Home / IB DP / IB DP Chemistry 2026, 2027 & 2028 / IB DP Chemistry SL & HL Study Notes / IB DP Chemistry -R3.4.2 Nucleophilic substitution reactions – Study Notes

IB DP Chemistry -R3.4.2 Nucleophilic substitution reactions – Study Notes

IB DP Chemistry - R3.4.2 Nucleophilic substitution reactions - Study Notes - New Syllabus - 2026, 2027 & 2028

IB DP Chemistry – R3.4.2 Nucleophilic substitution reactions – Study Notes – New Syllabus

IITian Academy excellent Introduction to the Proton transfer reactions – Study Notes and effective strategies will help you prepare for your IB DP Chemistry  exam.

IB DP Chemistry Study Notes – All Topics

Reactivity 3.4.2 — Nucleophilic Substitution Reactions

Reactivity 3.4.2 — Nucleophilic Substitution Reactions

 A nucleophilic substitution reaction is a type of reaction in which a nucleophile (electron-pair donor) attacks an electrophilic carbon, replacing a leaving group (usually a halide). It involves the formation of a new bond as another breaks.

General Mechanism:

  • The nucleophile uses its lone pair of electrons to form a new bond with a carbon atom.
  • At the same time, the leaving group (a halogen, for example) departs, taking the electrons from the bond with it.
  • This process is a substitution because one group is replaced by another.

General Equation:

\( \text{R-X} + \text{Nu}^- \rightarrow \text{R-Nu} + \text{X}^- \)

where:
\( \text{R} \) = alkyl group, \( \text{X} \) = leaving group (halide), \( \text{Nu}^- \) = nucleophile

Types of Nucleophilic Substitution:

  • SN1 Mechanism (Unimolecular Nucleophilic Substitution)
  • SN2 Mechanism (Bimolecular Nucleophilic Substitution)

SN1 Mechanism:

 

  • Occurs in two steps
  • First, the leaving group leaves forming a carbocation intermediate
  • Second, the nucleophile attacks the carbocation
  • Favoured by tertiary halogenoalkanes and polar protic solvents

Mechanism Example (SN1):

\( (\text{CH}_3)_3\text{CBr} + \text{H}_2\text{O} \rightarrow (\text{CH}_3)_3\text{COH} + \text{HBr} \)

SN2 Mechanism:

  • Occurs in a single concerted step (no intermediate)
  • Nucleophile attacks the carbon atom from the opposite side of the leaving group (backside attack)
  • Favoured by primary halogenoalkanes and polar aprotic solvents

Mechanism Example (SN2):

\( \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^- \)

Movement of Electron Pairs:

  • Shown with curly arrows (double-barbed arrows) indicating the movement of an electron pair
  • From the lone pair of the nucleophile to the electrophilic carbon
  • From the bond between carbon and leaving group to the leaving group itself

Diagrammatic Representation:

  • Nucleophile approaches the carbon bonded to a halogen
  • Lone pair forms a new bond to carbon
  • Simultaneously, the carbon–halogen bond breaks and the halide ion leaves

Example :

Explain the Reaction of ethyl bromide with hydroxide ion

▶️Answer/Explanation

This is a classic SN2 mechanism. The hydroxide ion, \( \text{OH}^- \), attacks the electrophilic carbon in ethyl bromide \( \text{CH}_3\text{CH}_2\text{Br} \), resulting in substitution:
\( \text{CH}_3\text{CH}_2\text{Br} + \text{OH}^- \rightarrow \text{CH}_3\text{CH}_2\text{OH} + \text{Br}^- \)
The reaction occurs in a single step with inversion of configuration at the carbon atom.

Example :

Explain the Hydrolysis of tert-butyl chloride in water

▶️Answer/Explanation

This is an SN1 mechanism. First, the leaving group \( \text{Cl}^- \) departs, forming a stable carbocation:
Step 1: \( (\text{CH}_3)_3\text{CCl} \rightarrow (\text{CH}_3)_3\text{C}^+ + \text{Cl}^- \)
Step 2: \( (\text{CH}_3)_3\text{C}^+ + \text{H}_2\text{O} \rightarrow (\text{CH}_3)_3\text{COH}_2^+ \)
Step 3: \( (\text{CH}_3)_3\text{COH}_2^+ \rightarrow (\text{CH}_3)_3\text{COH} + \text{H}^+ \)

Example :

Explain the Reaction of cyanide ion with bromoethane

▶️Answer/Explanation

The nucleophile \( \text{CN}^- \) performs a backside attack on the electrophilic carbon of ethyl bromide:
\( \text{CH}_3\text{CH}_2\text{Br} + \text{CN}^- \rightarrow \text{CH}_3\text{CH}_2\text{CN} + \text{Br}^- \)
This is an SN2 mechanism, and the product is propanenitrile.

Example

A student mixes aqueous sodium hydroxide with 1-bromopropane in a polar aprotic solvent.

Identify the nucleophile, write a balanced equation for the reaction, and explain the steps of the substitution mechanism using curly arrows.

▶️Answer/Explanation

Nucleophile: \( \ce{OH^-} \)

Equation: \( \ce{CH3CH2CH2Br + OH^- -> CH3CH2CH2OH + Br^-} \)

Mechanism (SN2):
1. The hydroxide ion donates a lone pair to the electrophilic carbon (opposite side of the Br).
2. A transition state forms with both \( \ce{OH} \) and \( \ce{Br} \) attached.
3. The \( \ce{Br^-} \) leaves with the bonding pair of electrons.

Curly Arrows:
From lone pair on \( \ce{OH^-} \) to the carbon
From C–Br bond to the \( \ce{Br} \)

IBDP Tip: In exams, clearly draw curly arrows showing electron pair movement and indicate transition states (for SN2) or carbocation intermediates (for SN1). You may be asked to compare rates of reaction based on the type of haloalkane.

Scroll to Top