Nucleophilic Substitution SN1 and SN2 Summary Questions
Introduction to Nucleophilic Substitution
What is a nucleophile? Give some examples of nucleophiles…
Why do nucleophiles attack the carbon-halogen bond of a halogenoalkane?
What is the general equation for the reaction of a halogenoalkane and a strong base?
Write the general equation for Nucleophilic Substitution (showing the nucleophile and Halogen (X))
What is heterolytic fission?
Drawing the SN2 mechanism
SN2, 1 Step, 1o (primary)… In the diagrams below, show the movement of the electron pairs with the curly arrow…
Nucleophile attacking
Unstable transition state (with
Formation of alcohol and the release of
halogenoalkane from the back
nucleophile and halogen both
the leaving group (halide)
attached by weak bonds)… Show the
charge on the molecule…
Why does the SN2 mechanism favour primary halogenoalkanes and not tertiary? (Think about steric hindrance)
Explain why the SN2 mechanism is bimolecular. Write the rate equation for this mechanism.
What happens to the arrangement of the atoms around the carbon atom after SN2?
Drawing the SN1 mechanism
SN1, 2 Step, 3o (tertiary)… in the diagrams below, show the movement of the electron pairs with the curly arrow…
Step 1: Heterolytic fission of the carbon-halogen bond
Step 2: Carbocation intermediate attacked by nucleophile
(ionizes the halogenoalkane).
leading to new bond…
Halide ion will leave, producing a carbocation
intermediate.
The presence of 3 alkyl groups on the tertiary halogenoalkane has what effect on the carbocation?
Explain why the SN1 mechanism is unimolecular. Write the rate equation for this mechanism.
Which nucleophilic substitution mechanism do 2o (secondary) halogenoalkanes undergo?
Factors that affect the rate of nucleophilic substitution
What are the three factors that affect the rate of nucleophilic substitution?
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Give the order of rates according to the type of halogenoalkane. Start with the fastest halogenoalkane and annotate
which SN mechanism is applicable under each halogenoalkane.
Explain how the influence of the leaving group involves:
(a) polarity of the carbon-halogen bond
(b) strength of the carbon-halogen bond
Describe which (polarity or strength) of the carbon-halogen bond is the dominant factor for determining rate.
Explain which solvents (polar & protic/polar & aprotic) are favoured by:
SN1:
SN2:
‘The fastest reactions will be tertiary iodoalkanes in polar, protic solvents’
Explain the statement above using understanding about:
 1o, 2o and 3o halogenoalkanes
 the mechanisms they favour
 which is the best carbon-halogen bond (in terms of rate)
 the choice of solvent that each mechanism favours
Mind Map/Summary Page for Nucleophilic Substitution
SN1
SN2