Download Mechanistic notation

Mechanistic notation
The mechanism just described is an
example of an SN1 process.
SN1 stands for substitution-nucleophilicunimolecular.
The molecularity of the rate-determining
step defines the molecularity of the
overall reaction.
Mechanistic notation
The molecularity of the rate-determining
step defines the molecularity of the
overall reaction.
H
+
(CH3)3C
O
+
H
Rate-determining step is unimolecular
dissociation of alkyloxonium ion.
Effect of Alcohol Structure
on Reaction Rate
slow step is:
ROH2+  R+ + H2O
The more stable the carbocation, the faster
it is formed.
Tertiary carbocations are more stable than
secondary, which are more stable than primary,
which are more stable than methyl.
Tertiary alcohols react faster than secondary,
which react faster than primary, which react faster
than methanol.
Hammond's Postulate
If two succeeding states (such as a
transition state and an unstable intermediate)
are similar in energy, they are similar in structure.
Hammond's postulate permits us to infer the
structure of something we can't study (transition
state) from something we can study
(reactive intermediate).
carbocation
formation
carbocation
capture
R+
proton
transfer
ROH
+
ROH2
RX
carbocation
formation
R+
proton
transfer
ROH
+
ROH2
Rate is
carbocation
governed by
capture
energy of this
transition state.
Infer structure of
this transition
state from
structure of
state of closest
energy; in this
case the
nearest state is
the
RXcarbocation.
4.13
Reaction of Primary Alcohols with
Hydrogen Halides.
The SN2 Mechanism
Preparation of Alkyl Halides
(CH3)3COH + HCl
25°C
(CH3)3CCl + H2O
78-88%
OH + HBr
80-100°C
Br + H2O
73%
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
87-90%
Preparation of Alkyl Halides
Primary carbocations are too high in energy to
allow SN1 mechanism. Yet, primary alcohols
are converted to alkyl halides.
Primary alcohols react by a mechanism called
SN2 (substitution-nucleophilic-bimolecular).
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
87-90%
The SN2 Mechanism
Two-step mechanism for conversion
of alcohols to alkyl halides:
(1) proton transfer to alcohol to form
alkyloxonium ion
(2) bimolecular displacement of water
from alkyloxonium ion by halide
Example
CH3(CH2)5CH2OH + HBr
120°C
CH3(CH2)5CH2Br + H2O
Mechanism
Step 1:
Proton transfer from HBr to 1-heptanol
CH3(CH2)5CH2
..
O: + H
..
:
Br
..
H
fast, bimolecular
H
+
CH3(CH2)5CH2 O :
H
Heptyloxonium ion
+
.. –
: Br:
..
Mechanism
Step 2:
Reaction of alkyloxonium ion with bromide
ion.
H
.. –
+
: Br: + CH3(CH2)5CH2 O :
..
H
slow, bimolecular
H
CH3(CH2)5CH2
..
Br
.. :
1-Bromoheptane
+
:O:
H
+
–
Br
CH2
OH2
CH3(CH2)4 CH2
proton
transfer
ROH
+
ROH2
RX
4.14
Other Methods for Converting
Alcohols to Alkyl Halides
Reagents for ROH to RX
Thionyl chloride
SOCl2 + ROH  RCl + HCl + SO2
Phosphorus tribromide
PBr3 + 3ROH  3RBr + H3PO3
Examples
CH3CH(CH2)5CH3
SOCl2
K2CO3
CH3CH(CH2)5CH3
Cl
OH
(81%)
(pyridine often used instead of K2CO3)
(CH3)2CHCH2OH
PBr3
(CH3)2CHCH2Br
(55-60%)