H - Carbon Rules

Nuc
Product(s)
R LG
Solvent
Four factors that affect which mechanism predominates:
1. The substrate
2. The nucleophile
3. The leaving group
4. Solvent
Substrate
CH3
H C Br
H
H
H C Br
H
methyl
alkyl halide
1
alkyl halide
Favors SN2
CH3
H3C C Br
H
2
alkyl halide
Neither mechanism
is favored
CH3
H3C C Br
CH3
3
alkyl halide
Favors SN1
Nucleophile
 A strong nucleophile favors SN2
 A weak nucleophile disfavors SN2, and is therefore more likely to go
through an SN1 mechanism
1.
Charge: charged is better than uncharged
HO– vs H2O
2.
Electronegativity: less electronegative atoms
don’t hold on to electrons as tightly and are
better nucleophiles
NH3 vs H2O
I–
4.
vs
Br–
Common Nucleophiles
Strong
Weak
I-
HS-
HO-
F-
Br-
H2S
RO-
H2O
Cl-
RSH
N≡C-
ROH
Size of the nucleophile: the smaller the
nucleophile, the better
HO– vs –OCH3

The importance of these factors generally follows the order they were
presented: charge, electronegativity, and then size.
Leaving Group
A good leaving group is the conjugate base of a strong acid.
Acid
Strongest
acid
Weakest
acid
Conjugate
base
pKa
Most stable
base
I-
H–I
-11
H–Br
-9
Br-
H–Cl
-7
Cl-
TsO–H
-3
TsO-
H3O+
-2
H2O
H2O
15.7
HO-
CH3CH2OH
16
CH3CH2O-
NH3
38
Least stable
base
NH2-
Good
leaving
groups
Bad
leaving
groups
Solvent
Best for SN1
Best for SN2
Polar protic: contain at least 1 H
atom directly connected to an
electronegative atom
Polar aprotic: contain no H atoms
directly connected to an electronegative
atom
Examples:
Examples:
H
O
Me
H
water
O
H
Et
methanol
H
H
N
O
H
ethanol
O
S
Dimethylsulfoxide
(DMSO)
acetonitrile
O
O
N P N
N
O
H
ammonia
O
H
acetic acid
Why? Solvent can stabilize the polar
intermediates and transition states
H3C C N
H
N
Dimethylforamide
(DMF)
Hexamethylphosphoramide
(HMPA)
Why? Cations are solvated, but the anion
is not. As a result the anion (nucleophile)
is higher in energy and therefore more
reactive.