ORGANIC CHEMISTRY CHM 207

CHAPTER 4:AROMATIC COMPOUNDS

(BENZENE AND TOLUENE)

NOR AKMALAZURA JANI

Aromatic compounds

• Organic compound that contains a benzene ring in its molecule is known as an aromatic compounds.

• Sometimes called arenes.• Molecular formula: C6H6

• Represented as a regular hexagon containing an inscribed circle.

• The corner of each hexagon represents a carbon and a hydrogen atom.

• Can be represented in two abbreviated ways.

Structure of Benzene

Kekulé Structure of Benzene

Each carbon atom must have four covalent bonds.

Molecular formula is C6H6

All the hydrogen atoms are equivalent

Resonance Structure

• Resonance theory: the structure of benzene is a resonance hybrid structure of two Kekulé cononical forms.

• The hybrid structure is often represented by a hexagon containing an inscribed circle.

represents a resonance hybrid between the two

• Hexagonal ring – 6 carbon-carbon bonds are equal.

• Circle – delocalised electrons of the benzene ring

CRITERIA OF AROMATIC COMPOUNDS

• Structure must be cyclic, containing some number of conjugated pi bonds.

• Each atom in the ring must have an unhybridized p orbital. (The ring atoms are usually sp2 hybridized or occasionally sp hybridized).

• The unhybridized p orbitals must overlap to form a continuous ring of parallel orbitals. The structure must be planar (or nearly planar) for effective overlap to occur.

• Delocalization of the pi electrons over the ring must lower the electronic energy.

* Antiaromatic compound: fulfills the first three criteria, but delocalization of the pi electrons over the ring increase the electronic energy.

Huckel’s rule

• Used to determine aromaticity for planar, cyclic organic compounds with a continous ring of overlapping p-orbitals.

• If the number of pi (π) electrons in the monocyclic system is (4N+2), the system is aromatic. N is 0, 1, 2, 3…..

• Systems that have 2, 6 and 10 pi electrons for N = 0, 1, 2 is a aromatic.

• Systems that have 4, 8, and 12 pi electrons for N = 1, 2, 3 are antiaromatic.

Naming Aromatic Compounds

Naming Aromatic Compounds

• A substituted benzene is derived by replacing one or more of benzene’s hydrogen atoms with an atom or group of atoms.

• A monosubstituted benzene has the formula C6H5G where G is the group that replaces a hydrogen atom.

• All hydrogens in benzene are equivalent.

• It does not matter which hydrogen is replaced by G.

Monosubstituted Benzenes

Monosubstituted Benzenes

• Some monosubstituted benzenes are named by adding the name of the substituent group as a prefix to the word benzene.

• The name is written as one word.

nitrobenzene

nitro group

ethylbenzene

ethyl group

• Certain monosubstituted benzenes have special names.

• These are parent names for further substituted compounds.

methyl group

toluene

hydroxy group

phenol

carboxyl group

benzoic acid

aniline

amino group

Disubstituted BenzenesDisubstituted Benzenes

• Three isomers are possible when two substituents replace hydrogen in a benzene molecule.

• The prefixes ortho-, meta- and para- (o-, m- and p-) are used to name these disubstituted benzenes.

ortho-dichlorobenzene(1,2-dichlorobenzene)mp –17.2oC, bp 180.4oC

ortho disubstituted benzene

substituents on adjacent carbons

meta-dichlorobenzene(1,3-dichlorobenzene)mp –24.82oC, bp 172oC

meta disubstituted benzene

substituents on adjacent carbons

para-dichlorobenzene(1,4-dichlorobenzene)mp 53.1, bp 174.4oC

para disubstituted benzene

substituents are on opposite sides of the benzene ring

phenol 3-nitrophenol

When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.

When one substituent corresponds to a monosubstituted benzene with a special name, the monosubstituted compound becomes the parent name for the disubstituted compound.

toluene 3-nitrotoluene

Tri- and Polysubstituted Benzenes

Tri- and Polysubstituted Benzenes

• When a benzene ring has three or more substituents, the carbon atoms in the ring are numbered.

