1. Reactivity Considerations
A compound with an sp3 hybridized carbon bonded to a halogen can undergo two types of reactions
There are two fundamentally different mechanisms for substitution reactions
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2. Substitution Reactions
SN2 mechanism: C–X bond weakens as nucleophile approaches
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3. The SN2 Reaction Bimolecular nucleophilic substitution
rate = k [alkyl halide][nucleophile]
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4. The SN2 Reaction
The inversion of configuration resembles the way an umbrella turns inside out in the wind, and
Is also known as the Walden inversion
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5. The SN2 Reaction
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6. The SN2 Reaction: Leaving Group
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7. The SN2 Reaction: Leaving Group
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8. The SN2 Reaction: Nucleophile
stronger base weaker base better nucleophile poorer nucleophile
HO– > H2O
CH3O– > CH3OH
–NH2 > NH3
CH3CH2NH– > CH3CH2NH2
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9. The SN2 Reaction: Nucleophile Atom
Comparing nucleophiles with attacking atoms of approximately the same size, the stronger base is also the stronger nucleophile
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10. The SN2 Reaction: Nucleophile Size
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11. The SN2 Reaction: Nucleophile Size
Size is related to polarizability
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12. The SN2 Reaction: Nucleophile Bulkiness
Nucleophilicity is affected by steric effects
A bulky nucleophile has difficulty getting near the back side of a sp3 carbon
ethoxide ion tert-butoxide ion
better nucleophile stronger base
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13. The SN2 Reaction: Reversibility
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14. The SN2 Reaction: Reversibility
For bases of relatively equal strength, an equilibrium results
This particular reaction will proceed to completion in acetone because sodium iodide is soluble in acetone but sodium bromide is not
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15. Substitution Reactions
SN1 mechanism: C–X bond breaks first without any help from nucleophile
This is a two-step process
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16. Substitution Reactions
Both mechanisms are called a nucleophilic substitution
Which one takes place depends on
the structure of the alkyl halide
the reactivity and structure of the nucleophile
the concentration of the nucleophile, and
the solvent in which reaction is carried out
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