Arenes

Introduction

1. Hydrocarbon that contain rings which stabilised by electron delocalisation are called aromatic hydrocarbon or arenes

2. The simplest yet most important member of the aromatic hydrocarbons is benzene

Naming Aromatic Hydrocarbon

• Monosubstituted alkylbenzene is named as derivatives of benzene. For example:

CH3

methylbenzene(Toluene)

CH2CH3

ethylbenzene

OH

Phenol

COOH

Benzeoic acid

CHO

Benzaldehyde

NH2

Aniline(Phenylamine)

CH=CH2

Phenylethene(Styrene)

• The IUPAC system retains certain common names for some simpler alkylbenzenes. For example: Aniline, styrene, toluene.

• When two or more substituents on a benzene ring, three structural isomers are possible.a) Location of two substituents may be indicated by the

prefixs ortho, meta or para– 1,2 is equivalent to ortho, o-– 1,3 is equivalent to meta, m-– 1,4 is equivalent to para, p-

b) The order of decreasing priorities of common substituents is:COOH > SO3H> CHO> CN > C=O> OH> NH2> R> NO2> XWhere R= alkyl group ;

X= halogen

CH3

1,2-dimethylbenzeneo-xylene

CH3 OH

Phenol

COOH

Benzeoic acid

CH312

3

4

5

6

CH3

1,4-dimethylbenzenep-xylene

CH3

Br

CH3

Br

CH3

Br

1

2

34

5

6

2-bromomethylbenzeneo-bromotoluene

3-bromomethylbenzenem-bromotoluene

4-bromomethylbenzenep-bromotoluene

OH

NO2

OH

NO2

1

2

34

5

6

2-nitrophenolo-nitrophenol

4-nitrophenolp-nitrophenol

• When three or more groups are on the benzene ring, a numbering system must be used to locate them. For example,

CH3

NO2

NO2

1

2

3

45

6

CH3

Br

NO2

1

2

3

4

5

6

2,4-dinitromethylbenzene 2-bromo-4-nitrophenol

• In more complex molecules, the benzene ring is named as a substituent on a parent chain. In this case, the C6H5- group is called a phenyl group. For example

CH2CH CHCH3

1 2 3 4

1-phenylbut-2-ene

CHCH2CH2CH3

CH31

2 3 4 5

2-phenylpentane

Exercise

• Name the following aromatic compoundsCH2CH3 NO2 Cl CH3

Cl

CH3

NO2

CH3

O2N

NO2

COOH

H3C

COOH

HO

Reaction of benzene

1. Nitration• Nitration is the substitution of a nitro group, -

NO2, for one of the hydrogen atoms on the benzene ring

• Benzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid

• The product is nitrobenzene, a pale yellow liquid

+ HNO3

NO2

concentrated H2SO4

<55oCrefulx

+ H2O

2. Halogenation– This reaction involves substitution of a halogen

atom for a hydrogen atom on the aromatic ring– In the presence of a catalyst or halogen carrier

such as aluminium chloride, AlCl3, benzene is substituted by chlorine or bromine at room temperature.

+ Cl2

AlCl3

room temp

Cl

+ HCl

• The catalyst aluminium chloride, AlCl3, an electron-deficient compound which has only six electrons in its outer shell, accepts a lone pair of electrons from one of the chlorine atoms, which induces polarisation in the chlorine molecule.

• The positively charged end of the chlorine molecule is now an electrophile and attacks the benzene ring.

Cl Cl -

AlCl3

3. Alkylation: Friedel-Crafts reaction• This reaction substitutes an alkyl group for a

hydrogen atom on an aromatic ring• When benzene is warmed with

chloromethane, CH3Cl and aluminium chloride (catalyst) under anhydrous conditions, a substitution reaction occurs and methylbenzene is formed

+ CH3Clanhydrous AlCl3

warm

CH3

+ HCl

– AlCl3 polarise the CH3 Cl molecule

– The positively charged methyl group attacks the benzene ring and electrophilic substitution occurs

AlCl3H3C Cl

AlCl3H3C Cl

CH3

+ + AlCl4-

CH3

CH3HCH3

+ H+

4. Acylation• Involves substitution of an acyl group, RCO,

for a hydrogen atom on the aromatic ring.• When benzene is added to ethanoyl chloride

and AlCl3, under anhydrous conditions, a substitution reaction occurs and phenylethanone is formed.

