Unimolecular Nucleophilic Substitution Reaction – SN1 Reaction

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 

of SN1 reaction

• 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                

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