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Halogenation: Conditions Matter

$X_2$ in UV Light/Heat vs $X_2$ with Lewis Acid ($FeCl_3/AlCl_3$).

By chemca Team • Updated Jan 2026

The reaction of halogens ($Cl_2$ or $Br_2$) with hydrocarbons depends entirely on the reaction conditions. Light ($h\nu$) or Heat ($\Delta$) promotes Free Radical Substitution on alkyl chains, while Lewis Acids ($FeCl_3, AlCl_3$) promote Electrophilic Substitution on aromatic rings.

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

Free Radical Substitution

High energy photons or heat cause Homolytic Fission of the halogen bond ($Cl-Cl \to 2Cl^\bullet$). The halogen radical attacks $sp^3$ hybridized carbons (alkanes or side chains).

A. Alkanes: Hydrogen atoms are replaced by Halogen. Reactivity of H: $3^\circ > 2^\circ > 1^\circ$.

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

B. Alkyl Benzenes (Side Chain Halogenation): Substitution occurs at the Benzylic position due to the stability of the benzylic radical.

$$ \underset{\text{Toluene}}{C_6H_5-CH_3} + Cl_2 \xrightarrow{h\nu \text{ or } \Delta} \underset{\text{Benzyl Chloride}}{C_6H_5-CH_2Cl} + HCl $$

Further chlorination gives Benzal chloride ($Ph-CHCl_2$) and Benzotrichloride ($Ph-CCl_3$).

C. Alkenes (Allylic Halogenation): At high temperatures ($>500^\circ C$), substitution at the Allylic position is favored over addition to the double bond.

$$ CH_3-CH=CH_2 + Cl_2 \xrightarrow{773 K} \underset{\text{Allyl Chloride}}{Cl-CH_2-CH=CH_2} + HCl $$

2. $X_2$ + Lewis Acid ($FeCl_3, AlCl_3, Fe$)

Electrophilic Aromatic Substitution (EAS)

Lewis acids accept electron pairs, causing Heterolytic Fission of the halogen bond to generate an electrophile ($X^+$).

$$ Cl-Cl + FeCl_3 \rightarrow Cl^+ \text{ (Electrophile)} + FeCl_4^- $$

Reaction with Benzene/Derivatives: Substitution occurs on the Aromatic Ring ($sp^2$ carbon).

Example: Chlorination of Toluene The methyl group is ortho-para directing.

$$ \underset{\text{Toluene}}{C_6H_5-CH_3} + Cl_2 \xrightarrow{FeCl_3, \text{Dark}} \underset{\text{o-Chlorotoluene}}{o\text{-Cl-C}_6H_4\text{-CH}_3} + \underset{\text{p-Chlorotoluene}}{p\text{-Cl-C}_6H_4\text{-CH}_3} $$

Condition: These reactions are typically carried out in the Dark to prevent free radical side reactions.

3. Summary: Toluene as the Case Study

Reagents	Condition	Mechanism	Active Species	Site of Attack	Product
$Cl_2 + h\nu / \Delta$	Sunlight / Heat	Free Radical Substitution	$Cl^\bullet$ (Radical)	Side Chain ($sp^3$ C)	Benzyl Chloride
$Cl_2 + FeCl_3$	Dark / Catalyst	Electrophilic Substitution	$Cl^+$ (Electrophile)	Benzene Ring ($sp^2$ C)	o/p-Chlorotoluene

4. Note on Addition Reactions

If $X_2$ is used in an inert solvent like $CCl_4$ in the dark without a Lewis acid, it undergoes Electrophilic Addition across double bonds (alkenes/alkynes).

$$ CH_2=CH_2 + Br_2 \xrightarrow{CCl_4, \text{Dark}} \underset{\text{Vicinal Dihalide}}{Br-CH_2-CH_2-Br} $$

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