Chapter 3 - Lecture notes 3 PDF

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Course was instructed by Dr. Rajani Srinivasan for a 6 week summer course....

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Chapter 3: Acids and Bases An Introduction to Organic Reactions and their Mechanisms

Acid-Base Reactions Bronsted-Lowry Acids and Bases Bronsted-Lowry acid-base reactions involve the transfer of protons Acid- donates or loses a proton Base- accepts or removes a proton Example: H2O + H-Cl -> H3O + ClH3O (hydronium ion) – strongest acid that can exist in water HO (hydroxide ion)- strongest base that can exist in water

Lewis Acids and Bases Lewis acids are electron pair acceptors Lewis bases are electron pair donors Example: Cl – H + NH3 -> Cl - + H—NH3

Electrophiles and Nucleophiles   

Because carbocations are electron-seeking reagents, chemists call them electrophiles Electrophiles seek electrons (E+) All Lewis acids are electrophiles

Carbanions are Lewis bases (Nu-) pKa = -log [Ka] Keq = equilibrium constant Keq = [c][d]/[a][b]

Large value of Ka = strong acid Small value of Ka = weak acid Larger value of pKa = weak acid Smaller value of pKa = strong acid Example CH3CO2H > CF3CO2H > HCl pKa= 4.75 -

pKa= 0

pKa= 7

The stronger the acid, the weaker its conjugate base The weaker the acid, the stronger the base The stronger the H-X bond, the weaker the acid

Example: HF > HCl > HBr > HI Increasing acidity

The Effect of Hybridization H-C ≡ C-H

C=C

C—C

pKa = 25

pKa = 44

pKa = 50

50% s character

33.3% s character

25% s character

sp

sp2

sp3

More s character means electrons of anion will be lower in energy and anion will be more stable.

Relative Acidity of Hydrocarbons H-C ≡ C-H

>

C=C

>

C—C

>

H-C ≡ C-H

Relative Basicity of Hydrocarbons C—C

>

C=C

Inductive Effects   

Electronic effects transmitted through bonds The inductive effect of a group can be electron donating or electron withdrawing Inductive effects weaken as the distance from the group increases

Energy changes   

The two fundamental types of energy are kinetic and potential energy Kinetic energy = ½ mv2 Potential energy = mgh

Potential energy is stored energy Chemical energy is a form of PE More PE > less stable Less PE > more stable

Potential Energy and Covalent Bonds Atoms and molecules possess potential energy—often called chemical energy—that can be released as heat when they react. Δ G = -RT ln Keq R= gas constant (8.314 J K-1) ΔG=ΔH-TΔS Δ H enthalpy- total energy in system Δ S entropy- disorder in system - Δ H makes Δ G negative...