Halogenation: Heat/Light vs Lewis Acid

Master the difference between Halogenation in sunlight (Free Radical) and with a Lewis Acid catalyst (Electrophilic Substitution). A crucial concept for Side-chain vs. Ring chlorination in Toluene.

The reaction of halogens ($Cl_2, Br_2$) with hydrocarbons produces vastly different products depending on the conditions. Heat or Light favors Radical pathways (alkanes/side-chain), while Lewis Acids favor Ionic pathways (aromatic ring).

1. $X_2$ with Heat ($\Delta$) or Light ($h\nu$)

Free Radical Substitution

High energy conditions generate halogen radicals ($X^\bullet$), which attack $sp^3$ hybridized C-H bonds.

A. Alkanes:

Direct substitution of Hydrogen.

$$ R-H + Cl_2 \xrightarrow{h\nu} R-Cl + HCl $$

B. Alkyl Benzenes (Side Chain Substitution):

Specific substitution at the highly reactive benzylic position.

$$ \underset{\text{Toluene}}{C_6H_5CH_3} + Cl_2 \xrightarrow{h\nu} \underset{\text{Benzyl Chloride}}{C_6H_5CH_2Cl} \xrightarrow{Cl_2} \dots \xrightarrow{Cl_2} \underset{\text{Benzotrichloride}}{C_6H_5CCl_3} $$

C. Benzene (Addition Reaction):

Exception: Under drastic conditions (excess $Cl_2$, strong UV light), Benzene loses aromaticity to add chlorine.

$$ C_6H_6 + 3Cl_2 \xrightarrow{UV, 500K} \underset{\text{BHC (Gammaxene)}}{C_6H_6Cl_6} $$

2. $X_2$ with Lewis Acid ($FeCl_3, AlCl_3$)

Electrophilic Aromatic Substitution (EAS)

The Lewis acid accepts a lone pair from the halogen molecule, generating a strong electrophile ($X^+$ or complex), which attacks the aromatic ring.

A. Benzene (Ring Substitution):

$$ C_6H_6 + Cl_2 \xrightarrow{\text{Anhyd. } AlCl_3} \underset{\text{Chlorobenzene}}{C_6H_5Cl} + HCl $$

B. Alkyl Benzenes (Ring Substitution):

Toluene directs the incoming halogen to Ortho and Para positions due to hyperconjugation and inductive effects.

$$ \underset{\text{Toluene}}{C_6H_5CH_3} + Cl_2 \xrightarrow{FeCl_3} \underset{\text{o-Chlorotoluene}}{o\text{-}ClC_6H_4CH_3} + \underset{\text{p-Chlorotoluene}}{p\text{-}ClC_6H_4CH_3} $$

Mechanism Note: Generation of the Electrophile. $$ Cl-Cl + AlCl_3 \rightleftharpoons Cl^+ \dots AlCl_4^- $$

3. Toluene: The Critical Distinction

The clearest way to understand these reagents is to observe their effects on Toluene, a molecule with both an aromatic ring and an aliphatic side chain.

Conditions	Mechanism Type	Target Site	Major Product
$Cl_2$ / Sunlight ($h\nu$)	Free Radical Substitution	Side Chain ($sp^3$ C)	Benzyl Chloride
$Cl_2$ / Lewis Acid ($Fe/FeCl_3$)	Electrophilic Substitution	Benzene Ring ($sp^2$ C)	o/p-Chlorotoluene

Explore More Chemistry Resources

Knowledge Check

Test your understanding of Reaction Conditions

Search This Blog