+ H3C C

O

Cl

AlCl3

heat

C

O

CH3

+ HCl

Exercise

• Write the structural formula for the producta formed from Friedal Crafts alkylation or acylation of benzene with

(a) CH3CH2CH2Cl

(b) CH3CH2CH2COCl

5. Sulphonation• Involves substitution of an –SO3H group for a

hydrogen atom on the aromatic ring.• If benzene and [H2SO4] are reflux together for

several hours, benzenesulphonic acid is formed:

+ concentrated H2SO4reflux SO3H

+ H2O

Benzenesulphonicacid

Reaction of methylbenzene

1. Reaction with chlorine• When chlorine, Cl2, is bubbled through

boiling methylbenzene in strong sunlight or uv light, substitution takes place on the methyl group:CH3 CH2Cl CHCl2 CCl3

Cl2, boil

uv light

• If Cl2 is bubbled through methylbenzene in the absence of uv light but in the presence of AlCl3, the ring is substitutes. A mixture of two isomers is obtained.

• White fumes of HCl are given off• Electron density of the benzene ring more strongly at the 2-

and 4- positionsCH3

+ Cl2AlCl3

room temperature

CH3

Cl

+ HCl

AlCl3

room temperature

CH3

Cl

+ HCl

2. Reaction with HNO3

• When methyl benzene is added to HNO3 and concentrated H2SO4, at room temperature, a mixture of 2- and 4-nitromethylbenzeneCH3

+ HNO3

H2 SO

4

room temperature

CH3

NO2

+ H2O

room temperature

CH3

NO2

+ H2O

H2SO4

• If the temperature is raised, two or three nitro groups, NO2 are introduced

CH3

NO2

NO2

CH3

NO2

NO2

O2N

2,4-diphenylmethylbenzene 2,4,6-triphenylmethylbenzene

3. Reaction with CH3I (Friedal craft alkylation)

• When methlbenzene is added to CH3I in AlCl3 at 20 oC, a mixture of 1,2-dimethylbenzene and 1,4-dimethylbenzene is formed

CH3

room temperature

CH3

CH3

CH3

CH3

AlCl3+ CH3I + HI

+ HI

4. Reaction with CH3COCl (FriedalCraft Acylation)

• The reaction of an acyl chloride with an aromatic hydrocarbon is known as FriedalCraft acylation.

• Involves substitution of an acyl group, RCO-, for a hydrogen atom on the aromatic ring in the presence of AlCl3.

CH3

AlCl3

80oC

CH3

C

CH3

AlCl3

80 oC

+ H3C C Cl

O

ethanoylchloride

O

CH3+ HCl

C

OCH3

+ HCl

Exercise

• Write the structural formula for the products formed by Friedal Crafts acylation or alkylation of methylbenzene with

(a)C6H5COCl

(b) CH3CH2Br

5. Reaction with SO3

• When methylbenzene is added to fuming H2SO4 containing SO3 at 0oC, a mixture of 2- and 4-methylbenzenesulphonic is obtained.CH3

+ SO3

H2SO4

0 oC

CH3

SO3H

CH3

SO3H

1

4

1

2

+ H+

+ H+

H2 SO

40 oC

2-methylbenzenesulphonic acid

4-methylbenzenesulphonic acid

6. Oxidation of alkylbenzene• Alkylbenzenes are oxidised to benzoic acid on

prolonged reflux with KMnO4 or K2Cr2O7

RKMnO4 / K2Cr2O7

COOH + H2O + CO2

• Example

CH3KMnO4 / K2Cr2O7

COOH + H2O+ 3 [O]

reflux

CH2CH3KMnO4 / K2Cr2O7

COOH + 2H2O + CO2+ 6 [O]reflux

Exercise

• Predict the products from treatment of each of the following compounds with KMnO4 / H2SO4 under reflux.CH3

CH3

CH3

CH3

CH2CH2CH3

CH3

(a) (b) (c)

Exercise

• Name compound P, Q, R and S belowCH3

NO2

CH3 CH3

Br

4-methylbenzenesulphonic acid

fumingH2SO4

P + Q

R

AlCl3

(S)

Exercise

• Draw the structural formulae of the organic products formed when methylbenzene reacts with

(a)Concentrated nitric acid and concentrated sulphuric acid at room temp

(b)Chlorine in the presence of anhydrous aluminium chloride

(c)Chloromethane in the presence of aluminium chloride