Reactivity Considerations

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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 © Prentice Hall 2001 Chapter 9

Substitution Reactions SN2 mechanism: C–X bond weakens as nucleophile approaches © Prentice Hall 2001 Chapter 9

The SN2 Reaction Bimolecular nucleophilic substitution rate = k [alkyl halide][nucleophile] © Prentice Hall 2001 Chapter 9

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 © Prentice Hall 2001 Chapter 9

The SN2 Reaction © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Leaving Group © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Leaving Group © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Nucleophile stronger base weaker base better nucleophile poorer nucleophile HO– > H2O CH3O– > CH3OH –NH2 > NH3 CH3CH2NH– > CH3CH2NH2 © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Nucleophile Atom Comparing nucleophiles with attacking atoms of approximately the same size, the stronger base is also the stronger nucleophile © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Nucleophile Size © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Nucleophile Size Size is related to polarizability © Prentice Hall 2001 Chapter 9

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 © Prentice Hall 2001 Chapter 9

The SN2 Reaction: Reversibility © Prentice Hall 2001 Chapter 9

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 © Prentice Hall 2001 Chapter 9

Substitution Reactions SN1 mechanism: C–X bond breaks first without any help from nucleophile This is a two-step process © Prentice Hall 2001 Chapter 9

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 © Prentice Hall 2001 Chapter 9