11 . Reactions of Alkyl Halides: Nucleophilic Substitutions and Eliminations
11. Reactions of Alkyl Halides: Nucleophilic …webpages.iust.ac.ir/naimi/Lectures/Organic...
Transcript of 11. Reactions of Alkyl Halides: Nucleophilic …webpages.iust.ac.ir/naimi/Lectures/Organic...
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Organic Chemistry
M. R. Naimi-Jamal
Faculty of Chemistry
Iran University of Science & Technology
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Chapter 7-2. Reactions of Alkyl Halides: Nucleophilic Substitutions
Based on McMurry’s Organic Chemistry, 6th edition
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Alkyl Halides React with Nucleophiles and Bases
Alkyl halides are polarized at the carbon-halide bond, making the carbon electrophilic
Nucleophiles will replace the halide in C-X bonds of many alkyl halides (reaction as Lewis base)
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Alkyl Halides React with Nucleophiles and Bases
Nucleophiles that are Brønsted bases produce
elimination
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Substitution vs. Elimination
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The Nature of Substitution
Substitution requires that a "leaving group", which is also a Lewis base, departs from the reacting molecule.
A nucleophile is a reactant that can be expected to participate as a Lewis base in a substitution
reaction.
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The Discovery of the Walden Inversion
In 1896, Paul Walden showed that (-)-malic acid
could be converted to (+)-malic acid by a series of
chemical steps with achiral reagents
This established that optical rotation was directly
related to chirality and that it changes with
chemical alteration
Reaction of (-)-malic acid with PCl5 gives (+)-
chlorosuccinic acid
Further reaction with wet silver oxide gives (+)-malic acid
The reaction series starting with (+) malic acid gives (-)
acid
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The Walden Inversion (1896)
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Significance of the Walden Inversion
The reactions involve substitution at the
chiral center
Therefore, nucleophilic substitution can
invert the configuration at a chirality center
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Stereochemistry of Nucleophilic Substitution
A more rigorous Walden
cycle using 1-phenyl-2-
propanol (Kenyon and
Phillips, 1929)
Only the second and fifth
steps are reactions at
carbon
Inversion must occur in
the substitution step
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Two Stereochemical Modes of Substitution
Substitution with inversion:
Substitution with retention:
X
H3C
RH
HO
CH3
RH
OH-
+ X-
X
H3C
RH
OH-
+ X-OH
H3C
RH
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Substitution Mechanisms
SN1
Two steps with carbocation intermediate
Occurs in 3°, allyl, benzyl
SN2
Two steps combine - without intermediate
Occurs in primary, secondary
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Kinetics of Nucleophilic Substitution
Rate is the change in concentration with time
Depends on concentration(s), temperature,
inherent nature of reaction (energy of
activation)
A rate law describes the relationship
between the concentration of reactants and
the overall rate of the reaction
A rate constant (k) is the proportionality
factor between concentration and rate
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Kinetics of Nucleophilic Substitution
Rate = d[CH3Br]/dt = k[CH3Br][OH-1]
This reaction is second order: two
concentrations appear in the rate law
SN2: Substitution Nucleophilic 2nd order
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The SN2 Reaction
Reaction occurs with inversion at reacting center
Follows second order reaction kinetics
Ingold nomenclature to describe rate-
determining step:
S=substitution
N (subscript) = nucleophilic
2 = both nucleophile and substrate in rate-
determining step (bimolecular)
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SN2 Process
The transition state for the rate-determining
(and only) step contains both reactants
(substrate alkyl halide and nucleophile).
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SN2 Transition State
The transition state of an SN2 reaction has a
planar arrangement of the carbon atom and
the remaining three groups
Hybridization is sp2
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Characteristics of the SN2 Reaction
Sensitive to steric effects
Methyl halides are most reactive
Primary are next most reactive
Unhindered secondary halides react under some conditions
Tertiary are unreactive by this path
No reaction at C=C (vinyl or aryl halides)
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Steric Effects on SN2 Reactions
X
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Order of Reactivity in SN2
The more alkyl groups connected to the reacting carbon, the slower the reaction
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Vinyl and Aryl Halides:
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The Nucleophile
Neutral or negatively charged Lewis base
Reaction increases coordination (adds a new bond) at the nucleophile
Neutral nucleophile acquires positive charge
Anionic nucleophile becomes neutral
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For example:
Br+ CN-
C N+ Br-
CH2 Cl + H2O CH2 OH2 + Cl-
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Relative Reactivity of Nucleophiles
Depends on reaction and conditions
More basic nucleophiles react faster
Better nucleophiles are lower in a column of the
periodic table
Anions are usually more reactive than neutrals
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The Leaving Group
A good leaving group reduces the energy of
activation of a reaction
Stable anions that are weak bases (conjugate
bases of strong acids) are usually excellent
leaving groups
Stronger bases (conjugate bases of weaker
acids) are usually poorer leaving groups
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Poor Leaving Groups
If a group is very basic or very small, it does
not undergo nucleophilic substitution.
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Summary of SN2 Characteristics:
Substrate: CH3->1o>2o>>3o (Steric effect)
Nucleophile: Strong, basic nucleophiles favor the reaction
Leaving Groups: Good leaving groups (weak bases) favor the reaction
Solvent: Aprotic solvents favor the reaction; protic reactions slow it down by solvating the nucleophile
Stereochemistry: 100% inversion
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Prob.: Arrange in order of SN2 reactivity
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The SN1 Reaction
Tertiary alkyl halides react rapidly in protic
solvents by a mechanism that involves
departure of the leaving group prior to the
addition of the nucleophile.
Reaction occurs in two distinct steps, while SN2
occurs in one step (concerted).
Rate-determining step is formation of
carbocation:
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SN1 Reactivity:
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SN1 Energy Diagram
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Rate-Limiting Step
The overall rate of a reaction is controlled by
the rate of the slowest step
The rate depends on the concentration of the
species and the rate constant of the step
The step with the largest energy of activation
is the rate-limiting or rate-determining step.
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SN1 Energy Diagram
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Stereochemistry of SN1 Reaction
The planar carbocation intermediate leads to
loss of chirality
Product is racemic or has some inversion
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Characteristics of the SN1 Reaction
Tertiary alkyl halide is most reactive by this
mechanism
Controlled by stability of carbocation
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Relative Reactivity of Halides:
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Allylic and Benzylic Halides
Allylic and benzylic intermediates stabilized by
delocalization of charge
Primary allylic and benzylic are also more
reactive in the SN1 mechanism
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Effect of Leaving Group on SN1
Critically dependent on leaving group
Reactivity: the larger halides ions are better leaving
groups
In acid, OH of an alcohol is protonated and leaving
group is H2O, which is still less reactive than halide
p-Toluensulfonate (TosO-) is an excellent leaving group
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Summary of SN1 Characteristics:
Substrate: Benzylic~allylic > 3o > 2o
Nucleophile: Does not affect reaction (although
strong bases promote elimination)
Leaving Groups: Good leaving groups (weak
bases) favor the reaction
Solvent: Polar solvents favor the reaction by
stabilizing the carbocation.
Stereochemistry: racemization (with some
inversion)
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Prob.: Arrange in order of SN1 reactivity
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Practice Problem: SN1 or SN2?
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Problem: SN1 or SN2?
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Chapter 7-2, Questions
27, 29, 31, 32,
46, 47