CHEM 215 Chapter Notes - Chapter 13: Algorithmic Skeleton, Nucleophile, Reagent
Reaction mechanisms for ROH -> RX
There are many reagents that can be used to cause the one-step transformation of alcohols to halides. The
general mechanistic scheme involves the reaction of the alcohol, as a nucleophile, with the reagent,
displacing a halide and producing an oxygen atom leaving group at the same time. In the subsequent step,
the halide comes back and displaces this leaving group. Simple hydrohalic acids are the exemplar for this:
CHEM 215
O
H
Br
H
O
H
H
Br
favorable
acid-base
reaction
SN2
Br O
H
H
+
Reagents such as PCl3, PCl5, PBr3, (CH3)3SiI, etc., use the above mechanism as a strong analogy:
O
H
Cl
P
Cl
Cl
O
H
P
Cl
SN2
Cl O
H
P
+
Cl Cl
Cl Cl
The overall observation for reagents such as thionyl chloride (SOCl2) and pyridine is the same basic idea.
The mechanism is typically drawn in a couple of different ways. For CHEM 215, both are equally fine.
O
H
Cl
S
O
Cl
(a) SOCl2 & pyridine treating S at a tetrahedral atom with good leaving groups on it:
O
H
S
+
OCl
SO2 + Cl
O
H
Cl
S
O
Cl
(b) SOCl2 & pyridine: treating SO as a pi bond, gets to the same intermediate:
O
H
S
OCl
Cl
O
S
OCl
N
Cl
Cl
O
H
S
+
OCl
SO2 + Cl
O
S
OCl
N
Cl
Cl
In fact, both of these correspond to a simplification of what people think happens here. The “SO2Cl”
group is not considered to be a good enough leaving group for being displaced by chloride, and another
pyridine gets involved to make this a better leaving group.
O
S
OCl
NO
S
ON
Cl
Cl
N
SO2
SN2
SN2
SN2
The evidence for this is pretty interesting. If you
do not have a base like pyridine present, the
reaction is not SN2, but an SN
1 with retention of
configuration, called an SNi reaction.
H
O
S
O
Cl
H
O
S
O
Cl
H
Cl
SO2
SNi: nucleophilic substitution
with internal return
Cl
X
Document Summary
There are many reagents that can be used to cause the one-step transformation of alcohols to halides. The general mechanistic scheme involves the reaction of the alcohol, as a nucleophile, with the reagent, displacing a halide and producing an oxygen atom leaving group at the same time. In the subsequent step, the halide comes back and displaces this leaving group. Simple hydrohalic acids are the exemplar for this: Reagents such as pcl3, pcl5, pbr3, (ch3)3sii, etc. , use the above mechanism as a strong analogy: The overall observation for reagents such as thionyl chloride (socl2) and pyridine is the same basic idea. The mechanism is typically drawn in a couple of different ways. For chem 215, both are equally fine. (a) socl2 & pyridine treating s at a tetrahedral atom with good leaving groups on it: Cl (b) socl2 & pyridine: treating so as a pi bond, gets to the same intermediate:
