• Properties of aromatic compounds appears in A2/A-level Chemistry unit 3.4
• We will go through basic properties about aromatic compounds
•  Aromatic reactions including electrophillic substitution Reactions, chemistry of Phenol, di-substitution and directing groups

Benzene (C₆H₆)

Basic background

• first discovered by Michael Faraday in 1825
• colourless, sweet smelling, highly flammable
• found naturally in crude oil, petrol and cigarette smoke
• Consists of a hexagonal ring of 6 carbons with each carbon bonded to 2 carbons + 1 hydrogen

Kekulé Model (1865)

• 6 carbon ring with alternating double and single bonds
• Lengths of carbon-carbon bonds: 0.139 nm
• Length between a double bond: 0.134 nm
• Length of a single bond: 0.153 nm
• the model was disproved by two major reasons:

(a) Lack of reactivity

• C=C double bond in benzene DOES NOT decolourize bromine under normal conditions
• benzene DOES NOT undergo electrophilic addition

(b) Hydrogenation enthalpies

• benzene hydrogenation enthalpy should be equal to three times of cyclohexene if Kekulé Model is true
• theoretical result: -306kJ mol^-1
• actual experimental result: -208 kJ mol^-1 

Delocalized Model of Benzene

• planar, cyclic hexagonal ring of carbons
• each carbon used 3 electrons to bond to 2 carbons and 1 hydrogen
• each carbon has one electron in a p-orbital
• p-orbitals overlap above and below the plane
• a system of pi-bonds with 6 delocalized electrons

Electrophillic substitution

• an electrophile displaces a functional group in a compound
• in aromatic electrophilic substitution, the atom being replaced by electrophile would usually be hydrogen

Benzene reactions

• undergoes substitution of hydrogen with other atoms/atom groups

Nitration of benzene

• Benzene + nitric acid (sulfuric acid catalyst at 50°) ---> nitrobenzene + water
• H is replaced with NO₂
• Nitric acid and sulfuric acid react and give out NO²⁺ electrophile
• H⁺ and HSO₄ react to form H₂SO₄

Halogenation of Benzene

• C=C double bond on benzene DOES NOT undergo halogenation in normal condition like alkene does
• Special Condition: presence of a halogen carrier e.g. AlCl₃
• e.g. Bromination of Benzene
• Room temperature with a halogen carrier
• hydrogen on benzene is replaced with Br
• a type of Electrophilic substitution

Alkylation of Benzene

• Benzene + haloalkane (in the presence of halogen carrier) ---> alkylbenzene + an acid
• may refer to Friedel-Crafts Reaction
• H replaced with alkyl group

Acylation Reactions

• Benzene + acyl chloride (in the presence ofAlCl₃) ---> aromatic ketone + HCl

Chemistry of Phenol

Phenol

• -OH group bonded directly to benzene ring

Phenol as a Weak Acid

• Less soluble in water than alcohol
• Reason: non-polar benzene ring
• partially dissociates in water ---> phenoxide ion + a proton(/H+)
• Phenols and carboxylic acids DO react with strong bases
• DOES NOT react with sodium carbonate (Na₂CO₃)
• Phenol + sodium hydroxide ---> sodium phenoxide salt + water (neutralization)

Electrophilic substitution of Phenol

Bromination

• NO halogen carrier required
• Phenol + bromine water ---> 2,4,6-tribromophenol (precipitate)
• Decolourised bromine water and forms white precipitate

Nitration of Phenol

• Phenol + dilute nitric acid ---> mixture of 2-nitrophenol + 4-nitrophenol

Reactivity of Phenol

• Bromine and nitric acid react more readily with phenol than benzene
• Reason: lone pair of electrons from OH being donated to pi-system
• electron density increases and effective delocalization of charge
• phenol attracts electrophiles more /phenol is more electrophilic

Directing groups

• Activation: aromatic ring reacts more readily with electrophiles
• Deactivation: aromatic ring reacts less readily with electrophiles
• Different groups have a directing effect on any second substitutions

2,4 (ortho/para) directing groups

• NHR/NH₂
• OH
• R (alkyl group)
• Halogens (F, Br, Cl, I)

3 (meta) directing groups

• mostly carbonyl compounds
• RCOR (ketone)
• COOR (ester)
• SO₃H
• CHO (aldehyde)
• COOH (carboxylic acid)
• CN
• NO₂
• NR³⁺

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Image Reference:

• Benzene structure - https://socratic.org/questions/how-many-sigma-bonds-are-in-benzene
• Wikipedia: Electrophilic aromatic substitution - https://en.wikipedia.org/wiki/Electrophilic_aromatic_substitution

• Electrophilic Aromatic Substitution: Are protonated OH-groups ortho-/para- or meta-directing? - https://chemistry.stackexchange.com/questions/8509/electrophilic-aromatic-substitution-are-protonated-oh-groups-ortho-para-or-me

• ResearchGate: Directing groups (para/ortho/meta) - https://www.researchgate.net/figure/Structure-and-nomenclature-for-phenol-and-substitution-patterns-of-phenolic-compounds_fig1_319961465

Drafted by Yoyo Lau (Chemistry)