Nucleophilic Substitution
Quick Notes
- Halogenoalkanes contain polar C–X bonds (C is δ+ and X is δ−).
- Nucleophiles (electron pair donors, such as OH−, CN−, NH3) can attack the δ+ carbon, replacing the halogen.
- Mechanism involves a curly arrow from the nucleophile to the C–X bond.
- The rate of reaction depends on the carbon–halogen bond enthalpy
- Weaker bonds break more easily giving faster rate of reaction
- C–Cl bonds stronger than C–Br bonds and C–Br bonds stronger than C–I bonds. Meaning iodoalkanes react fastest, chloroalkanes slowest.
Full Notes
Nucleophilic substitution of halogenoalkanes has been outlined in more detail here and SN1 and SN2 mechanisms have been outlined in this video.
This page is just what you need to know for AQA A-level Chemistry :)
Why Do Halogenoalkanes Undergo Nucleophilic Substitution?
Halogenoalkanes contain a polar C–X bond because halogens are more electronegative than carbon.
The carbon is δ+, making it susceptible to attack by nucleophiles that can replace the halogen in substitution reactions.
Nucleophiles are electron pair donors and they must have a lone pair of electrons.
Example Nucleophiles:
OH− (Hydroxide ion) → Forms alcohols.
CN− (Cyanide ion) → Forms nitriles.
NH3 (Ammonia) → Forms amines.
Nucleophilic Substitution Mechanism
Primary and secondary halogenoalkanes follow the following mechanism when they react with nucleophiles:
1. Curly arrow from nucleophile to δ+ carbon.
2. Curly arrow from C–X bond to halogen (X− leaves).
3. New bond forms between nucleophile and carbon.
There are several key nucleophilic substitution reactions of halogenoalkanes you need to know:
Reaction with OH− (Hydrolysis to Alcohols)
Reagent: Aqueous NaOH/KOH.
Conditions: Warm, reflux.
Reaction with CN− (Formation of Nitriles)
Reagent: KCN in ethanol.
Conditions: Reflux.
Increases carbon chain length.
Reaction with NH3 (Formation of Amines)
Reagent: Excess NH3 in ethanol.
Conditions: Sealed tube, pressure.
Note that here there is a middle step in the mechanism as well - this is because another NH3 molecule will take a H+ ion from the NH3+ group in the intermediate if the NH3 is in excess, forming an NH4+ ion.
Effect of Carbon–Halogen Bond Enthalpy on Reaction Rate
The rate of substitution is dependent on the strength of the carbon–halogen bond as the bond has to break at the start of the reaction.
The weaker the bond, the faster the rate of reaction as less energy is needed to break the bond (lower activation energy).
C–F bond is the strongest → least reactive, slowest rate.
C–I bond is the weakest → most reactive, fastest rate.
Order of reactivity:
Iodoalkanes > Bromoalkanes > Chloroalkanes > Fluoroalkanes
It is possible to compare the rates of hydrolysis for different halogenoalkanes by adding aqueous silver nitrate and ethanol to the reaction mixture and timing how long it takes for a silver halide precipitate to form. The precipitate is formed by the halide ion released from the halogenoalkane and silver ions from the silver nitrate. The faster the forming of a precipitate, the faster the rate of reaction.
Ethanol is added to help the halogenoalkane dissolve in the aqueous mixture. Its OH group allows it to mix with polar substances (like water and Ag+ ions), while its ethyl group (CH3CH2) helps it dissolve non-polar substances, such as the hydrocarbon chain of a halogenoalkane.
Summary Table: Nucleophilic Substitution of Halogenoalkanes
| Reaction | Reagent | Conditions | Product |
|---|---|---|---|
| Reaction with OH− (Hydrolysis to Alcohols) | Aqueous NaOH / KOH | Warm, reflux | Alcohol |
| Reaction with CN− (Formation of Nitriles) | KCN in ethanol | Reflux | Nitrile (chain length increases) |
| Reaction with NH3 (Formation of Amines) | Excess NH3 in ethanol | Sealed tube, pressure | Primary amine + NH4X |