aromatic hydrocarbons

New Way Chemistry for Hong Kong A-Level Book 3A

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Aromatic Hydrocarbons

31.131.1 IntroductionIntroduction

31.231.2 Nomenclature of the Derivatives of BenzeneNomenclature of the Derivatives of Benzene

31.331.3 The Stability of BenzeneThe Stability of Benzene

31.431.4 Physical Properties of Aromatic HydrocarbonsPhysical Properties of Aromatic Hydrocarbons

31.531.5 Preparation of BenzenePreparation of Benzene

31.631.6 Reactions of BenzeneReactions of Benzene

31.731.7 AlkylbenzenesAlkylbenzenes

Chapter 31Chapter 31


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31.1 Introduction (SB p.147)

Benzene

• Highly unsaturated

• Six-membered ring compound with alternative single and double bonds between adjacent carbon atoms

• Chemically unreactive compared to alkenes


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31.2 Nomenclature of the Derivatives of Benzene (SB p.148)

1. Monosubstituted benzenes

(a) For certain compounds, benzene is the parent name and the substituent is simply indicated by a prefix


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(b)For other compounds, the substituent and the benzene ring taken together may form a new parent name



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2. Polysubstituted benzenes

(a) If more than one substituent are present and the substituents are identical, their relative positions are indicated by the use of numbers assigned on the ring. The prefixes ‘di-’, ‘tri-’, ‘tetra-’, and so on are used.


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(b) When more than one substituent are present and the substituents are different, they are listed in alphabetical order.


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(c) When a substituent is one that when taken together with the benzene ring gives a new parent name, that substituent is assumed to be in position 1 and the new parent name is used


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Example 31-1Example 31-1Draw the structural formula for each of the following compounds:

(a) 1,3,5-Trichlorobenzene

(b) 2,5-Dibromophenol

(c) 2,4-Dinitrobenzoic acidAnswer


Solution:

(a) (b)

(c)


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Check Point 31-1 Check Point 31-1

Give the IUPAC name for each of the following compounds:

(a) (b)

(c) (d)


Answer

(a) 1,2-Dimethylbenzene

(b) 1-Methyl-2-nitrobenzene or 2-nitrotoluene

(c) 3-Bromo-5-chlorobenzoic acid

(d) 4-Bromo-2,6-dinitrophenol


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31.3 The Stability of Benzene (SB p.151)

In 1865, Kekule proposed the structure of benzene:


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According to the Kekulé structure, there should be two different 1,2-dibromobenzenes:

Only one 1,2-dibromobenzene has been found!!


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According to the Kekulé structure, benzene should

• undergo addition reactions readily

• it gave substitution reaction products rather than addition reaction products

Kekulé structure cannot explain the behaviour of benzene


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Enthalpy change of hydrogenation of cyclohexene

= –119.6 kJ mol–1

Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene

Enthalpy Changes of Hydrogenation of Benzene and Cyclohexene


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The enthalpy change of hydrogenation of 1,3-cyclohexadiene i

s expected to be twice that of cyclohexene


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If benzene has the structure of 1,3,5-cyclohexatriene, The

enthalpy change of hydrogenation is expected to be three

times as much as that of cyclohexene


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• Benzene is more stable than Kekulé structure

• The energy difference for the stabilization of benzene is called resonance energy of benzene


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From X-ray crystallography,

The length of carbon-carbon bond in benzene is intermediate between C – C bond and C = C bond

0.134 nm > 0.139 nm > 0.154 nm C = C carbon bond in benzene C – C

The Resonance Explanation of the Structure of Benzene

The Resonance Explanation of the Structure of Benzene


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All carbon atoms in benzene are sp2-hybridized

The side-way overlap of unhybridized 2p orbitals on both sides gives a delocalized electron cloud above and below the plane of the ring


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The delocalization of electrons gives benzene extra stabili

ty and determines the chemical properties of benzene


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Structural formula of benzene:

The circle represents the six electrons that are delocalized

about the six carbon atoms of the benzene ring


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• All C atoms in the ring is sp2-hybridized

• The C atom in the methyl group is sp3-hybridized

• The delocalized electron clouds give rise to extra stability

Structure of MethylbenzeneStructure of Methylbenzene


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31.4 Physical Properties of Aromatic Hydrocarbons (SB p.155)

Physical properties of aromatic hydrocarbons:

• have a fragrant smell

• generally less dense than water at 20°C

• usually immiscible with water

• soluble in organic solvents


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Name Formula Boiling

point (°C)

Melting point (°C)

Density at 20°C (g cm–3)

Benzene 80.1 5.5 0.878

Methylbenzene 111 –95 0.867

Ethylbenzene 136 –94 0.867


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Name Formula Boiling

point (°C)

Melting point (°C)

Density at 20°C (g cm–

3)

1,2-Dimethylbenzene

144 –25.2 0.880

1,3-Dimethylbenzene

139 –47.4 0.864

1,4-Dimethylbenzene

138 13.3 0.861


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31.5 Preparation of Benzene (SB p.157)

Catalytic reforming converts alkanes and cycloalkanes int

o aromatic hydrocarbons

e.g.

