Chemistry Reference
In-Depth Information
S
N
1 reaction
R
R
H
2
O
RC
Cl
RC
OH
readily
R
R
R
R
H
slow
relatively poor nucleophile
attacks very good electrophile
R
C
Cl
RC
O
H
R
R
fast
H
R
O
R
H
H
RC
O
RC
OH
H
R
R
Finally, do appreciate that, depending upon con-
ditions, it is quite possible that both S
N
1andS
N
2
mechanisms might be operating at the same time,
with each contributing its own stereochemical char-
acteristics upon the product.
Box 6.4
Biological S
N
1 reactions involving allylic cations
The leaving groups most commonly employed in nature are
phosphates
and
diphosphates
. These good leaving
groups are anions of the strong acids phosphoric (p
K
a
2.1) and diphosphoric (p
K
a
1.5) acids respectively. The
p
K
a
values given refer to the first ionization of these polyfunctional acids (see Section 4.7).
O
P
O
P
HO
OH
OH
O
O
O
phosphoric acid
phosphate
O
P
O
O
P
O
P
P
O
O
O
HO
O
OH
OH
OH
O
O
diphosphoric acid
(pyrophosphoric acid)
diphosphate
(pyrophosphate)
The compound
dimethylallyl diphosphate
provides an excellent example of a natural product with a
diphosphate leaving group that can be displaced in a nucleophilic substitution reaction. Suitable nucleophiles are
hydroxyl groups, e.g. a phenol, though frequently an electron-rich nucleophilic carbon is employed. Dimethylallyl
diphosphate is a precursor of many natural products that contain in their structures branched-chain C
5
subunits
termed isoprene units.
