Nucleophilic substitution is one of the most important reaction types in IB Chemistry Topic 10 (Organic Chemistry) and Topic 20 (HL Organic). It explains how molecules exchange functional groups through attack by electron-rich species. These reactions form the basis of many organic transformations, including preparing alcohols, halogenoalkanes, and amines. Understanding nucleophilic substitution is essential for exam mechanisms, reaction prediction, and arrow-pushing skills.
What Is Nucleophilic Substitution?
Nucleophilic substitution is a reaction where a nucleophile replaces a leaving group in a molecule.
Breaking it down:
- Nucleophile = electron-rich species that donates a lone pair (e.g., OH⁻, CN⁻, NH₃).
- Leaving group = atom or group that departs with electrons (commonly a halide: Cl⁻, Br⁻, I⁻).
- Substitution = one group replaces another.
This reaction occurs most commonly in halogenoalkanes because the carbon–halogen bond is polar, making carbon electrophilic and vulnerable to attack.
Why Nucleophilic Substitution Happens
The carbon atom bonded to a halogen carries a partial positive charge due to electronegativity differences:
C–δ⁺ — X–δ⁻
The nucleophile is attracted to this electron-poor carbon and attacks it.
The halogen (X⁻) acts as the leaving group and departs with the bonding electrons.
This electron flow is the foundation of the mechanism.
Two Types of Nucleophilic Substitution: SN1 and SN2
IB Chemistry requires you to know both mechanisms, their conditions, and how structure affects them.
SN2 Mechanism (Bimolecular Nucleophilic Substitution)
Key features:
- Occurs in primary halogenoalkanes
- One-step mechanism
- Nucleophile attacks from the opposite side
- Backside attack causes inversion of configuration
- Rate depends on both the nucleophile and the substrate
Reaction rate:
Rate = k[halogenoalkane][nucleophile]
Mechanism steps:
- Nucleophile attacks carbon from the back.
- Transition state forms with partial bonds to both nucleophile and leaving group.
- Leaving group departs.
- Product forms with inverted geometry (Walden inversion).
Why primary halogenoalkanes favor SN2:
- Little steric hindrance
- Nucleophile easily reaches the carbon
SN2 is fast when nucleophiles are strong and unhindered.
SN1 Mechanism (Unimolecular Nucleophilic Substitution)
Key features:
- Occurs in tertiary halogenoalkanes
- Two-step mechanism
- Forms a carbocation intermediate
- Nucleophile can attack from either side → may produce racemic mixture
- Rate depends only on halogenoalkane concentration
Reaction rate:
Rate = k[halogenoalkane]
Mechanism steps:
- Carbon–halogen bond breaks → carbocation forms.
- Nucleophile attacks the carbocation.
- Product forms.
Why tertiary halogenoalkanes favor SN1:
- Tertiary carbocations are stabilized by alkyl groups
- Steric hindrance blocks SN2 attack
SN1 is fastest in polar protic solvents that stabilize ions.
Choosing Between SN1 and SN2
Primary halogenoalkanes → SN2
Low steric hindrance, strong nucleophile attack.
Tertiary halogenoalkanes → SN1
Stable carbocation formation.
Secondary halogenoalkanes
Can undergo both depending on solvent and nucleophile strength.
Role of Solvents
Polar protic solvents (water, alcohols)
- Stabilize carbocations
- Favor SN1
Polar aprotic solvents (acetone, DMSO)
- Do NOT stabilize carbocations
- Strengthen nucleophiles
- Favor SN2
Solvent choice is a major IB HL point.
Importance in Organic Synthesis
Nucleophilic substitution can produce:
- Alcohols (from halogenoalkanes + OH⁻)
- Nitriles (from halogenoalkanes + CN⁻)
- Amines (from halogenoalkanes + NH₃)
These products are crucial for building more complex molecules.
Common IB Misunderstandings
“Strong nucleophiles always cause SN2 reactions.”
Not true—substrate structure and solvent matter more.
“Carbocations form in all substitution reactions.”
Only SN1 mechanisms involve carbocations.
“Tertiary halogenoalkanes can do SN2.”
Steric hindrance makes SN2 nearly impossible for tertiary structures.
FAQs
Is SN1 or SN2 faster?
It depends—SN2 is fast for primary substrates, SN1 is fast for tertiary ones.
Can a reaction switch from SN1 to SN2?
Yes. Changing solvent or nucleophile strength can shift the mechanism.
Why do SN1 reactions give racemic mixtures?
The planar carbocation can be attacked from either side.
Conclusion
Nucleophilic substitution is a key organic reaction where a nucleophile replaces a leaving group. It occurs through two mechanisms: SN1 (carbocation-based, two-step) and SN2 (backside attack, one-step). Understanding when each mechanism occurs helps IB Chemistry students predict products, draw mechanisms, and analyze organic structures with confidence.
