Transcript Chapter 1
An Introduction to Organic Reactions
and Their Mechanism
Acids and Bases
3.7 Inductive Effect
An inductive effect is an electronic effect due to the
polarization of σ bonds within a molecule or ion.
In a carbocation, the positive C attracts the electrons in the
σ bonds towards itself and away from the atom at the other
end of the s bond.
Electrons in C-C bonds are more readily polarized than
those in a C-H bond.
Therefore, alkyl groups are better at stabilizing C+ than H
atoms.
Inductive Effect
electron drawing
Electron releasing
Inductive effects weaken as
the distance from the
substituent increases
3.8 Energy Changes
Energy is defined as the ability to do work
Kinetic energy and potential energy
Kinetic energy: energy of motion
E = ½ mv2
Potential energy: stored energy
Potential Energy
Energy Changes
Chemical energy is potential energy (aka relative potential
energy)
Attractive and repulsive forces exist between different pieces
of the molecules
The more potential energy an object has, the less stable it is
3.8 Potential energy and covalent bond
The state of greatest potential energy is the state of free
atoms
Forming a chemical bond is always accompanied by the
lowering of potential energy of atoms
3.8 Potential energy and covalent
bond
Potential energy
Enthalpies or heat contents, H: relative potential energy or
(heat) of molecules
ΔHo = change in potential energy between reactants and
products
+ ΔHo = endothermic
stronger bonds are formed in the product
- ΔHo = exothermic
Weaker bonds are formed in the product
3.9 The Relationship between the
Equilibrium Constant and the
Standard Free-Energy Change, ΔGo
ΔGo = -R T ln Keq
R = 8.314 J/Kmol
- ΔGo = formation of product is favored
Spontaneous
+ ΔGo = formation of product is unfavored
nonspontaneous
ΔGo = ΔHo - T ΔSo
Entropy
Entropy (S) – measure of
disorder or randomness
Randomness increase -> S
increases
Sgas> Sion >Smolecule>Ssolid
Ssolid = 0
E.gH2O(s) H2O (l) S = +
H2O (l) H2O (g) S = +
H2O (g) H2O (l) S = -
2nd law of thermodynamic: the
entropy of the universe is
always increasing
Example
State whether you would expect the entropy change ΔSo ,
to be positive, negative or approximate zero for each of the
following reactions. (Assume the reactions take place in the
gas phase)
A + B C
A+BC +D
A B +C
Example
3.10 The Acidity of Carboxylic acid
The Acidity of Carboxylic acids
The Acidity of Carboxylic acids
3.10A The effect of Delocalization
Resonance stabilization
The Inductive Effect
Inductive electron withdrawing effect
Carboxylate ion has two oxygen atoms who combined
electronegativity stabilizes the charge more than an alkoxide
ion
Lower energy barrier in forming carboxylate ion than alkoxide
Acetic acid is more acidic than ethanol
Electrostatic map
Inductive effects on Other groups
Substituent effect: the acid-strengthening effect of other
electron-attracting groups
Dispersal of charge always makes a species more stable
Any factor that stabilizes the conjugate base of an acid
increases the strength of the acid
Inductive effects on Other groups