Chemistry Form 6 Sem 3 Chapter 3

Chemistry form 6

organic chemistry

chapter 3 :

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

C

H

C

H

C

H

C

H

OH

Structural formula Molecular formula Common name IUPAC name

Nitrobenzene Nitrobenzene

Benzoic acid Benzenecarboxylic acid Benzaldehyde Phenylmethanal Aniline Phenylamine Naphthalene Naphthalene

C

H

NO

C

H

COOH

C

H

COH

C

H

NH

C

H

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-dinitrobenzene o-dinitrobenzene 1,3-dinitrobenzene m-dinitrobenzene 1,4-dinitrobenzene p-dinitrobenzene

2-nitrophenol 3-nitrophenol 4-nitrophenol

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 C₁ and 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

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 π

π π

π _{– 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 reaction Reagent used

and condition Equation

Halogenation

Chlorine gas, Cl₂ with AlCl₃ as halogen carrier (catalyst) ---Bromine gas, Br₂ with FeBr₃ as halogen carrier (catalyst)

Name of reaction Reagent used

and condition Equation

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 55o_C

benzene nitric acid nitrobenzene

Sulphonation

Concentrated sulphuric acid (H₂SO₄) and heat at

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–Cl}

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 [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

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 aluminium

chloride (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

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

3.2.3 Acylation of Benzene

When ethanoyl chloride (CH₃COCl) reacts with benzene under the presence of

AlCl₃, phenylethanone is produced (C₆H₅–COCH₃) at 80o_C.

The mechanism of acylation

Step 1 : Formation of electrophile by heterolytic fission

Step 2 : Electrophile attacking the benzene ring to form carbocation

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 55o_{C 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–NO}

2

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}

When nitration is carried out at higher temperature (above

200

_{C), 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}

Step 2 : Electrophile attacked benzene ring to form 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 250o_C

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 M Ring 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

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

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

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

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 condition Equation

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

Methylbenzene reacts with strong oxidising agent such as acidified potassium

manganate (VII) [KMnO₄ / H+_{] or acidified potassium dichromate (VI) [K}

2Cr2O7

/ 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

3.4.2 Reaction of methylbenzene in the benzene ring

Name of reaction

Reagent used

And condition Equation

Halogenation Cl₂ / AlCl₃ or Br₂ / FeBr₃ o-chlorotoluene p-chlorotoluene Friedel – Crafts Alkylation CH3Cl / AlCl3 o-xylene p-xylene Friedel – Crafts

Acylation CH3COCl / AlCl3

Other types of alkylbenzene synthesis and reaction

Formation of phenol

Formation of aniline Nitration Conc. 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.

4.

5.

7.

Step 1 :H₂SO₄ + HNO_{3  NO2}+ _{+ HSO}

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

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 + HNO_{3  NO2}+ _{+ HSO}

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