Chemistry form 6

organic chemistry

chapter 3 :

benzene and its compound

3.0 Introduction

� Organic compounds which contain benzene are categorise as aromatic compounds (arene)

� For most of simple aromatic compounds, it will end with –benzene.

� There are basic type of aromatic compounds, structural formula, common name and IUPAC name

Structural formula Molecular formula Common name IUPAC name

Benzene Benzene

Toluene Methylbenzene

Ortho-xylene 1,2-dimethylbenzene

Phenol Phenol

C6H6

C7H8

C8H10

C6H5OH

Structural formula Molecular formula Common name IUPAC name

Nitrobenzene Nitrobenzene

Benzoic acidBenzenecarboxylic

acid

Benzaldehyde Phenylmethanal

Aniline Phenylamine

Naphthalene Naphthalene

C6H5NO2

C6H5COOH

C6H5COH

C6H5NH2

C10H8

3.1 Nomenclature of aromatic compounds

� For simple aromatic compound, it is as describe in the table above

� Benzene can also be considered as a branched group.

� Branched benzene is called as phenyl (C6H5–)

� When there are 2 or more substituents on benzene ring, 3 structural isomers are possible. The substituents may be located by numbering the atoms of the ring, or may be indicates by prefixes of ortho, meta, or para

Position of the 2 substituents in benzene ring

1,2-position [ortho (o)] 1,3-position [meta (m)] 1,4-position [para (p)]

1,2 – dichlorobenzene

ortho-dichlorobenzene

1,3 – dichlorobenzene

meta-dichlorobenzene

1,4 – dichlorobenzene

para-dichlorobenzene

1,2-dinitrobenzeneo-dinitrobenzene

1,3-dinitrobenzenem-dinitrobenzene

1,4-dinitrobenzenep-dinitrobenzene

2-nitrophenol 3-nitrophenol 4-nitrophenol

2-bromotoluene 3-hydroxybenzoic acid 4-methylbenzaldehyde

� When 3 or more groups are on benzene ring, a numbering system must be used to name them. Usually a smaller number of groups will be C1and the other will be numbered accordingly.

� If there are 3 different groups, the one which have a common name will be given priority. The other 2 will be name and numbered base on alphabetical order.

2,3-dichlorotoluene 5-bromo-3-nitrotoluene 4-chloro-2-ethylphenol

2,4,6-tribromonitrobenzene2-hydroxy-5-methylbenzoic

acid 3-chloro-2-phenylbutane

3.2 Reaction of Benzene

� Even though in benzene contain 3 double bonds, but as explained in Kekule’s structure, it give an extra stability due to delocalised

ππππ – electrons in the ring and the resonance structure.

� Thus, benzene usually undergoes substitution reaction instead of addition reaction.

� The substitution reactions of benzene with an electrophilic reaction include : 1. Halogenation 2. Alkyation

3. Acylation 4. Nitration 5. Sulphonation

Name of reactionReagent used

and conditionEquation

Halogenation

Chlorine gas, Cl2 with

AlCl3 as halogen

carrier (catalyst)

-----------------

Bromine gas, Br2with FeBr3 as

halogen carrier

(catalyst)

benzene halogen halobenzene

Name of reactionReagent used

and conditionEquation

Friedel – Crafts

Alkylation

Haloalkane (R – X)

with AlCl3 as

halogen carrier

(catalyst)benzene haloalkane alkylbenzene

Friedel – Crafts

Acylation

Acyl chloride

with AlCl3 as

halogen carrier

(catalyst) benzene acyl chloride

Nitration

Concentrated

Nitric acid (HNO3)

catalysed by

concentrated

sulphuric acid and

reflux at 55oC

benzene nitric acid nitrobenzene

Sulphonation

Concentrated

sulphuric acid

(H2SO4) and heat at

55oC under reflux benzene sulphuric acid benzenesulphonic acid

3.2.1 Halogenation

� Chlorine react with benzene under aluminium chloride as catalyst under room condition

� Bromine reacts with benzene only under the presence of catalyst iron (III) bromide and some hear

� The mechanism of halogenation of benzene

� Step 1 : Formation of halogen ion (X+) as electrophile using heterolytic fission reaction. In chlorine, aluminium chloride (electron deficient compound) is readily to receive lone pair electron (act as Lewis acid) from chlorine

� Step 2 : Electrophilic attack on benzene ring to form a carbocation. Cl+ ion attack the benzene ring and the delocalise π-electron form a C–Clbond in benzene. This will result a carbocation formed as intermediate and disturb the ring (cause benzene ring become unstable)

� Step 3 : Proton lost from carbocation. Carbocation transfers a proton to [AlCl4]

− and the benzene ring is stabilised back. This results in the formation of chlorobenzene and HCl.

