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DIRECTIVITY AND RING ACTIVATION / DEACTIVATION. ACTIVATED RING. Nitration of Anisole. HNO 3. +. H 2 SO 4. anisole. ortho. para. Reacts faster than benzene. = “ACTIVATED”. The -OCH 3 group when it preexists on the ring gives only

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Presentation Transcript
slide1

DIRECTIVITY AND

RING ACTIVATION / DEACTIVATION

slide2

ACTIVATED RING

Nitration of Anisole

HNO3

+

H2SO4

anisole

ortho

para

Reacts faster

than benzene

= “ACTIVATED”

The -OCH3 group when it preexists on the ring gives only

ortho and para products, and no meta.

Substituents that cause this result are called o,p directors

and they usually activate the ring.

slide3

DEACTIVATED RING

Nitration of Methyl Benzoate

HNO3

H2SO4

meta

methyl benzoate

Reacts slower

than benzene

= “DEACTIVATED”

The -COOMe group when it preexists on the ring gives only

meta, and no ortho or para products.

Substituents that cause this result are called m directors

and they usually deactivate the ring.

slide4

SUBSTITUENT CATEGORIES

Most ring substituents fall into one of these two categories:

o,p - directors

m- directors

activate the ring

deactivate the ring

We will look at one of each kind in order to

understand the difference…..

slide6

Nitration of Anisole

ortho

meta

para

activated

ring

actual

products

para

ortho

+

slide7

:

ortho

EXTRA!

meta

:

para

EXTRA!

slide8

RECALL:

HAMMOND

POSTULATE

Ea

Energy Profiles

NITRATION OF ANISOLE

benzenium

intermediate

meta

benzenium

intermediates

have more

resonance

ortho

para

ortho-para

director

slide9

BENZENIUM IONS GIVE ELIMINATION INSTEAD OF ADDITION

ADDITION REACTION

doesn’t happen

resonance would be lost

X

ELIMINATION REACTION

restores aromatic ring

resonance

( 36 Kcal / mole )

slide11

Nitration of Methyl Benzoate

ortho

meta

para

deactivated

ring

actual

product

meta

slide12

d-

d+

BAD!

ortho

meta

d-

d+

para

BAD!

slide13

Energy Profiles

NITRATION OF METHYL BENZOATE

some

benzenium

resonance

structures

have a bad

situation

ortho

para

meta

meta

director

slide15

ortho, para - Directing Groups

Groups that donate

electron density

to the ring.

PROFILE:

:

E+

increased

reactivity

+I Substituent

+R Substituent

..

These groups also

“activate” the ring, or

make it more reactive.

CH3-O-

..

CH3-

..

R-

CH3-N-

..

-NH2

..

The +R groups activate

the ring more strongly

than +I groups.

-O-H

..

slide16

meta - Directing Groups

Groups that withdraw

electron density from

the ring.

PROFILE:

d+

d-

E+

decreased

reactivity

-I Substituent

-R Substituent

These groups also

“deactivate” the ring,

or make it less reactive.

+

+

-

-SO3H

slide17

THE EXCEPTION

Halides - o,p Directors / Deactivating

Halides represent a special case:

..

:

:

They are o,p directing groups

that are deactivating

E+

They are o,p directors (+R effect )

They are deactivating ( -I effect )

Most other other substituents fall

into one of these four categories:

+R / -I / o,p / deactivating

1) +R / o,p / activating

-F

-Cl

-Br

-I

2) +I / o,p / activating

3) -R / m / deactivating

4) -I / m / deactivating

slide18

PREDICT !

o,p

m

o,p

m

slide20

GROUPS ACTING IN CONCERT

steric

crowding

o,p director

m-director

very

little

formed

HNO3

H2SO4

When groups direct to the

same positions it is easy to

predict the product.

major

product

slide21

GROUPS COMPETING

o,p-directing groups win

over m-directing groups

too

crowded

X

HNO3

+

H2SO4

slide22

RESONANCE VERSUS INDUCTIVE EFFECT

+R

HNO3

H2SO4

major

product

+I

resonance effects are more

important than inductive effects

slide23

SOME GENERAL RULES

1) Activating (o,p) groups (+R, +I) win over

deactivating (m) groups (-R,-I).

2) Resonance groups (+R) win over inductive (+I) groups.

3) 1,2,3-Trisubstituted products rarely form due to

excessive steric crowding.

4) With bulky directing groups, there will usually be more

p-substitution than o-substitution.

5) The incoming group replaces a hydrogen, it will not

usually displace a substituent already in place.

slide25

BROMINE - WATER REAGENT

PHENOLS AND ANILINES

slide26

BROMINE IN WATER

This reagent works only with highly-activated rings

such as phenols, anisoles and anilines.