• Numbering starts at one of the substituent groups.• The numbering direction can be clockwise or

counterclockwise.• Numbering must be in the direction that gives the

substituent groups the lowest numbers.

4

6

5

2

3

1

clockwise numbering

1,4,6-trichlorobenzene

4-chloro

1-chloro

6-chloro

4

2

3

6

5

1

counterclockwise numbering

1,2,4-trichlorobenzene

4-chloro

1-chloro

2-chloro

chlorine substituents have lower numbers

• When a compound is named as a derivative of the special parent compound, the substituent of the parent compound is considered to be C-1 of the ring.

toluene

5

16

34

2 5

16

34

2

2,4,6-trinitrotoluene

(TNT)

• When the hydrocarbon chain attached to the benzene ring is small, the compound is named as benzene derivative.

• Example:

CH2CH3

ethylbenzene

Naming compounds that cannot be easily named as benzene derivatives

diphenylmethane4-phenyl-2-pentene

Benzene named as a substituent on a molecule with another functional group as its root by the prefix phenyl.

The phenyl group, C6H5-

CH=CH2 NH2 CH2Cl

CH2

phenylethene phenylamine benzyl chloridecommonname

phenyl benzyl

• If the hydrocarbon chain contains more than three carbon atoms, phenyl is used as part of the name.

• Examples:

CH2(CH2)5CH3

1-phenylheptane

C

Br

CH3

CH2 CH3

2-bromo-2-phenylbutane

PHYSICAL PROPERTIES OF BENZENE AND ITS DERIVATIVES

• Benzene derivatives tend to be more symmetrical than similar aliphatic compounds, and pack better into crystals and have higher melting points.

• Density:- Slightly dense than non-aromatic analogues, but still less dense than water.- halogenated benzenes are denser than water.

• Insoluble in water• Boiling points depends on the dipole moments of

compounds.

REACTION OF BENZENEELECTROPHILIC SUBSTITUTION REACTIONS OF

BENZENE

stability of π-electron system is lost when benzene undergoes addition reactions.

benzene and its derivatives undergo substitution reaction rather than addition reactions.

product of substitution reactions: aromatic compounds and not saturated compounds.

Mechanism of electrophilic substitution Mechanism of electrophilic substitution of benzeneof benzene

Step 1: Electrophilic addition of the benzene ring

E+E

H

slow

arenium ion (a carbocation)

Step 2: Deprotonation of the arenium ion

EH

Nu- fast

nucleophile

E

H Nu

ELECTROPHILIC SUBSTITUTION REACTIONS

H

H

H

X2

HNO3

SO3

H2SO4

H2SO4

H2SO4

X

NO2

SO3H

HX

2H2O

a) Halogenation

or FeX3

b) Nitration

halobenzene

nitrobenzene

c) Sulphonation

benzenesulphonic acid

H

H

CH3Cl

CH3CCl

O

AlCl3

AlCl3

CH3

C CH3

O

HCl

HCl

d) Friedel-Crafts alkylation

e) Friedel-Crafts acylation

toluene

acetophenone

ELECTROPHILIC SUBSTITUTION REACTIONS

Reagents, electrophiles and catalysts in electrophilic substitution reactions

Reactions Reagents Catalysts Electrophiles

Halogenation Cl2 or Br2 AlCl3, AlBr3, FeCl3 or FeBr3

Cl , Br

Nitration HNO3 H2SO4 NO2

Alkylation RCl

RCH=CH2

AlCl3

H2SO4

R

RCH-CH3

Acylation RCOCl AlCl3

RCO

Sulphonation SO3 H2SO4 SO3H

HALOGENATION OF BENZENE

Cl2

Br2

AlCl3

FeBr3

Cl

Br

HCl

HBr

a)Chlorination

b)Brominationchlorobenzene

bromobenzene

1/2I2

I

NO2

c) Iodination

iodobenzene

HNO3H2O

MECHANISM: BROMINATION OF BENZENE

H

H

H

H

H

H

Br Br

H

H

H

H

H

HBr

FeBr3

FeBr4-

Br Br FeBr3

Br Br FeBr3

H

Br

H

H

H

H

H

H

H

H

H

HBr

FeBr4-

HBr

H

H

H

H

H

HBr

FeBr3

H

H

H

H

H

HBr

Step 1: Formation of a stronger electrophile

Br2.FeBr3 intermediate(a stronger electrophile than Br2)

