# Halogenoalkanes and Nucleophilic Substitution
Halogenoalkanes contain a C−X bond (where X = F, Cl, Br, or I). The carbon is electron-deficient (δ+) due to the electronegativity of the halogen, making it susceptible to attack by nucleophiles. This leads to two key types of reaction: nucleophilic substitution and elimination.
1. Nucleophilic Substitution
A nucleophile is an electron-pair donor that attacks an electron-deficient carbon.
Common nucleophiles: OH⁻, CN⁻, NH₃, H₂O
SN2 Mechanism (Primary Halogenoalkanes)
- One step — the nucleophile attacks at the same time as the halide leaves
- Bimolecular — rate depends on both nucleophile and halogenoalkane concentration
- Rate = k[RX][Nu⁻]
- Curly arrow from nucleophile lone pair to δ+ carbon; another from C-X bond to X
SN1 Mechanism (Tertiary Halogenoalkanes)
- Two steps — the halide leaves first to form a carbocation, then the nucleophile attacks
- Unimolecular — rate depends only on halogenoalkane concentration
- Rate = k[RX]
- Step 1: C−X bond breaks heterolytically → carbocation + X⁻
- Step 2: Nucleophile attacks the carbocation
Secondary Halogenoalkanes
React via a mixture of SN1 and SN2.
2. Key Nucleophilic Substitution Reactions
| Nucleophile | Conditions | Product | Name |
|---|---|---|---|
| OH⁻ (NaOH/KOH, aqueous) | Reflux | Alcohol (R-OH) | Hydrolysis |
| CN⁻ (KCN in ethanol/water) | Reflux | Nitrile (R-CN) | — |
| NH₃ (excess, in ethanol) | Sealed tube, heat | Amine (R-NH₂) | — |
The CN⁻ reaction is important because it increases the carbon chain length by one carbon.
3. Elimination vs Substitution
With ethanolic NaOH (NaOH dissolved in ethanol), halogenoalkanes undergo elimination rather than substitution:
| Conditions | Reaction Type | Product |
|---|---|---|
| Aqueous NaOH, reflux | Nucleophilic substitution | Alcohol |
| Ethanolic NaOH, reflux | Elimination | Alkene |
4. Reactivity of Halogenoalkanes
Bond Strength
C−F > C−Cl > C−Br > C−I (bond enthalpy decreases)
Since C−I is the weakest bond, iodoalkanes react fastest and fluoroalkanes react slowest.
Hydrolysis Rates (Practical)
React RCl, RBr, RI with aqueous AgNO₃:
- RI produces precipitate fastest (AgI = yellow)
- RCl reacts slowest (AgCl = white)
5. Ozone Depletion
Chlorofluorocarbons (CFCs) were used as refrigerants and aerosols. In the upper atmosphere, UV light breaks C-Cl bonds:
Chlorine radicals catalytically destroy ozone:
One Cl radical can destroy thousands of O₃ molecules. CFCs are now banned (Montreal Protocol).
6. Practice Questions
- Draw the SN2 mechanism for the reaction of 1-bromopropane with NaOH(aq).
- Draw the SN1 mechanism for 2-bromo-2-methylpropane with water.
- Explain why aqueous NaOH gives substitution but ethanolic NaOH gives elimination.
- Place CH₃F, CH₃Cl, CH₃Br, CH₃I in order of increasing rate of hydrolysis.
- Write equations showing how chlorine radicals from CFCs destroy ozone.
Want to check your answers and get step-by-step solutions?
Summary
- Nucleophilic substitution: nucleophile replaces halogen
- SN2: one step, primary halogenoalkanes; SN1: two steps, tertiary
- Aqueous NaOH → substitution (alcohol); ethanolic NaOH → elimination (alkene)
- Reactivity: R-I > R-Br > R-Cl > R-F (weaker C-X bond reacts faster)
- CFCs deplete ozone via chlorine radical chain reactions