Chapter 9.2: Mechanisms of Nucleophilic Aliphatic Substitution
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Define bimolecular reaction
a reaction in which two species are involved in the rate-determining step
What is the stereochemistry of the Sn1 reaction?
o Stereochemistry: stereochemistry is scrambled (not 1:1 exactly)
What is a reaction in which only one species is involved in the rate-determining step?
unimolecular reaction
Why is the Sn1 reaction unimolecular?
Only the haloalkane is involved in the rate-determining step
When is the Sn2 mechanism preferred? (steric hindrance and carbocation stability)
o Favored when steric hinderance is low or carbocation stability is low
What is a nucleophilic substitution in which the solvent is also the nucleophile?
solvolysis
What is a reaction in which two species are involved in the rate-determining step?
bimolecular reaction
Define solvolysis
a nucleophilic substitution in which the solvent is also the nucleophile
Define Sn2 reaction
a bimolecular nucleophilic substitution reaction
What is a unimolecular nucleophilic substitution reaction?
Sn1 reaction
Define unimolecular reaction
a reaction in which only one species is involved in the rate-determining step
What is the stereochemistry of the Sn2 mechanism?
o Stereochemistry: inversion at site of reaction
Describe the Sn1 mechanism
Step 1: Break a bond to give stable molecules or ions • Formation of a carbocation intermediate. Because no nucleophile is assisting the departure of the halide anion, this is the relatively slow, rate-determining step of the reaction. Step 2: Make a new bond between a nucleophile and an electrophile Step 3: Take a proton away
What is a bimolecular nucleophilic substitution reaction?
Sn2 reaction
Define Sn1 reaction
a unimolecular nucleophilic substitution reaction
Describe the Sn2 mechanism
Make a new bond between a nucleophile and an electrophile and simultaneously break a bond to give stable molecules or ions • The nucleophile attacks the reactive center from the side opposite the leaving group; involves backside attack of the nucleophile
Why is the Sn2 mechanism bimolecular?
• Both the haloalkane and nucleophile are involved in the rate-determining step
When is the Sn1 mechanism preferred? (steric hindrance and carbocation stability)
o Favored when steric hindrance is high or carbocation stability is high