AROMATIC HYDROCARBONS
aromatic h/c= h/c w/ benzene ring
Alkylbenzenes
general formula: C_6+nH_6+2n (n = 0,1,..)
Benzene
molecular formula: C₆H₆
structural formula:
-consists of 6 sp²-hybridised C atoms, 6 π electrons delocalised forming 2 electron cloud rings above + below benzene ring, all C-C bonds same (longer than C-C double bond, but shorter than C-C single bond)
benzene has resonance stabilised structure

Physical properties
-colourless liquid w/ characteristic smell
-benzene less dense than water & immiscible w/ water
-carcinogenous (toxic vapour, excessive inhalation of vapour can led to anaemic leukaemia)
(benzene burns w/ yellow sooty flame)

Nitration of benzene
reagents: nitration mixture (conc HNO₃ + conc H₂SO₄)
conditions: temperature: < 55°C, heat under reflux
equation: C₆H₆ + HNO₃ >(conc HNO₃ + conc H₂SO₄, under reflux < 55°C)> C₆H₅-NO₂ + H₂O
        [benzene + nitric acid > nitrobenzene + water]
observatyion: lower yellow oily liquid (nitrobenzene) formed

Mechanism of electrophilic subtstitution (nitration of benzene)
- electrophilic nitronium/nitryl ion, NO₂⁺, formed
--protonating nitric acid

HNO₃ (conc) + H₂SO₄ (conc) > HSO₄^- + H₂O + NO₂⁺
--removal of water molecule from protonated nitric acid > electrophilic nitronium ion

-Electrophilic nitronium ion attacks benzene ring > positive carbocation intermediate

-H⁺ removed from carbocation intermediate by nucleophilic HSO₄^- > nitrobenzene


delocalisation of 6 π electrons preserves benzene ring structure > substitution, not addition, occurs in nitration
temperature of reaction: 50-55°C, higher temp: formation of dinitro-, trinitro- benzene
Energy diagram of nitration of benzene


Halogenation
reagent: X [Br(l)/Cl(g)]
condition: catalyst: 'halogen carrier'- finely divided iron or iron(III) bromide, FeBr₃, room temp
equation: C₆H₆ + X₂ >(Fe)> C₆H₅-X + HX [benzene + halogen molecule > halogenoarene + hydrogen halide]
observation: excess benzene > red Br / yellow -green Cl decolourised (colourless products)
bromination of benzene w/ Fe: electrophilic subs mechanism like nitration (electrophilic Br⁺/Cl⁺ produced)
2Fe + 3Br₂ > 2FeBr₃


Methylbenzene
molecular formula: C₆H₅CH₃

C₆H₅ part is planar
C-CH₃ part is tetrahedral

Physical properties
-colourless liquid w/ characteristic smell
-insoluble in water > upper immiscible layer
-good organic solvent (replace toxic benzene)

Reactions of methylbenzene:
Electrophilic subs of H in benzene ring
-nitration
-halogenation
   --bromination
   --chlorination
Side-chain methyl group
-FR substitution of H atoms of methyl group
-oxidation of methyl group

Electrophilic substitution
methyl group, -CH₃: more reactive than benzene
Nitration
reagent: nitrating mixture of conc HNO₃ + conc H₂SO₄
conditions: under reflux, ~ 30°C
equation:

nitrating of methylbenzene produces 2 isomers: 1-methyl-2-nitrobenzene & 1-methyl-4-nitrobenzene (ratio 2:1)

Position of substitution in benzene ring
Group G in benzene ring determines position of substitution


substitution occurs at carbon 2,4,6 if G is
-alkyl group
-hydroxy group
-amino group
-halogeno group
(groups don't have double / triple bonds)


substitution occurs at carbon 3/5 if G is
-cyano group
-nitro group
-carboxylic group
-carbonyl group
(groups have double / triple C bonds)

Halogenation
substitution of H atom of benzene ring by halogen occurs through electrophilic subs mechanism
reagent: X [Br(l)/Cl(g)]
condition: catalyst: 'halogen carrier'- finely divided iron or iron(III) halide, [FeBr₃/FeCl₃], room temp
equation:

[1-X-2-methylbenzene/1-X-4-methylbenzene formed in ration= 2:1]
observation: excess methylbenzene > red Br / yellow -green Cl decolourised (colourless products)

Reactions involving side-chain methyl group of methylbenzene
Halogenation: Free radical substitution
reagent: halogen, Cl₂ (g) / Br₂(l)
condition: boiling methylbenzene (in absence of halogen carrier) + UV light
equation: C₆H₅-CH₃ + X₂ >(reflux at bp of methylbenzene)> C₆H₅-CH₂X+ HX
[methylbenzene + halogen > halogenomethylbenzene + hydrogen halide]
excess halogen > multi substituted product

Oxidation
methylbenzene oxidised to carboxylic acid
reagent: acidified potassium manganate (VII), KMnO₄(aq)/H₂SO₄(aq) (not HCl/HNO₃ as they react w/ KMnO₄)
condition: heat under reflux
equation:

[methylbenzene >(oxidation)> benzoic acid + water]
observation: if excess methyl benzene:
  -purple KMnO₄ decolourised
  -on cooling > white needle-shaped crystals of benzoic acid (benzoic acid + PbCl₂: soluble in hot water)
(only C atom attached to benzene ring will form acid, other C atoms oxidised to CO₂ + water)


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