Step 2: Electrophilic attack and formation of the sigma complex

sigma complex

Step 3: Loss of a proton gives the products

Step 1: Formation of the nitronium ion, NO2+

Step 2: Formation of an arenium ion as a result of electrophilic addition

Step 3: Loss of a proton gives the products

HO SO3 H HO NO2 H2O + NO2+ + HSO4

-

NO2+

H NO2

arenium ionnironium ion

slow

H NO2

HSO4-

fast

NO2

H2SO4

MECHANISM: NITRATION OF BENZENE

C ClH

CH3CH3

AlCl3

C CH3

H

CH3

C

H

CH3

CH3

H CH(CH3)2

AlCl4-

CH(CH3)2

H CH(CH3)2

HCl + AlCl3

AlCl4-

Step 1: Formation of electrophile

Step 2: Formation of an arenium ion

Step 3: Loss of a proton

arenium ion

carbocation (electrophile)

MECHANISM: FRIEDEL-CRAFTS ALKYLATION

CH3 C Cl

O

AlCl3

CH3 C

O

AlCl4-

H C

O

CH3

CH3 C

O

C

O

CH3

H C

O

CH3

AlCl4-

HCl + AlCl3

Step 1: Formation of electrophile

Step 2: Formation of an arenium ion

Step 3: Loss of a proton

MECHANISM: FRIEDEL-CRAFTS ACYLATION

Ortho-Para and Meta Directing Substituents

• When substituted benzenes undergo further substituents, the substituent group present in the benzene derivative will influence electrophilic substitution in 2 ways which are:i) Reactivityii)Orientation

EFFECTS OF SUBSTITUENTS ON THE REACTIVITY OF ELECTROPHILIC

AROMATIC SUBSTITUTION

• Substituent group present in the benzene ring can influence the rate of reaction of further substitutions.

• Electron-donating groups make the ring more reactive (called activating groups) thus influence the reaction become faster.

• Electron-withdrawing groups make the ring less reactive (called deactivating groups) thus influence the reaction become slower.

• A substituents group already in the ring influences the position of further electrophilic substitution whether at ortho, meta or para position.

• Ortho-para directors: the groups that tend to direct electrophilic substitution to the C2 and C4 positions.

• Meta directors: the groups that tend to direct electrophilic substitution to the C3 position.

EFFECTS OF SUBSTITUENTS ON THE ORIENTATION OF

ELECTROPHILIC AROMATIC SUBSTITUTION

Effetcs of substituent groups on the benzene ring

Activating groups (electron donating)

Deactivating groups

(electron-withdrawing)

-NH2 -R

-OH

-OR

-NHCOCH3

-F

-Cl

-Br

-I

ortho-para directors ortho-para directors

meta directors

C

O

R

C

O

OH

C

O

OR

SO3H

C N

NO2

NR3

CH2CH3

Br2

FeBr3

CH2CH3Br

CH2CH3

Br

CH2CH3

Br

Example:

ortho position para position meta position

major products minor product

-CH2CH3 = ortho and para directors

NO2

Br2

FeBr3

NO2

Br

NO2Br

NO2

Br

Example:

ortho position para positionmeta position

minor productsmajor product

-NO2 = meta director

REACTIONS OF BENZENE DERIVATIVES

• Alkylbenzene such as toluene (methylbenzene) resembles benzene in many of its chemical properties.

• It is preferable to use toluene because it is less toxic.

