BENZENE AND AROMATICITY

2ND SHIFTING - ORGCHEM LEC

Aromatic hydrocarbons

• do not undergo electrophilic addition

• Considering the structure of benzene ring, the structures has a special type of bonding depicting both localized and delocalized bonds.

Benzene

• C6H6

• simplest aromatic hydrocarbon with a special type

  of bonding that stabilizes the structures

-benzene

used to refer to benzene ring serving as a parent name

e.g. nitrobenzene, methylbenzene, aminobenzene

“phenyl-” (Ph)

used to refer to benzene ring serving as a substituent.

e.g. phenylamine, ethylphenyl ether

“aryl group” (Ar)

general aromatic hydrocarbon substituent of benzene

monosubstitution

benzene is used as parent name

e.g. methylbenzene, chlorobenzene, hydroxybenzene

disubstitution

-ortho, -meta, and -para are terms used to refer to the

position of one of the substituents in reference to the position of the other.

ortho (o-)

1,2-substitution on the ring

meta (m-)

1,3-substitution on the ring

para (p-)

1,4-substitution on the ring

trisubstitution

numbers are assigned to refer to the position

of the substituents in the ring. The lowest set of numbering system is used

and substituents are arranged in alphabetical order.

Aromatic compounds

• represented by benzene (simplest) are cyclic, conjugated, stable, planar and undergoes substitution reaction.

• follow Hückel’s Rule.

Hückel’s Rule

• predicts that a compound which is planar, cyclic, has

  a conjugated double bonds will have total of 4n + 2 pi-electrons.

• important determinant of aromaticity.

Carboxylic Aromatic Compounds

• toluene

• aniline

• phenol

• xylene

• cresol

• styrene

• benzaldehyde

• anthracene

• phenanthrene

• benzo [a] pyrene

Heterocyclic Aromatic Compounds

• pyrrole

• imidazole

• furan

• thiophene

• pyridine

• pyrazine

• pyrimidine

• indole

• purine

• quinoline

toluene

aniline

phenol

xylene

cresol

styrene

benzaldehyde

anthracene

phenanthrene

benzo [a] pyrene

pyrrole

imidazole

furan

thiophene

pyridine

pyrazine

indole

purine

pyrimidine

quinoline

polar mechanism

Aromatic compound (e.g. benzene) reaction follows a _____

which creates an electrophile (R+). The specific mechanism is called

electrophilic substitution reaction (SE).

Halogenation, Nitration, Sulfonation, Friedel-Crafts Alkylation, Friedel-Crafts Acylation

common electrophilic substitution reaction (SE)

Halogenation

• substitution of halogens (X2) to a benzene ring

• Aromatic compound can be substituted with a polarized halogen which serves as an electrophile (+X).

FeBr3

HALOGENATION:

catalyst of Br2

FeCl3

HALOGENATION:

catalyst of Cl2

H2O2 or CuCl2

HALOGENATION:

catalyst of I2

heat (~500°C)

HALOGENATION:

catalyst of CuF2

polarized halogen (X+)

STEPS IN HALOGENATION:

1. The catalyst aids in creating a (a=?). This creates the electrophile to be added to the aromatic compound.

2. The polarized electrophile (b=?) electrons

   and breaking it in order for the electrophile to attach to

   the ring and create a carbocation intermediate.

3. The polarized catalyst attracts the ring -H and the electrons

   are delocalized back to the carbocation. The (c=?) in the process.

a=

attracts the pi bond

STEPS IN HALOGENATION:

1. The catalyst aids in creating a (a=?). This creates the electrophile to be added to the aromatic compound.

2. The polarized electrophile (b=?) electrons

   and breaking it in order for the electrophile to attach to

   the ring and create a carbocation intermediate.

3. The polarized catalyst attracts the ring -H and the electrons

   are delocalized back to the carbocation. The (c=?) in the process.

b=

catalyst is restored

STEPS IN HALOGENATION:

1. The catalyst aids in creating a (a=?). This creates the electrophile to be added to the aromatic compound.

2. The polarized electrophile (b=?) electrons

   and breaking it in order for the electrophile to attach to

   the ring and create a carbocation intermediate.

3. The polarized catalyst attracts the ring -H and the electrons

   are delocalized back to the carbocation. The (c=?) in the process.

c=

Nitration

substitution of a nitro group (+NO2) to a benzene ring

H2SO4

catalyst of Nitration

ArNO2

product of Nitration

Fe/SnCl2

reducing agents of Nitration

arylamine (ArNH2)

product of Nitration with reducing agents

+NO2

STEPS IN NITRATION:

1. The catalyst removes a -OH from HNO3 producing the (a=?).

   The pi-electrons then bonds with +NO2.

2. Water acts as a base and bonds with the ring -H. The electrons

   from the C-H bonds moves back to the carbocation forming

   the final product, (b=?).

a=

nitrobenzene

STEPS IN NITRATION:

1. The catalyst removes a -OH from HNO3 producing the (a=?).

   The pi-electrons then bonds with +NO2.

