied

carbocation

E1

halogen

first

proton

second

carbanion

E1cb

proton

first

halogen

second

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