• The methyl group activates the benzene nucleus.• Toluene reacts faster than benzene in all

electrophilic substitutions.

Reactions of toluene

Reactions of the methyl group

Reactions of the benzene ring

Substitution-halogenation

Oxidation

Electrophilic substitutions- Halogenation- Nitration- Friedel-Crafts reactions- Sulfonation

Addition reaction-hydrogenation

SIDE-CHAIN REACTIONS

OXIDATION REACTION OF ALKYLBENZENE

CH2 R C

O

OHhot, conc., KMnO4/H+

reflux

examples:

CH3 C

O

OHhot, conc., KMnO4/H+

reflux

CH2 CH3 C

O

OHhot, conc., KMnO4/H+

reflux

CH3hot, conc., KMnO4/H+

refluxCH3 COOH

COOH

HALOGENATION OF TOLUENE

CH3

Cl2

CH2 Cl

Cl2

CHCl2

Cl2

CCl3

CHCl2

CH2 Cl

HCl

HCl

HCluv light

(chloromethyl)benzene

uv light

(dichloromethyl)benzene

uv light

(trichloromethyl)benzene

Side chain substitution

* Bromination of toluene takes place under similar conditions to yield corresponding bromine derivatives.

SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS

NO2

Cl

?

Synthesis m-chloronitrobenzene starting from benzene

• Two substituents: -NO2 (meta-directing) and –Cl (ortho- and para-directing)• Cannot nitrate chlorobenzene because the wrong isomer (o- and p-chloronitrobenzenes) would formed.

NO2

Cl

HNO3

H2SO4

NO2

NO2

Cl

Cl2

FeCl3

NO2

Cl

HNO3, H2SO4

Cl2, FeCl3 m-chloronitrobenzene

chlorobenzene

nitrobenzene

TWO STEPS:

nitrobenzene m-chloronitrobenzenebenzene

SYNTHESIZING A SUBSTITUTED AROMATIC COMPOUNDS

COOH

Br

?

Synthesis p-bromobenzoic acid starting from benzene

• Two substituents: -COOH (meta-directing) and –Br (ortho- and para-directing)• Cannot brominated benzioc acid because the wrong isomer (m-bromobenzoic acid) would formed.• Oxidation of alkylbenzene side chains yields benzoic acids.• Intermediate precursor is p-bromotoluene

COOH

BrBr

CH3KMnO4

Immediate precursor of p-bromotoluene:i)Bromination of toluene

orii) Methylation of bromobenzene

CH3Br2

FeCl3

CH3

Br

CH3

Br

separate the isomeror

CH3Cl

AlCl3

CH3

Br

CH3

Br

separate the isomer

Br

Immediate precursor of toluene:i)Benzene was methylated in a Friedel-Crafts reaction

CH3CH3Cl

AlCl3

toluenebenzene

Immediate precursor of bromobenzene:i)Bromination of benzene

Br2

FeBr3

bromobenzenebenzene

Br

Br2

FeBr3

CH3Cl

AlCl3

Br

CH3

AlCl3

CH3Cl

Br2

FeBr3

Br

CH3KMnO4

Br

COOH

benzene

TWO WORKABLE ROUTES FROM BENZENE TO p-BROMOBENZOIC ACID

• Benzene:Benzene:- as solvent for oils and fats - starting material for making other chemicals. For example, benzene is used in the cumene process to produce phenol.- making organic compounds such as phenylethene (styrene) and nitrobenzene. These organic compounds are then used to make plastics (polystyrene), dyes and nylon.

USES OF BENZENE AND TOLUENE

• Toluene:Toluene:

- A common solvent, able to dissolve paints, paint thinners, silicone sealants, many chemical reactants, rubber, printing ink, adhesives (glues), lacquers, leather tanners and disinfectants.- As a solvent to create a solution of carbon nanotubes.- Dealkylation to benzene (industrial uses).- As an octane booster in gasoline fuels used in internal combustion engines.-As a coolant in nuclear reactor system loops.

USES OF BENZENE AND TOLUENE