..

..

..

..

..

..

-

:

:

:

:

:

..

..

..

..

+

..

..

:

+

..

+

bromonium

ion

etc

slide27

PHENOLS AND ANILINES REACT

Br2

H2O

All available

positions are

bromiated.

Br2

H2O

slide28

AROMATICITY

THE HUCKEL RULE

slide29

AROMATICITY

QUESTION:

Are all fully-conjugated, cyclic systems aromatic?

36 kcal/mole RE

?

?

?

KNOWN

AROMATIC

?

?

Do these other rings have the

same kind of stability as benzene?

slide30

AROMATICITY

..

HUCKEL 4n+2 RULE

Prediction: Compounds that have 4n+2 pi electrons in

a cyclic array will be aromatic.

4n+2 series = 2, 6, 10, 14, 18, 22, 26, 30 …….. etc.

The rule was derived by observation of

POLYCYCLIC AROMATIC COMPOUNDS

6

10

14

benzene

naphthalene

anthracene

14

18

slide31

Aromatic Compounds Have Special Properties

Aromatic compounds:

1) Must be cyclic and fully conjugated

2) Must have 4n+2 p electrons in the system

3) Must have the entire system planar

4) Will have no unpaired electrons in

the p system molecular orbitals

planar

system

Characteristic Properties:

1) Special chemical stability

2) Give substitution reactions instead of addition

3) Show a ring currrent in the NMR

(Chapter 13, Section 13.8)

slide32

A proton placed in the

middle of the ring would

be shielded!

Ring Current in Benzene

Circulating p electrons

Deshielded

H

H

Secondary magnetic field

generated by circulating p

electrons deshields aromatic

protons

Bo

slide33

RING CURRENTS CAN BE SEEN IN THE NMR

AROMATIC - SHOWS

A RING CURRENT

-1.0 ppm

2.0 ppm

18 p

inner hydrogens -1.8 ppm

outer hydrogens 8.9 ppm

-1.4 ppm

slide34

HUCKEL MNEMONIC

CYCLIC p MOLECULAR ORBITAL ARRAYS

1) Draw a circle

2) Inscribe the ring in the circle point down

3) Each point where the polygon (ring)

touches the circle represents an

energy level.

4) Place the correct number of electrons in

the orbitals, starting with the lowest

energy orbital first.

E

N

E

R

G

Y

slide35

BENZENE

Aromatic

compounds

will have all

of the occupied

p M.O. levels

completely

filled with no

unpaired

electrons.

6p electrons

AROMATIC

E

N

E

R

G

Y

closed*

shell

(*completed

level)

slide36

CYCLOBUTADIENE

ANTI-AROMATIC

Does not have a completed

shell and has unpaired electrons.

4p electrons

Does not have 4n+2 p electrons.

open*

shell

E

N

E

R

G

Y

.

.

(*incomplete

level)

DIRADICAL

slide37

CYCLOOCTATETRAENE

ANTI-AROMATIC

8p electrons

Does not have a completed

shell and has unpaired electrons.

Does not have 4n+2 p electrons.

open*

shell

E

N

E

R

G

Y

not planar

(*incomplete

level)

slide38

SOME CYCLIC POLYENES

ANNULENES

10 p = 4(2) + 2

16 p

BUT CANNOT BE PLANAR

(see the hydrogens)

[10]-annulene

[16]-annulene

12 p

18 p = 4(4) + 2

AROMATIC

[12]-annulene

[18]-annulene

[20]-annulene

14 p = 4(3) + 2

20 p

AROMATIC

[14]-annulene

slide39

HETEROCYCLIC COMPOUNDS

The unshared pairs are in the

cyclic pi system (one pair in each case).

..

..

..

..

..

..

pair not in

p system

pyridine

pyrrole

furan

thiophene

All have 6p electrons in a cyclic array.

These compounds have reactions similar to benzene,

rather than to alkenes. They will give substitution reactions

under conditions similar to those for benzene.

slide40

CYCLOPENTADIENYL ANION AND CATION

The methylene

hydrogens are

acidic.

This compound

does not dissolve

in water.

NaOEt

EtOH

X

6p

4p

..

AROMATIC

ANTI-AROMATIC

-

+

-

Cl

The cation does not

form at all.

The anion forms readily.

slide41

CYCLOHEPTATRIENYL ANION AND CATION

This compound

ionizes easily in

water.

The methylene

hydrogens are

not acidic.

X

NaOEt

EtOH

8p

6p

..

+

-

ANTI-AROMATIC

AROMATIC

-

Cl

Doesn’t form

easily.

Dissolves in water.