Aromatic Substitution & Formylation
How new groups are stitched onto a benzene ring — almost the whole Part runs on one machine, electrophilic aromatic substitution (EAS): an electrophile adds to the aromatic π-system to make a resonance-stabilised arenium (Wheland) ion, then a proton is lost to restore aromaticity. Two layers on every reaction: a hand-held beginner path (plain “what it does”, arrow-by-arrow mechanism, graded worked examples, trap boxes) and a research-grade advanced/reference path (arenium-ion resonance, directing-effect rationale, activating/deactivating scales, partial-rate factors and selectivity, acylium vs carbenium electrophiles, Meisenheimer-complex stabilisation, benzyne regiochemistry, and modern variants). EAS fundamentals: arenium ion, activating/deactivating, ortho/para vs meta. Friedel–Crafts: alkylation (rearrangement/polyalkylation) & acylation. Formylation & ring functionalisation: Gattermann, Gattermann–Koch, Vilsmeier–Haack, Reimer–Tiemann, Kolbe–Schmitt, Houben–Hoesch. Diazonium chemistry: Sandmeyer, Balz–Schiemann, Bucherer, azo coupling & Japp–Klingemann. Advanced mechanisms: SNAr (Meisenheimer) and benzyne (elimination–addition) — closing with a directing-effects master table and a “how do I put a group here?” decision map.
The 18 sections in Part 6
- 1The arenium (Wheland) ion — do this first Free below
- 2Directing & activating effects — the master logic
- 3The Friedel–Crafts alkylation
- 4The Friedel–Crafts acylation
- 5The Gattermann & Gattermann–Koch formylations
- 6The Vilsmeier–Haack formylation
- 7The Reimer–Tiemann reaction
- 8The Kolbe–Schmitt reaction (phenol carboxylation)
- 9The Houben–Hoesch reaction (ketone synthesis)
- 10Diazotisation — making the diazonium salt
- 11The Sandmeyer reaction (and related diazonium substitutions)
- 12The Balz–Schiemann reaction (aryl fluorides)
- 13The Bucherer reaction (naphthol ↔ naphthylamine)
- 14Azo coupling & the Japp–Klingemann reaction
- 15SNAr — the addition–elimination mechanism
- 16The benzyne (elimination–addition) mechanism
- 17Directing-effects master table
- 18Aromatic-functionalisation decision guide
The arenium (Wheland) ion — do this first
Free extractSection M.0 of Part 6, reproduced in full from the book — figures and all. No sign-in, no paywall on this section.
One mechanism underlies the entire Part: the ring donates a pair of π-electrons to E⁺, making a charged non-aromatic intermediate, which then loses H⁺ to become aromatic again.
In one line: in EAS, the aromatic ring uses two of its π-electrons to bond to an electrophile E⁺, forming a resonance-stabilised arenium ion (also called the σ-complex or Wheland intermediate); a base then removes the proton from the carbon that picked up E, and the ring becomes aromatic again. Net result: a ring H is replaced by E.
Mechanism — The two steps of EAS
- Step 1 (slow, rate-determining): electrophilic attack. Two π-electrons of the ring attack E⁺. The attacked carbon becomes sp³ (it now carries both H and E); the ring loses aromaticity and becomes a positively charged arenium/Wheland ion, whose + charge is spread over the ortho and para carbons. This step has a high barrier because aromatic stabilisation is temporarily lost.
- Step 2 (fast): deprotonation. A base (e.g. the conjugate base of the acid catalyst, or the AlCl₄⁻ counter-ion) removes the proton from the sp³ carbon. The C–H bonding electrons fall back into the ring, restoring the aromatic sextet. The product is the substituted arene.
Draw the three resonance forms of the arenium ion from nitration of benzene Easy
Is the first or the second step rate-determining, and how do we know? Medium
⚠ Common mistakes & exam traps
- EAS is substitution: an H is replaced. Do not draw the ring ‘adding’ E and a nucleophile across a double bond (that is alkene chemistry).
- The arenium ion’s + charge is on the carbons ortho and para to the point of attack — never meta, never on the sp³ carbon. Getting this wrong wrecks every directing-effect argument.
- The slow step is attack on E⁺, not the loss of H⁺; hence usually no C–H/C–D isotope effect.
- Aromaticity is only lost temporarily. Step 2 always restores it — that is the whole reason benzene substitutes rather than adds.
π-complex vs σ-complex, the Hammond picture, and the quantitative language of EAS (partial rate factors, selectivity, the Hammett relationship).
Before the σ-complex (arenium ion) forms, a weak π-complex (electrophile loosely associated with the π-face) can precede it; the π-complex is usually not product-determining. The arenium (σ-complex) is a true intermediate — it has been directly observed as stable salts at low temperature (e.g. the heptamethylbenzenium ion; the benzenium ion in superacid). By Hammond’s postulate, because the rate-determining transition state resembles the high-energy arenium ion, anything that stabilises the arenium ion stabilises that TS and accelerates the reaction — this is exactly why the arenium-ion resonance picture predicts both rate (activation) and regiochemistry (direction).
Quantifying reactivity: partial rate factors and selectivity
The reactivity of a specific ring position is captured by a partial rate factor (fo, fm, fp): the rate at that position relative to one position of benzene. A factor >1 means activated, <1 deactivated. From these one builds the observed isomer ratio and the substrate’s selectivity. More selective (milder) electrophiles discriminate more sharply between o/p and m; very reactive electrophiles are less selective (the reactivity–selectivity principle). Toluene’s partial rate factors in nitration (often quoted as roughly fo≈42, fm≈2.5, fp≈58 under one set of conditions) illustrate strong o/p activation by CH₃.
Read the rest of Part 6
The remaining 17 sections of this part — Directing & activating effects — the master logic, The Friedel–Crafts alkylation, The Friedel–Crafts acylation, The Gattermann & Gattermann–Koch formylations — and all nine parts of Named Reactions are part of ChemVidya Full Access, along with the other books, 55 Study Notes and 6,000+ practice questions.
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