1. Structure / Reactivity
Structure / Reactivity
Relationships
Relationships
Basic Premise of Organic Chemistry - The
Basic Premise of Organic Chemistry - The
nature of the chemical reaction an organic
nature of the chemical reaction an organic
molecule undergoes is dependent upon the
molecule undergoes is dependent upon the
functional groups present in the molecule
functional groups present in the molecule.
Corollary - Structural features in the
Corollary - Structural features in the
molecule at sites that are not directly
molecule at sites that are not directly
involved in a particular chemical
involved in a particular chemical
transformation can affect both the kinetics
transformation can affect both the kinetics
and the equilibrium of the chemical reaction.
and the equilibrium of the chemical reaction.
2. Structure / Reactivity
Structure / Reactivity
Relationships
Relationships
Organic chemists generally classify
Organic chemists generally classify
the manner in which substituents
the manner in which substituents
modify chemical reactivity into
modify chemical reactivity into
three categories
three categories:
Inductive and Field effects (Polar effects)
Inductive and Field effects (Polar effects)
Resonance effects (Conjugative effects)
Resonance effects (Conjugative effects)
Steric effects
Steric effects
4. Summary of Common Substituents grouped according to their
Summary of Common Substituents grouped according to their Inductive effect
Inductive effect
-I (electron withdrawing) +I (electron donating)
NH3
+
, NR 3
+
NO2
C N
C
O
OH , C
O
NH2
C
O
OR , C
O
X
C
O
H, C
O
R
OH , OR
SH , SR
C C, C C
F
Cl
Br
I
CH3
CH2R
CHR2
CR3
C
O
O
5. Examples of Resonance Effects
Examples of Resonance Effects
Case I: COOH
Ka
= 6.3x10
-5
COOH
HO
Ka
= 2.9x10
-5
Case II: OH
O2N
Ka meta para
OH
O2N
Ka
<
Case III: K
b
for C6H5CH2NH2 >> K
b
for C6H5NH2
Case IV:
K
b
for R C
O
NH2 < K
b
for R NH2
K for R C
O
OH >
a
K for R OH
a
Case V:
H3C
COOH
Ka meta para
H3C COOH
Ka
>
6. Summary of Common Substituents grouped according to
Summary of Common Substituents grouped according to Resonance
Resonance and
and Inductive
Inductive effects
effects
NO2
C N
C
O
OH , C
O
NH2
C
O
OR , C
O
X
C
O
H, C
O
R
F Cl Br I CH3
CH2R
CHR2
CR3
, , ,
OH , OR
SH ,
CF3
SR
-R, -I
NH2 NHR NR 2
, ,
O C
O
R , NH C
O
R
+R, -I +R, +I
O-
, S-
7. Examples of Steric Inhibition
Examples of Steric Inhibition
of Resonance
of Resonance
Case I: O2N NH2 is less basic than O2N NH2
Br
Br
despite the
presence of more electron withdrawing (base weakening) groups in the
latter compound.
Case II: O2N Br + R2NH O2N NR 2 + HBr
O2N Br
H3C
H3C
+ R2NH O2N NR 2
H3C
H3C
+ HBr
8. Special Substituent effect -
Special Substituent effect -
Hydrogen bonding
Hydrogen bonding
Case I:
N
N
O
OCH2C6H5
C6H5CH2
N
N
OCH2C6H5
C6H5CH2
O
N
N
O
C6H5CH2 OH
N
N
C6H5CH2
O
OH
Case II: C
H3C CH3
O
C
H3C
OH
CH2
K eq
C
CH3
O
C
O
H3C
K eq
C
C
O
H3C
OH
CH2
'
K eq
' K eq
>>
Case III: H
C C
H
COOH
HOOC
maleic acid
H
C C
HOOC
COOH
H
fumaric acid
Maleic acid has an unusually large first acid ionization constant
and an unusually small second acid ionization constant relative to
fumaric acid.
