Section 10.4 The Reversibility of an SN 2 Reaction 373
(Section 3.7). Le Châtelier’s principlestates that if an equilibrium is disturbed, the
system will adjust to offset the disturbance. In other words, if the concentration of
product C is decreased, A and B will react to form more C and D to maintain the value
of the equilibrium constant.
For example, the reaction of ethyl chloride with methanol is reversible because the
difference between the basicities of the nucleophile and the leaving group is not very
large. If the reaction is carried out in a neutral solution, the protonated product will
lose a proton (Section 1.20), disturbing the equilibrium and driving the reaction
toward the products.
fast
H+Cl−
CH 3 CH 2 Cl + CH 3 OH CH 3 CH 2 OCH 3 CH 3 CH 2 OCH 3
+
+ H+
AD++C
Keq =
B
[C][D]
[A][B]
SO 3 H
HI
HBr
HCl
H 2 SO 4
CH 3 OH 2
H 3 O+
CH 3 COH
−10.0
−9.0
−7.0
−5.0
−2.5
−1.7
−0.6
I−
Br−
Cl−
−OSO
3 H
CH 3 OH
H 2 O
HF
O
CH 3 CO−
O
3.2
4.8
H 2 SHS 7.0 −
HC N
HC CH C−
9.1 −C
NH 4 9.4 NH 3
CH 3 CH 2 SH 10 .5 CH 3 CH 2 S−
(CH 3 ) 3 NH 10 .8 (CH 3 ) 3 N
CH 3 OH 15 .5 CH 3 O−
H 2 OHO 15 .7 −
25 HC
NH 3 36 −NH 2
H 2 ~ 40 H−
F−
SO 3 −
+
N
+
+
Table 10.3 The Acidities of the Conjugate Acids of Some Leaving Groups
Acid pKa Conjugate base (leaving group)
If an equilibrium is disturbed,
the system will adjust to offset the
disturbance.