3. 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

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
1,2-Dimethylbenzene		144	–25.2	0.880
1,3-Dimethylbenzene		139	–47.4	0.864
1,4-Dimethylbenzene		138	13.3	0.861

5 Preparation of Benzene

Industrial Preparation

Catalytic Reforming of Alkanes

Catalytic reforming converts alkanes and cycloalkanes into aromatic hydrocarbons

e.g.

Pt

C ₆H₁₄ ---® C₆H₆ + 4H₂

500°C, 10 – 20 atm

Destructive Distillation of Coal

• 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

Laboratory Synthesis

Decarboxylation of Sodium Salt of Benzoic Acid

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

Reduction of Phenol

Phenol vapour is passed slowly over heated zinc dust to produce benzene and zinc(II) oxide

Trimerization of Alkyne

• (lecture 5.3)

Preparation of Alkylbenzenes

Interaction of metallic Na with mixture of halogenalkanes and halogenbenzenes

6 Reactions of Benzene

Comparative Investigation of Chemical Properties of Cyclohexane, Cyclohexene and Benzene

Reaction	Cyclohexane (a saturated alicyclic hydrocarbon)	Cyclohexene (an unsaturated alicyclic 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
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

• 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

Electrophilic Aromatic Substitution Reactions

Most characteristic reaction of aromatic compounds:

Electrophilic 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 (d+)

Mechanism of Electrophilic Aromatic Substitution Reactions

The electrophiles attack the benzene ring and unstable aromatic π- complex is formed.

A new C-E bond is formed in a slow, rate-limiting step to yield a nonaromatic carbocation intermediate, σ- complex:

The carbocation intermediate loses a proton and the neutral substitution prodact is formed.

Same Electrophilic Aromatic Substitututijn Reaction

Halogenation

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

Nitration

Conc. H₂SO₄ increases the rate of reaction by increasing the concentration of the electrophile, NO₂⁺ (nitronium ion)

Sulphonation

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

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

Friedel–Crafts Alkylation

When benzene is warmed with a haloalkane in the presence of catalysts such as AlCl₃, an alkylbenzene is formed

Aromatic ring must be at least as reactive as a halobenzene. Deactivated ring do not react.

Alkyl halide can be methyl, ethyl, 2⁰, or 3⁰; primary halides undergo carbocation rearrangemeht.

Friedel–Crafts Acylation

An acyl group, -COR is introduced onto the ring when an aromatic compound reacts with a carboxylic acid chloride or anhydrides

The mechanism of Friedel–Crafts Acylation is similar to that of Friedel–Crafts Alkylation

Substituent Effect in Substitututed Aromatic Ring

Substitutents already present on the ring have tow effects:

1. Substitutents effect the reactivity of the aromatic ring. Some substitutents activate the ring, making it more reactive than benzene, and some deactivate the ring making it less reactive than benzene.

2. Substitutents effect the orientation of the reaction. The nature of the substitutent already present on the benzene ring determines the position of the second substitutent.

Classification of Substitutuent Effect in Electrophilic Aroma Aromatic Substitutution

Substituent can be classified into three groups:

Reactivity

Examples:

Trisubstituted Benzenes: Additivity of Effect

1. If the directing effects of tow groups reinforce each other:

2. If the directing effects of tow groups oppose each other, the more powerful activation group has the dominant influence, but mixtures of products often esult.

If there are two different deactivatorу substituents, more powerful deactivator directs next substituent to meta- position

3. Further rarely occurs between two groups in a meta- disubstituted compound because this site is too hindered.

Nucleophilic aromatic substitution

Aryls that have strong electron-withdrawing substituents can undergo nucleophilic aromatic substitution

Aryl halides that have electron-withdrawing substituents in a position ortho- or para- to the halogen can undergo nucleophilic aromatic substitution

Aryl halides without electron-withdrawing substituents are inert to nucleophiles under most condition.

At high temperature and pressure chlorobenzene can be forced to react.

Oxidation of alkylbenzene side chains

Aromatic rings are inert to oxidation under most condition. Alkylbenzenes are oxidized to benzoic acid by strong oxidizing agents such as hot alkaline potassium manganate(VII)

Examples:

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

e.g.

Ozonolysis

Like alkenes, aromatic compounds can undergo oxidation reaction by ozone.

Ozonolysis is a widely used method for locating the subtituents of benzene

Redaction of aromatic compounds

Aromatic rings are inert to catalytic hydrogenation under conditions that reduce typical alkene double bond. As a result, it’s possible to selectively reduce alkene double bonds in the presence of aromatic ring:

To hydrogenate an aromatic ring, it’s necessary either to use a platinum catalyst with hydrogen gas at several hundred atmospheres pressure or to use a more powerful catalyst such as rhodium on carbon.