Addition reactions and aromatic substitution

A π pair can contribute to a new bond

For a familiar conceptual polar alkene addition, the alkene π pair contributes to a bond with an electrophilic partner, and any coupled bond cleavage must also be followed. A proposed cation or other intermediate then needs its own subsequent capture step. A drawn E⁺ is a bookkeeping symbol for electrophilic interaction rather than a guarantee that a bare free ion exists in the actual medium.

Radical addition uses one-electron ledgers and compatible radical-generation/chain conditions. It cannot be predicted by copying a polar mechanism’s pair arrows. A balanced starting and product formula is a constraint both pathways must respect; it does not choose their rate, intermediates or product distribution.

Two site maps can share one elemental balance

The symbolic net C₃H₆+HCl→C₃H₇Cl permits distinct 1-chloropropane and 2-chloropropane connectivities at the atom-balance level. Both have the same molecular formula and net charge zero. That statement neither asserts that both are formed nor supplies their proportions.

Regioselectivity compares which constitutional site relationship is favoured under a stated process. Intermediate energetics, reagent identity, solvent, reversibility and other conditions matter. A familiar orientation mnemonic has a particular mechanistic scope and is not a universal law for every polar, radical or other addition.

A site ratio needs a kinetic model or actual evidence

In an invented irreversible two-site model with the same reactant dependence, constant medium and no product interconversion, departure coefficients k_A=0.0300 s⁻¹ and k_B=0.0100 s⁻¹ give product fractions k_A/(k_A+k_B)=0.750 and 0.250. Its ratio A/B is three.

Those are stipulated abstract channels rather than real alkene measurements. Equilibrium interconversion or another loss route would change the result. A formula balance, a proposed intermediate’s qualitative stability or a diagram without kinetic inputs does not establish that numerical selectivity.

Restoration distinguishes substitution from a retained addition

In a selected electrophilic aromatic substitution model, a ring π pair forms a new bond to the electrophilic partner, giving a sigma-bonded nonaromatic adduct. A subsequent proton-removal ledger restores the conjugated aromatic ring while retaining the substituent. A simple benzene contributor’s three π pairs become two in the sigma-adduct contributor and return to three after restoration.

Contributor drawings of that same sigma adduct are not separate sequential steps. A catalyst or reagent activation, when relevant, needs its own balanced context rather than an unexplained free-ion label. A net addition that permanently changes the conjugated ring would be a different transformation. Aromatic stabilization informs the pathway discussion but does not establish a universal reaction guarantee for all rings and conditions.

Tetrahedral carbon is not automatically a stereocenter

An addition can change a trigonal local environment into a tetrahedral one. To decide whether the product carbon is stereogenic in these simple cases, compare all four attached ligands. Propan-2-ol has OH, H and two identical methyl groups at its central carbon, so that center does not become chiral merely because it is tetrahedral.

A center carrying OH, H, CH₃ and CH₂CH₃ instead has four distinguishable ligands and needs a spatial configuration for an R/S claim. A chiral environment can make face choices inequivalent, but a flat unmarked drawing does not supply an enantiomeric ratio. Retain the actual atom map, geometry and conditions before describing stereoselectivity.

Worked examples

Try it yourself

1. Can a one-electron radical addition ledger be drawn using only ordinary two-electron pair arrows?

   Show the answer and reasoning

   No.

   The arrowheads must match the actual one- or two-electron bookkeeping being proposed.

2. Does conservation of C₃H₇Cl prove which carbon carries Cl in the product?

   Show the answer and reasoning

   No.

   The molecular formula does not determine that atom connectivity.

3. In the stated aromatic substitution model, what restores the ring’s conjugated aromatic system after sigma-adduct formation?

   Show the answer and reasoning

   The compatible proton-removal and bond-pair redistribution step.

   It restores the ring π system while retaining the new substituent.

4. Do multiple resonance contributors of one sigma adduct count as multiple successive intermediates?

   Show the answer and reasoning

   No.

   They describe one entity’s electronic structure rather than additional mechanistic steps.

What to carry forward

• Follow every pair or single electron with the appropriate notation.
• Possible product formulas do not determine site preference.
• Normalize branching fractions only under a stated kinetic model.
• Distinguish aromatic restoration from a retained addition.
• Count distinguishable ligands before assigning a new stereocenter.