Chapter 9 Flashcards
Nucleophilic substitution
Any reaction in which a nucleophile replaces another electron-rich group called a leaving group
Nucleophile
An electron-rich molecule or ion that donates a pair of electrons to another atom or ion to form a new covalent bond
If the nucleophile is negatively charged in a substitution reaction, the atom donating the pair of electrons becomes ______ in the product.
neutral
If the nucleophile is uncharged in the substitution reaction, the atom donating the electrons becomes _________ in the product.
positively charged
What are the two limiting mechanisms for nucleophilic substitution?
SN2 and SN1
In the SN2 reaction mechanism, bond forming and bond breaking occur _________.
simultaneously
SN2 reactions are bimolecular because:
both the nucleophile and haloalkane concentrations influence reaction rate
The nucleophile must approach from the ________ in SN2 reactions because:
backside, this has to occur in order for it to populat the C-Lv antibonding orbital and allow the reaction
In the SN1 mechanism, the leaving group departs first in the _________, leaving a _______ intermediate that reacts with the nucleophile in a second step.
rate-determining step, carbocation
SN1 reactions are unimolecular because:
only the haloalkane concentration influences the reaction rate
In SN2 reactions, both the ______________ are involved in the transition state.
nucleophile and leaving group
SN2 reactions result in ________ of configuration at the reaction center.
inversion
SN2 reactions are accelerated more in ________ solvents.
polar aprotic
The relative rates of SN2 reactions are governed by _______ factors, namely:
steric, the degree of crowding around the site of the reaction
In SN2 reactions, departure of the leaving group is assisted by:
the incoming nucleophile
Backside attack by the nucleophile is facilitated in two ways:
Fist, because of the polarization of the C-Lv bond, the carbon atom has a partial positive charge and therefore attracts the electron-rich nucleophile. Second, the electron density of the nucleophile entering from the backside assists in breaking the C-Lv bond, thereby helping the leaving group leave.
An SN1 reaction occurs in two steps. Step 1 is a slow, _________ ionization of the C-Lv bond to form a carbocation intermediate.
rate-determining
In SN1 reactions, the reaction at a chiral center gives:
largely racemization, often accompanied with a slight excess of inversion of configuration
SN1 reactions often involve carbocation ________ and are accelerated by ________ solvents.
rearrangements, polar protic
SN1 reactions are governed by ______ factors, namely:
electronic, the relative stabilities of carbocation intermediates
Solvolysis
A nucleophilic substitution in which the solvent is also the nucleophile
Because an SN2 reaction is bimolecular, doubling the concentration of either the haloalkane or nucleophile:
doubles the rate of the reaction
Haloalkanes that can form more stable carbocations react faster if an:
SN1 mechanism occurs
Steric hindrance on the backside of the C-Lv bond of a haloalkane:
slows down or possibly prevents an SN2 mechanism