� [As extra note, benzene also react with chlorine in the presence of UV and some heat to form 1,2,3,4,5,6-hexachlorocyclohexane (addition reaction)]

Friedel–Crafts reaction

� Similar to halogenation, Friedel – Crafts reaction also required a halogen carrier to act as catalyst

� Depending on the type of haloalkane used, the halogen carrier is also different.

� If chloroalkane (R–Cl) is used, the halogen carrier will be aluminiumchloride (AlCl3)

� If bromoalkaane (R–Br) is used, the halogen carrier will be iron (III) bromide (FeBr3)

3.2.2 Alkylation of Benzene

� When chloroethane (CH3CH2Cl) react with benzene with the presence of AlCl3, ethylbenzene is produced (C6H5–CH2CH3) under room temperature

The mechanism of alkylation is very similar in ways of how halogenation occur.

Step 1 : Formation of electrophile by heterolytic fission

Step 2 : Electrophile attacking the benzene ring to form carbocation

Step 3 : Proton lost from the unstable carbocation formed earlier.

3.2.3 Acylation of Benzene

� When ethanoyl chloride (CH3COCl) reacts with benzene under the presence of AlCl3, phenylethanone is produced (C6H5–COCH3) at 80

oC.

� The mechanism of acylation

Step 1 : Formation of electrophile by heterolytic fission

Step 2 : Electrophile attacking the benzene ring to form carbocation

Step 3 : Proton lost from the unstable carbocation formed earlier

� For nitration and sulphonation of benzene, halogen carrier is not used, as the reagent used for the reaction is an acid. The mechanism of nitration and sulphonation are also nearly similar to each other.

3.2.4 Nitration of benzene

� Concentrated nitric (V) acid, HNO3 will only react with benzene under the presence of a little concentrated sulphuric acid (H2SO4) at 55

oC heated under reflux, to produce nitrobenzene

� The mechanisms of nitration are explained below

Step 1 : Production of nitronium ion, NO2+. In nitration of benzene,

nitric (V) acid act as Bronsted-Lowry base where it accept a proton donated by sulphuric acid

Step 2 : Electrophile attacked benzene ring to form carbocation. NO2

+ ion attack the benzene ring and delocalise π-electron form a C–NO2

bond in benzene. This will result a carbocation formed as intermediate and disturb the ring (cause benzene ring become unstable)

Step 3 : Proton lost from carbocation. Carbocation transfers a proton to HSO4

− and the benzene ring is stabilised back. This results in the formation of nitrobenzene and H2SO4 (catalyst)

� When nitration is carried out at higher temperature (above 200oC), a 1,3,5-trinitrobenzene can be formed where :

3.2.5 Sulphonation of benzene

� The mechanisms occur for sulphonation of benzene is more or less the same with nitration of benzene. Unlike nitration, sulphonation does not required a catalyst as the reagent used, sulphuric acid (H2SO4) act as a catalyst itself

� Step 1 : Formation of electrophile from sulphuric acid. The protonation of sulphuric acid when it received one H+ (Bronsted-Lowry base) from another sulphuric acid

Step 2 : Electrophile attacked benzene ring to form carbocation.

Step 3 : Proton lost from carbocation

Other chemical reaction of benzene

� Unlike alkene, benzene is stabilised by the delocalised π electrons. So, it does not react easily as in alkene. For example, if benzene react with acidified potassium manganate (VII), KMnO4 (H2SO4)

� When react with hydrogen gas with presence of nickel as catalyst at 180oC, it form cyclohexane. The reaction is an additional reaction.

benzene cyclohexane

� Benzene also reacts with propene to give isopropylbenzene (well known as cumene) which is a starting material to synthesis phenol. Concentrated H3PO4 serve at catalyst under 250

oC

3.3 Influence of Substitution Group on Reactivity and

Orientation of Substituted Benzene

� When benzene ring contained a substituents M, the reaction of C6H5–M may be faster / slower compare to benzene

Group of MRing activating groups

(ortho, para directing)

Ring deactivating groups

(meta directing)

Effect of

groups

Cause ring more reactive (

increase rate)

Cause ring less reactive (

decrease rate)

Examples

– CH3 – NH2 – OH – NO2 – COOH – COH

– CH2CH3 – NH2R – OR – SO3H – COR –X (Cl, Br)

Type of director

ortho director para director meta director

� Properties of ring activate group

� Electron donating groups have positive inductive effect (+I)

� When electrophile attacked the benzene ring, carbocation is formed.

� Since a more stable carbocation form faster than a less stable one, when electrophile attacked at ortho & para position.