Industrial PreparationIndustrial Preparation

Catalytic Reforming of Alkanes

PtC6H14 C6H6 + 4H2500°C, 10 – 20 atm


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• Heating coal in the absence of air gives out coal gas, ammoniacal liquor, coal tar and coke

• Coal tar is a mixture of many organic compounds, mainly aromatic ones

• Benzene and methylbenzene can be obtained

Destructive Distillation of Coal


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When sodium benzoate is fused with sodium hydroxide, the carboxylate group is removed and benzene is formed

Decarboxylation of Sodium Salt of Benzoic Acid

Laboratory SynthesisLaboratory Synthesis


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Phenol vapour is passed slowly over heated zinc dust to p

roduce benzene and zinc(II) oxide

Reduction of Phenol


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31.6 Reactions of Benzene (SB p.158)

Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene

Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene

ReactionCyclohexane (a saturated alicyclic hy

drocarbon)

Cyclohexene (an unsaturated alicycli

c hydrocarbon)

Methylbenzene (an aromatic

hydrocarbon)

Action of Br2 in CH3Cl3 (in dark)

No reaction Br2 decolourized and no HBr evolved

No reaction with Br2 alone

In the presence of FeBr3, Br2 decolourized and HBr fumes evolved

Action of H2 (with Ni catalyst)

No reaction 1 mole of cyclohexene reacts with 1 mole of H2 at room temperature

1 mole of methylbenzene reacts 3 moles of H2 at high temperature and pressure


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ReactionCyclohexane (a saturated alicyclic hy

drocarbon)

Cyclohexene (an unsaturated alicycli

c hydrocarbon)

Methylbenzene (an aromatic

hydrocarbon)

Action of acidified KMnO4

No reaction KMnO4

decolourized

No reaction

Action of conc. HNO3 and conc. H2SO4

No reaction Cyclohexene oxidized and colour darkens

A yellow liquid is formed


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• Methylbenzene is highly unsaturated, but it is resistant to oxidation and addition reactions


• The resistance of oxidation and addition reactions of aromatic compounds is used to distinguish from unsaturated alkenes

• Methylbenzene reacts with Br2 in the presence of

FeBr3. It is through substitution reaction


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Electrophilic Aromatic Substitution ReactionsElectrophilic Aromatic Substitution Reactions

• The electrophiles attack the benzene ring, replacing one of the hydrogen atoms in the reaction

• Electrophiles are either a positive ion (E+) or some other electron-deficient species with a partial positive charge (+)

Most characteristic reaction of aromatic compounds:

Electrophilic substitution reactions


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• Conc. H2SO4 increases the rate of reaction by increasing

the concentration of the electrophile, NO2+ (nitronium io

n)

Nitration


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• Benzene reacts with fuming sulphuric(VI) acid at room temperature to give benzenesulphonic acid

Sulphonation

• Heating aqueous solution of benzenesulphonic acid above 100°C, benzene and sulphuric(VI) acid are formed


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Benzene reacts with chlorine and bromine in the presence of catalysts such as AlCl3, FeCl3, FeBr3, to give chlorobenzen

e and bromobenzene respectively

Halogenation


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• When benzene is warmed with a haloalkane in the presence

of catalysts such as AlCl3, an alkylbenzene is formed

Alkylation

• Important step in chemical industry to produce

polystyrene, phenol and detergents


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Example 31-2Example 31-2Complete each of the following by supplying the missing reactant or product as indicated by the question mark:

(a)

(b)

(c)

Answer


Solution:

(a)

(b) conc. H2SO4, conc. HNO3

(c) fuming H2SO4


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(a) One mole of benzene reacts with three moles of chlorine under special conditions. What is the reaction condition required for the reaction?

(b) Draw the structure of the reaction product in (a).

Answer


(a) UV radiation or diffuse sunlight must be

present for the free radical addition

reaction to take place.

(b)


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31.7 Alkylbenzenes (SB p.162)

• Alkylbenzenes are a group of aromatic hydrocarbons in which an alkyl group is bonded directly to a benzene ring

• also known as arenes

e.g.


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Alkylbenzenes are oxidized to benzoic acid by strong oxidizing

agents such as hot alkaline potassium manganate(VII)

This type of oxidation is limited to those molecules with


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Examples:


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The C = C double bond and acyl groups in the side chain are oxidized by hot alkaline potassium manganate(VII)

e.g.


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Example 31-3Example 31-3State the conditions under which methylbenzene can be converted in the laboratory to

(a) C6H5CH2Cl

(b) C6H5COOH Answer

Solution:

(a) Reagent: Cl2

Condition: in the presence of light

(b) Reagent: (1) KMnO4–, OH–, (2) H3O+

Condition: heating under reflux



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Methylbenzene undergoes two different types of chlorination reaction by different mechanisms. Compare the two different types of chlorination reaction in terms of reaction conditions as well as the products formed.

Answer



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Two different types of chlorination reaction of methylbenzene are:

Type I: free radical substitution reaction

Type II: electrophilic aromatic substitution reaction



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The END

aromatic hydrocarbons

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