Nucleophilicity, Basicity And Leaving Group Ability Page 3
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3CHM 331/332 handout (page 3 of 3)
Leaving Group Ability
Leaving Group
Approx. Relative Rate
Conjugate Acid
Approx. pKa
–
HO
very small
H
O
15
2
–
F
0.00001
HF
3.5
–
Cl
1
HCl
-7
–
Br
10
HBr
-9
+
H
O
10
H
O
-1.7
2
3
–
I
100
HI
-10
–
TsO
100,000
TsOH
-2.8
–
CF
SO
O
100,000,000
CF
SO
H
-6
3
2
3
3
Note that here there is a rough relationship between leaving group ability and
basicity. Very basic groups are poor leaving groups. Consider the reactions of
–
alcohols. Substitution does not take place for alcohols (HO
is too poor a leaving
group, the pKa of the conjugate acid, H
O, is approx. 15) unless the alcohol is
2
protonated, then the leaving group becomes H
O (pKa of the conjugate acid,
2
+
H
O
, is approx. -2).
3
A good leaving group should have the following properties:
1. It should polarize the bond which connects it to the carbon so that the
carbon has some positive charge, making it more reactive towards the
nucleophile; i.e., it should be electron withdrawing.
2. It should want to leave with a pair of electrons; i.e., it should be a stable
"anion" (not all leaving groups are anionic), which means it is unlikely to be a
strong base.
3. It should be polarizable to distribute charge in the transition state.
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