Aromatic Hydrocarbons — Revision Notes

⚡ 30-Second Revision

Aromaticity — Cyclic, planar, conjugated, $(4n+2)$ $\pi$ electrons.
Hückel's Rule — $n=0 \rightarrow 2 \pi$; $n=1 \rightarrow 6 \pi$; $n=2 \rightarrow 10 \pi$.
Benzene Structure — Delocalized $6 \pi$ electrons, all C-C bonds equal (139 pm).
EAS Mechanism — Electrophile generation $\rightarrow$ Arenium ion formation $\rightarrow$ Proton loss.
Nitration — Conc. $HNO_3$/Conc. $H_2SO_4 \rightarrow NO_2^+$.
Halogenation — $X_2$/Lewis acid ($FeX_3$) $\rightarrow X^+$.
Sulfonation — Conc. $H_2SO_4$/Oleum $\rightarrow SO_3$.
Friedel-Crafts Alkylation — $R-X$/Lewis acid ($AlCl_3$) $\rightarrow R^+$. Limitations: rearrangement, polyalkylation, deactivated rings.
Friedel-Crafts Acylation — $RCOCl$/Lewis acid ($AlCl_3$) $\rightarrow RCO^+$. No rearrangement/polyacylation.
Ortho/Para Directors (Activating) — $-OH, -OR, -NH_2, -R$. (Except halogens).
Ortho/Para Directors (Deactivating) — Halogens ($-F, -Cl, -Br, -I$).
Meta Directors (Deactivating) — $-NO_2, -COOH, -CHO, -CN, -SO_3H$.
Side-chain Oxidation — Alkylbenzene + $KMnO_4/H^+ \rightarrow$ Benzoic acid (if benzylic H present).

2-Minute Revision

Aromatic hydrocarbons are special cyclic compounds with enhanced stability due to delocalized $\pi$ electrons, following Hückel's Rule of $(4n+2)$ $\pi$ electrons. Benzene, the simplest, has 6 $\pi$ electrons in a planar, conjugated ring.

Their characteristic reactions are Electrophilic Aromatic Substitution (EAS), where an electrophile replaces a hydrogen, preserving aromaticity. Key EAS reactions include nitration (using $HNO_3/H_2SO_4$ to introduce $-NO_2$), halogenation ($X_2/FeX_3$ for $-X$), sulfonation ($H_2SO_4$ for $-SO_3H$), Friedel-Crafts alkylation ($R-X/AlCl_3$ for $-R$), and acylation ($RCOCl/AlCl_3$ for $-COR$).

Alkylation suffers from carbocation rearrangements and polyalkylation, unlike acylation. Substituents on the benzene ring influence the reactivity and regioselectivity of EAS. Electron-donating groups (like alkyls, $-OH$) are activating and ortho/para directing.

Electron-withdrawing groups (like $-NO_2$, $-COOH$) are deactivating and meta directing. Halogens are unique: deactivating but ortho/para directing. Alkyl groups on benzene can be oxidized to carboxylic acids if they have benzylic hydrogens.

5-Minute Revision

Aromatic hydrocarbons are defined by their aromaticity, a property of exceptional stability. This arises from a cyclic, planar, fully conjugated system containing $(4n+2)$ $\pi$ electrons, as per Hückel's Rule.

Benzene, with its 6 delocalized $\pi$ electrons, is the prime example. Its C-C bond lengths are intermediate due to resonance. Aromatic compounds predominantly undergo Electrophilic Aromatic Substitution (EAS) reactions, where an electrophile replaces a hydrogen atom, crucially maintaining the stable aromatic ring.

The general mechanism involves the generation of a strong electrophile, its attack on the electron-rich benzene ring to form a resonance-stabilized arenium ion (sigma complex), and subsequent deprotonation to restore aromaticity.

Key EAS reactions and their electrophiles are:

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Nitration — $HNO_3/H_2SO_4 \rightarrow NO_2^+$ (nitronium ion).
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Halogenation — $X_2/FeX_3 \rightarrow X^+$ (polarized halogen).
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Sulfonation — $H_2SO_4$ (conc.)/Oleum $\rightarrow SO_3$ (sulfur trioxide).
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Friedel-Crafts Alkylation — $R-X/AlCl_3 \rightarrow R^+$ (carbocation). This reaction is prone to carbocation rearrangements and polyalkylation, as the introduced alkyl group activates the ring.
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Friedel-Crafts Acylation — $RCOCl/AlCl_3 \rightarrow RCO^+$ (acylium ion). This is preferred over alkylation as acylium ions do not rearrange, and the acyl group deactivates the ring, preventing polyacylation.