� As discussed earlier, 3o carbocation is more stable than 2o carbocation. Using resonance, it is possible for cation to reside at 3o carbon.

� Since ortho / para position are more activated when a 30 carbocation formed, it increase the rate of reaction

� Properties of ring deactivate group

� Electron withdrawing groups have negative inductive effect (–I)

δ+ δ−

� Under (–I) effect, C – M, carbon had already bear partial positive charge δ+

� Unlike electron donating group, when the cation is placed at the directing group of electron withdrawing group, it will tend to become unstable

� So attacking at meta position is more stable than in ortho / para position.

� Still, since in react much slower than in benzene, so electron withdrawing group is to say deactivate benzene ring and cause the rate of reaction decrease.

3.4 Reaction of methylbenzene

� Methylbenzene resemble with benzene in many ways. As methylbenzene is less toxic, is often used as reagent instead of benzene. Moreover, methyl (CH3–) is ring activate group, it react faster and required lesser effort (lower temperature, concentration electrophile) compare to benzene.

� Unlike benzene, methylbenzene contain an aliphatic (CH3–) and aromatic (C6H6). In other words, methylbenzene undergoes 2 distinctive type of reaction :

⇒ reaction of the methyl group ⇒ reaction of the benzene ring

3.4.1 Reaction of the methyl group in methylbenzene

Name of

reaction

Reagent used

and conditionEquation

Oxidation of

methyl-

benzene

Acidified

potassium

manganate

(VII)

KMnO4 / H2SO4*Observation : (1) purple colour of potassium manganate

(VII) decolourised when react with toluene

Acidified

potassium

dichromate (VI)

K2Cr2O7 /

H2SO4

+ H2*Observation : Green colour of potassium dichromate (VI)

changed to orange colour

Chlorination

of

methylbenze

ne

Chlorine gas

under UV light

at room

temperature

* side product of reaction is HCl (g)

� Methylbenzene reacts with strong oxidising agent such as acidified potassium

manganate (VII) [KMnO4 / H+] or acidified potassium dichromate (VI) [K2Cr2O7

/ H+] to form benzoic acid. This is a method to distinguish between

benzene and methylbenzene.

� Under room temp, only H in methyl is substituted by Cl atom.

Step 1 : Initiation – Formation of Cl• (radical)

Step 2 : Propagation – Radical attack methylbenzene to form multiple form of

radical

Step 3 : Termination – chlorine radical react and methylbenzene radical

� If temperature increases to 200oC, then, even the H inside benzene ring may

be substituted by Cl.

3.4.2 Reaction of methylbenzene in the benzene ring

Name of

reaction

Reagent used

And conditionEquation

Halogenation

Cl2 / AlCl3or

Br2 / FeBr3o-chlorotoluene p-chlorotoluene

Friedel – Crafts

Alkylation CH3Cl / AlCl3

o-xylene p-xylene

Friedel – Crafts

AcylationCH3COCl / AlCl3

o-ethanoyltoluene p-ethanoyltoluene

Other types of alkylbenzene synthesis and reaction

� Formation of phenol

� Formation of aniline

NitrationConc. HNO3 +

conc. H2SO4

o-nitrotoluene p-nitrotoluene

Sulpho-

nation

Concentrated

H2SO4

o / p - methylbenzenesulphonic acid

� Practice : Suggest the methods of how to synthesis these products from benzene.

1.

2.

3.

4.

5.

6.

7.

8.

Step 1 :H2SO4 + HNO3 � NO2+ + HSO4

- + H2O [1]

� Reaction I is oxidation [1], where acidified potassium manganate (VII) [1]

under reflux [1]

� Reaction II is free radical substitution reaction [1], where bromine gas [1]

under the presence of sunlight [1] is required

� Reaction III is electrophilic aromatic substitution reaction [1], where bromine

gas react under the presence of iron (III) bromide [1]

A : chlorine gas under the presence of AlCl3 as catalyst

B : chlorine gas under the presence of UV

Reagent : Using acidified potassium manganate (VII)

Observation : A will decolourised purple colour of acidified KMnO4, while B won’t

Equation :

HNO3 catalysed by H2SO4 under reflux

Acidified KMnO4 under reflux

HCl under Sn as catalyst

Step 1 :H2SO4 + HNO3 � NO2+ + HSO4

- + H2O [1]

Reagent : Using acidified potassium manganate (VII)

Observation : methylbenzene will decolourised purple colour of acidified KMnO4, while

benzene will not.

Equation :

Reagent : Using nitric acid catalysed by concentrated sulphuric acid under reflux

Observation : benzene will turn from colourless to yellow liquid while cycloalkane will

remain colourless

Equation :