Chapter 3 An Introduction to Organic Reactions and Their Mechanism

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Presentation transcript:

Chapter 3 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