CHEM 0310 Lecture Notes - Lecture 7: Protic Solvent, Solvolysis, Rate-Determining Step

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6 Feb 2017

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Chapter 7: Further Reactions of Haloalkanes
Solvolysis of Tertiary/Secondary Haloalkanes
-SN2 reactions diminishes when the reacting center changes from primary tertiary
-This is only true for bimolecular substitution; secondary and tertiary undergo substitution but with
another mechanism: unimolecular substitution (SN1)
-This occurs when the substrate undergoes substitution by solvent molecules (solvolysis)
-Unlike bimolecular reactions, primary halides are very slow in their reactions with water while tertiary
is very fast
Unimolecular Substitution
-In contrast to SN2, solvolyses follows first order rate law, are not stereospecific, and are characterized
by the opposite order of reactivity
-The rate of a SN2 reaction depends on both ingredients while the rate of hydrolysis of the substrate is
proportional to the concentration of only the starting halide, not the water; rate = k[starting]
-The observed rate is that of the slowest step in the sequence or RDS
Mechanism of SN1
-The number on in this substitution refers to only one molecule, haloalkane, participating in the RDS
-Thus, the rate of the reaction does not depend on the concentration of the nucleophile
1. The rate determining step is the dissociation of the haloalkane to an alkyl cation and bromide
(heterolytic cleavage). The hydrocarbon product
contains a positively charged central atom with three
other groups and one electron group: carbocation
2. Nucleophilic attack by the solvent on the electron
deficient carbon
3. Deprotonation by the water in the reaction medium
to make carbocationOH and H3O+
-While SN2 is a one-step reaction, SN1 is three steps
-All three steps of the mechanism of solvolysis are
-A large excess of nucleophilic solvent ensures
complete solvolysis
-To minimize electron repulsion, the positively charges carbon assumes trigonal planar geometry, the
result of sp2 hybridization
-The intermediate is therefore achiral; to then obtain racemic products
The Solvent
-Heterolytic cleavage of the C-X bond in the RDS of the SN1 reaction entails a tradition state structure
that is highly polarized, leading eventually to two fully charged ions
-Due to this polar transition state, the rate of an SN1 reaction increases as solvent polarity increases
-This is particularly striking when the solvent is changed from aprotic to protic
-The protic solvent accelerates the SN1 reaction because it stabilizes the transition state by H bonding
with the leaving group
The Leaving Group
-The rate of the reaction increases as the leaving group ability of the departing group improves
-X = OSOR’ > I > Br > Cl
-Sufonates and lower elements are more reactive and readily undergo solvolysis
The Nucleophile
-The nucleophile deos not affect the rate of the SN1 reaction
-In a SN2 reaction, the rate of reaction increases significantly as the nucleophilicity of the attacking
species improves
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