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+BC +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