Unimolecular Nucleophilic
Substitution Reaction – SN1 Reaction
Contents
• Mechanism involved in SN1 reaction
• Factors affecting SN1 reaction
– Structure of the substrate
– Concentration and reactivity of the nucleophile
– Effect of the solvent
– Nature of the leaving group
• Stereochemistry
• SN1 vs SN2
Learning
Objectives
At the end of this
lecture, student will be able to
• Explain kinetics and mechanism involved in SN1 reaction
• Explain factors affecting SN1 reaction
• Compare SN1 and SN2 reaction
• Explain the stereochemistry of SN1 reaction
• Explain SN1 vs SN2
• Exlain mechanism using a few examples
SN1
Reaction
• Definition
– Unimolecular nucleophilic substitution
• Example:
Hydrolysis of 3°butyl bromide to 3°butyl alcohol
• Kinetics
– 1st order kinetics
– Rate α [substrate]
– Rate = k [RBr]
SN1
Reaction Mechanism
• Step 1
– Slow ionisation to give carbonium ion
– C-Br bond breaks heterolytically
– Nucleophile is not involved
– Rate determining step
• Step 2
– Fast attack of nucleophile on carbonium ion
Factors
affecting the rates of SN1 reaction
• Structure of the substrate
• Concentration and reactivity of the nucleophile
• Effect of the solvent
• Nature of the leaving group
Structure of the
substrate
– Reactivity order based on carbocation stability
Concentration &
strength of nucleophile
– Weak & low concentration of nucleophile
Solvents
– Polar protic solvents
Leaving group
– Good leaving groups
– Weak bases
– Halide ions
Stereochemistry
• Racemization plus
some net inversion
– Example 1
• Racemization plus
some net inversion
– Example 2
SN1 vs SN2
Comparison
between SN1 and SN2
Summary
• SN1 reaction is unimolecular – only nucleophile involved
in the rate determining step
• Mechanism of SN1 reaction – 2 steps – slow ionisation to
form carbocation & fast attack by nucleophile
• 3°alkyl halides are more reactive towards SN1 reaction
since it forms more stable 3°carbocation
• SN1 reaction is favoured by poor nucleophile in polar
protic solvents
• SN1 reaction involves racemisation plus some net inversion
of configuration
• Tertiary alkyl halides prefer unimolecular substitution
reaction
• Primary alkyl halides prefer bimolecular substitution
reaction