2. Water acts as a base and bonds with the ring -H. The electrons

   from the C-H bonds moves back to the carbocation forming

   the final product, (b=?).

b=

Sulfonation

substitution of +SO3H (sulfonic acid) to a benzene ring

fuming H2SO4

reagent and catalyst of Sulfonation

+SO3H, sulfonic acid

product of Sulfonation

activated electrophile +SO3H

STEPS IN SULFONATION:

Simultaneously, as the H2SO4 reacts with SO3 to form the (a=?),the electrons in the double-bond is drawn to the electrophilic +S atom.

As sulfonic functional group attaches to the ring, the -H attached to the target C is cleaved and the electrons is delocalized to the nearby carbon to restore the ring double bond.

a=

Friedel-Crafts Alkylation

• substitution with +R (alkyl group) to a benzene ring

• The reaction do not succeed on aromatic rings with initial

  substitution of highly electron-withdrawing groups or basic amino group for protonation e.g. —NO2, -CN, -SO3H, and/or -COR

• Hydride shift / Alkyl shift may occur to form more stable product.

aluminum halide (AlX3)

catalyst of Friedel-Crafts Alkylation

alkyl electrophile (R+, alkyl)

product of Friedel-Crafts Alkylation

electrophile, R+

STEPS IN FRIEDEL-CRAFTS ALKYLATION:

1. The AlCl3 catalyst aids in the break down of the alkyl halide producing the (a=?).

2. The pi electrons (b=?) to the electrophile, R+.

3. The catalyst (c=?), removing it from the ring. The electrons from the C-H bond delocalizes back to the ring.

a=

bonds

STEPS IN FRIEDEL-CRAFTS ALKYLATION:

1. The AlCl3 catalyst aids in the break down of the alkyl halide producing the (a=?).

2. The pi electrons (b=?) to the electrophile, R+.

3. The catalyst (c=?), removing it from the ring. The electrons from the C-H bond delocalizes back to the ring.

b=

binds the H

STEPS IN FRIEDEL-CRAFTS ALKYLATION:

1. The AlCl3 catalyst aids in the break down of the alkyl halide producing the (a=?).

2. The pi electrons (b=?) to the electrophile, R+.

3. The catalyst (c=?), removing it from the ring. The electrons from the C-H bond delocalizes back to the ring.

c=

Hydride/Methide shift

when a negatively-charged hydrogen (-H, hydride) or a negatively-charged alkyl, usually a methyl (-CH3, methide) is translocated to a nearby carbocation to form a more stable carbocation.

Friedel-Crafts Acylation

substitution with +COR (acyl group) to a benzene ring

aluminum halide (AlX3) [AlCl3 and AlBr3]

catalyst of Friedel-Crafts Acylation

alkyl electrophile (+COR, acyl)

product of Friedel-Crafts Acylation

Hydrogenation, Bromination of Alkyl Side Chain, Oxidation of Alkyl Side Chains, Reduction of Aryl Alkyl Ketones

Reaction of Benzene (Aromatics) that does not follow the SE mechanism

Hydrogenation

• addition of hydrogen (H2) to a benzene ring to create a saturated product.

H2 atom

added to Hydrogenation

Pt or Rh (Rhodium) metal catalyst under high pressure

catalyst of Hydrogenation

Bromination

• addition of bromine (Br2) to an alkyl side chain of a benzene ring

• occurs on the benzylic position with an alkylbenzene

N-bromosuccinimide (NBS) and benzoyl peroxide (Ph(CO2)2)

radical initiator of Bromination

Oxidation

• complete oxidation of alkyl side chain on a benzene ring.

• The alkyl side chain will only be oxidized if there is a benzylic hydrogen (H attached to the benzylic carbon) present in the structure.

KMnO4 in H2O

catalyst of Oxidation

COOH

product of Oxidation

Reduction

reduction oxygen bonds in a ketone (C=O) and nitro group (-NO2)

catalytic hydrogenation

Both aryl alkyl ketones (product of FC Acylation) and nitro substituted

(product of nitration) can be reduced

H2 / Pd

catalyst of reduction

reactivity and orientation

Substituents can affect SE reactions in 2 ways:

Reactivity

• can be affected by any substitution.

• It can activate (increase reactivity) and deactivate (decrease reactivity) of the ring.

• The reactivity of the ring can affect whether subsequent reactions can still take place on the ring.

Orientation

• consequent substitutions attached on the ring are affected by pre-existing substituents.

• Substitutions can be directed at the ortho-, meta-, and para- positions of the ring.

Acyl containing (aldehyde, ketone)`

Acid and Ester if benzene is directly bonded to carbon

Amino (3 degree)

Nitro

Cyano

Sulfonyl

Meta-Directing Deactivators

Halogens (-X)

I, Br, Cl, F

Ortho- and Para-Directing Deactivators

Alkyl (-R)

Aryl/Phenyl (Ar-)

Alkoxy (-OR)

Hydroxy (OH-)

Thiol (SH-)

Amino (-NH2, 1 degree, 2 degree)

Amido (-NO2)

Ester

Ortho- and Para-Directing Activators