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Chemical Reactivity and Mechanisms, Thermodynamics v.s. Kinetics,…
Chemical Reactivity and Mechanisms
Reactions
Kinetics
Factors of Reaction Rate
Temp.
A measure of a system's average kinetic energy
Geometry and Sterics
Activation Energy
Presence of Catalyst
Create a new reaction pathway.
Concentrations of Reactants
Rate Law Equation
Rate = k×[reactants]
Rate = k×([A]^a)([B]^b)
Equilibia
A + B ⇌ C + D
Equilibrium Constant, K_eq
K_eq = [Products] / [Reactants] = [C]×[D] / [A]×[B]
ΔG = − RT×ln(K_eq)
Thermodynamics
Entropy
ΔS, can be thought of as molecular disorder, randomness, or freedom
The # of states that a molecule's energy can be distributed over.
Higher # of vibrational, rotational, and translation states,
the sample will have higher entropy.
ΔS_tot = ΔS_sys + ΔS_surr
ΔS_tot > 0, i.e. ΔG < 0, the reaction is spontaneous.
The spontaneity of a process depends only on ΔS_tot.
ΔS_sys can be measured or estimated.
Gibbs Free Energy
ΔS_surr depends on ΔH_sys:
ΔS_surr = − ΔH_sys / T
ΔG = −T×ΔS_tot = ΔH_sys − T×ΔS_sys
Reactions
Exergonic
ΔG < 0, spontaneous
Endergonic
ΔG > 0, not spontaneous
Enthalpy
Bond
Bond Cleavage / Breaking
Heterolytic
Ions
Homolytic
Radicals
Bond Dissociation Energy (BDE)
H-X: Associated with electronegativity.
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C-C: Associated with structure, related to the stability of the radical from the broken bond.
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Bond Forming
Reaction
Exothermic
Surrounding temp. ↗
Most
Endothermic
Surrounding temp. ↘
ΔH or q, the heat energy exchange between
the reaction and its surroundings at constant pressure.
Electro-property
Nucleophiles
Election-rich
Lewis base: negative charge, lone-pair, or π-bond
Electrophiles
Electron-poor
Lewis acid: formal or partial positive charge
The predicting of ionic reaction products
Mechanisms
Four patterns
Nucleophilic Attack
Loss of a Leaving Group
Stability (e.g. ions) is the driving force.
Proton Transfers (Acid / Base)
Stability (e.g. resonance) is the driving force.
Rearrangements
Hyperconjugation
Slight orbital overlapping with neighboring groups causes the stability.
Induction
electronegativity
Arrow to show how electrons move
All for stability
Key rules
The arrow starts on a pair of electron.
The arrow ends on a nucleus or between two nuclei.
Avoiding breaking the octet rule.
Follow the 4 key patterns.
Reversibility
Nuc: is also a good L.G.
reversible attack
relatively low transition state energy
Nuc: is a poor L.G.
irreversible attack
relatively high transition state energy
If pKa difference is more than 10 units
Thermodynamics v.s. Kinetics
Thermodynamics control reaction
must need to be heat
lower activation energy, but less stable products
Kinetic control reaction
higher activation energy, but more stable products
Over two kinds of reactions for the same reactants.
Reactants
Transition State
Intermidiate
Transition State
Products
Rate Determine Step
The Hammond Postulate漢蒙假說
The intermediate looks like the compound (reactants, intermediates, or products) which is energy-closer.