Substituents on a monosubstituted benzene ring exert a 'directive influence' on incoming electrophiles and affect the reaction rate. Electron-donating groups (EDGs) like alkyls, $-OH$, $-NH_2$ are activating and direct to ortho/para positions.

Electron-withdrawing groups (EWGs) like $-NO_2$, $-COOH$, $-CHO$ are deactivating and direct to the meta position. Halogens are a special case: they are deactivating due to strong inductive withdrawal but ortho/para directing due to resonance donation of lone pairs.

Finally, alkyl groups attached to a benzene ring can be oxidized to carboxylic acids (e.g., toluene to benzoic acid) using strong oxidizing agents like alkaline $KMnO_4$, provided there's at least one benzylic hydrogen.

Prelims Revision Notes

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Aromaticity — Defined by Hückel's Rule: cyclic, planar, fully conjugated, and $(4n+2)$ $\pi$ electrons. Examples: Benzene (6 $\pi$), Naphthalene (10 $\pi$), Pyridine (6 $\pi$), Furan (6 $\pi$), Pyrrole (6 $\pi$), Cyclopropenyl cation (2 $\pi$).
2
Anti-aromaticity — Cyclic, planar, fully conjugated, $4n$ $\pi$ electrons (e.g., cyclobutadiene, cyclopentadienyl cation). Highly unstable.
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Non-aromaticity — Fails any of the first three criteria (e.g., cyclooctatetraene, cyclopentadiene).
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Benzene Structure — All C-C bonds are 139 pm, intermediate between single (154 pm) and double (134 pm) bonds, due to $\pi$-electron delocalization.
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Electrophilic Aromatic Substitution (EAS) — Characteristic reaction. Electrophile ($E^+$) replaces $H$. Mechanism: Electrophile generation $\rightarrow$ Arenium ion (sigma complex) formation $\rightarrow$ Proton loss to restore aromaticity.

* Nitration: Reagents: Conc. $HNO_3$ + Conc. $H_2SO_4$. Electrophile: $NO_2^+$. * Halogenation: Reagents: $X_2$ + Lewis acid ($FeX_3$, $AlX_3$). Electrophile: $X^+$. * Sulfonation: Reagents: Conc.

$H_2SO_4$ or Oleum. Electrophile: $SO_3$. * Friedel-Crafts Alkylation: Reagents: $R-X$ + Lewis acid ($AlCl_3$). Electrophile: $R^+$. Limitations: Carbocation rearrangement, polyalkylation, fails with deactivated rings.

* Friedel-Crafts Acylation: Reagents: $RCOCl$ or $(RCO)_2O$ + Lewis acid ($AlCl_3$). Electrophile: $RCO^+$. Advantages: No rearrangement, no polyacylation.

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Directive Influence

* Ortho/Para Directing & Activating: $-OH, -OR, -NH_2, -NR_2, -R$ (alkyl groups). Electron-donating groups. * Ortho/Para Directing & Deactivating: Halogens ($-F, -Cl, -Br, -I$). Inductive withdrawal > Resonance donation. * Meta Directing & Deactivating: $-NO_2, -CN, -CHO, -COOH, -COOR, -SO_3H$. Electron-withdrawing groups.

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Side-chain Oxidation — Alkylbenzenes with benzylic hydrogens are oxidized to benzoic acid by strong agents like alkaline $KMnO_4$ followed by acidification. Example: Toluene $\rightarrow$ Benzoic acid.
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Preparation of Benzene — From Ethyne (cyclization), Phenol (reduction), Benzoic acid (decarboxylation).

Vyyuha Quick Recall

To remember Hückel's Rule criteria: Can Planar Compounds Have Aromas?

Cyclic
Planar
Conjugated (fully)
Hückel's Rule (4n+2 $\pi$ electrons)
Aromatic (result)