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Hydrocarbons: Aromatic (Arenes) revision notes

Every idea in the Hydrocarbons: Aromatic (Arenes) chapter for JEE Main, with the usual mistakes and the fast methods. Topic 1 is open to everyone; sign in free to read all 10 and download the PDF.

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Benzene and aromaticity

Structure and stability of benzene
1× asked, last 2025
Kekuleˊ forms→hybrid: planar, all C sp2, six C–C=139 pm (C–C 154, C=C 133)resonance energy≈3(120)−208≈150 kJ mol−1;CX6HX6→Δ,P3 HX2,NiCX6HX12\begin{gathered}\text{Kekulé forms} \to \text{hybrid: planar, all C } sp^2,\ \text{six C--C} = 139\ \text{pm (C--C 154, C=C 133)}\\ \text{resonance energy} \approx 3(120) - 208 \approx 150\ \text{kJ mol}^{-1};\qquad \ce{C6H6 ->[3H2, Ni][\Delta, P] C6H12}\end{gathered}
6 π e⁻ delocalised over the ring make it extra stable, so it substitutes (ring kept) rather than adds. One mono-isomer.
benzene: 3 π bonds, takes 3 H₂
hexa-2,4-diyne: 4 π bonds (8 π e⁻),
takes 4 H₂; also one monobromo product
benzene: six C–C 139 pmKekulé form: 154 / 133 pm (not seen)
Drawn to scale: the real ring is a regular hexagon; one Kekulé structure would alternate long and short bonds
cyclohexanecyclohexene−120“cyclohexatriene”−360benzene−208152
Heats of hydrogenation (kJ mol⁻¹, sheet values): benzene releases 152 kJ mol⁻¹ less than three C=C would, its resonance energy (≈ 150)
  • Not benzene for every CX6HX6\ce{C6H6}. Benzene takes only 3 HX2\ce{H2}. Taking 4 HX2\ce{H2} means four π bonds and no ring: CHX3C≡CC≡CCHX3\ce{CH3C#CC#CCH3} (all 6 H alike, one monobromo product) has 8 π electrons.
Hückel's rule: aromatic or not
4× asked, last 2024
cyclic, planar, every ring atom with a p orbital:(4n+2) π e−=2, 6, 10⇒aromatic;4n π e−⇒antiaromatic;a condition fails⇒non-aromatic\begin{gathered}\text{cyclic, planar, every ring atom with a p orbital:}\\ (4n+2)\,\pi\ e^- = 2,\ 6,\ 10 \Rightarrow \text{aromatic};\quad 4n\,\pi\ e^- \Rightarrow \text{antiaromatic};\quad \text{a condition fails} \Rightarrow \text{non-aromatic}\end{gathered}
A lone pair counts only if it fills a ring p orbital: pyrrole N (6π). Pyridine's N pair is in-plane sp²; 6π from 3 π bonds.
C -
C₅H₅⁻: 6π, aromatic
C +
C₇H₇⁺ (tropylium): 6π, aromatic
cyclobutadiene: 4π, antiaromatic
N H
pyrrole: N pair in ring, 6π
RingNameRing π e⁻Verdict
C +
cyclopropenyl cation2aromatic
C -
cyclopentadienyl anion6aromatic
C +
cyclopentadienyl cation4antiaromatic
cyclobutadiene4antiaromatic
cyclopentadiene: non-aromatic (sp³ C)
C +
tropylium6aromatic
N H
pyrrole6aromatic
N
pyridine6aromatic
cyclooctatetraene8non-aromatic (tub, not planar)
C - C -
COT dianion10aromatic
π electrons counted atom by atom: a C in a ring double bond 1, C⁻ 2, C⁺ 0, ring N–H or O 2 (pyridine’s N is in a double bond: 1, its lone pair stays outside). COT: 8π would be antiaromatic if flat, so it puckers into a tub.
  • π electrons in a molecule: 2 per C=C or C=O, 4 per C≡C, 6 per benzene ring. Lone pairs on –OH or –OCH₃ outside the ring are not counted: vanillin has ring 6 + CHO 2 = 8 π.
  • Ions flip: CX3HX3X+\ce{C3H3+} (2π) and CX5HX5X−\ce{C5H5-} aromatic, but CX5HX5X+\ce{C5H5+} (4π) antiaromatic. Cyclooctatetraene is a non-planar tub (non-aromatic); its dianion (10π, planar) is aromatic.
  • Not 'aromatic needs 6 π'. n=0n=0 gives 2: the cyclopropenyl cation is aromatic. An sp3sp^3 ring C (cyclopentadiene) breaks conjugation: non-aromatic.
Preparing benzene; naming arenes
2× asked, last 2025
3 HC≡CH→873 Kred-hot Fe tubeCX6HX6,CX6HX5OH+Zn→ΔCX6HX6+ZnOCX6HX5COONa+NaOH→ΔCaOCX6HX6+NaX2COX3\begin{gathered}\ce{3HC#CH ->[\text{red-hot Fe tube}][873 K] C6H6},\qquad \ce{C6H5OH + Zn ->[\Delta] C6H6 + ZnO}\\ \ce{C6H5COONa + NaOH ->[CaO][\Delta] C6H6 + Na2CO3}\end{gathered}
Disubstituted: o = 1,2; m = 1,3; p = 1,4. Common names: toluene, xylenes, cumene, styrene, anisole (methoxybenzene).
O O H
o-toluic acid
O O H
m-toluic acid
O O H
p-toluic acid
O H O
phenylacetic acid
each of the four
NaOH / CaO
Δ, − COX2\ce{CO2}
toluene (CX7HX8\ce{C7H8})
  • Quick method. Soda lime only swaps –COOH for H, so the arene formed fixes the skeleton: a CX8HX8OX2\ce{C8H8O2} acid giving toluene is a toluic or phenylacetic acid.

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7 more concepts in 3 topics

Electrophilic aromatic substitution, Directive influence of substituents and Reactions of the side chain. Read the first part free. Sign in free with Google to read the rest and download the PDF.

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Electrophilic aromatic substitution

  • EAS mechanism: σ-complex2×
  • Halogenation, nitration, sulphonation4×
  • Friedel–Crafts alkylation2×
  • Friedel–Crafts acylation2×

Directive influence of substituents

  • Ortho/para and meta directors4×
  • Second substitution5×

Reactions of the side chain

  • Side-chain oxidation2×

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