Chemical Reactivity and Mechanisms
Reactions
Kinetics
Equilibia
Electro-property
Mechanisms
Thermodynamics
Entropy
Gibbs Free Energy
Enthalpy
Bond
Bond Cleavage / Breaking
Heterolytic
Homolytic
Ions
Radicals
Bond Dissociation Energy (BDE)
H-X: Associated with electronegativity.
C-C: Associated with structure, related to the stability of the radical from the broken bond.
The bigger the orbital is,
the more stable the radicals are.
Bond Forming
The higher the electronegativity of the atom is,
the more obvious the bond polarization,
the higher the energy needed to cleavage.
Reaction
Exothermic
Endothermic
ΔH or q, the heat energy exchange between
the reaction and its surroundings at constant pressure.
Nucleophiles
Electrophiles
Surrounding temp. ↗
Surrounding temp. ↘
Most
ΔS, can be thought of as molecular disorder, randomness, or freedom
ΔS_tot = ΔS_sys + ΔS_surr
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 > 0, i.e. ΔG < 0, the reaction is spontaneous.
ΔS_surr depends on ΔH_sys:
ΔS_surr = − ΔH_sys / T
The spontaneity of a process depends only on ΔS_tot.
ΔS_sys can be measured or estimated.
ΔG = −T×ΔS_tot = ΔH_sys − T×ΔS_sys
Reactions
Exergonic
Endergonic
ΔG < 0, spontaneous
ΔG > 0, not spontaneous
A + B ⇌ C + D
Equilibrium Constant, K_eq
K_eq = [Products] / [Reactants] = [C]×[D] / [A]×[B]
ΔG = − RT×ln(K_eq)
Factors of Reaction Rate
Temp.
Geometry and Sterics
Activation Energy
Presence of Catalyst
Concentrations of Reactants
Rate Law Equation
Rate = k×[reactants]
Rate = k×([A]^a)([B]^b)
A measure of a system's average kinetic energy
Create a new reaction pathway.
Thermodynamics v.s. Kinetics
Thermodynamics control reaction
Kinetic control reaction
Over two kinds of reactions for the same reactants.
must need to be heat
lower activation energy, but less stable products
higher activation energy, but more stable products
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.
The predicting of ionic reaction products
Election-rich
Electron-poor
Lewis base: negative charge, lone-pair, or π-bond
Lewis acid: formal or partial positive charge
Four patterns
Arrow to show how electrons move
Nucleophilic Attack
Loss of a Leaving Group
Proton Transfers (Acid / Base)
Rearrangements
Hyperconjugation
Induction
Stability (e.g. resonance) is the driving force.
Stability (e.g. ions) is the driving force.
All for stability
Slight orbital overlapping with neighboring groups causes the stability.
electronegativity
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.
Nuc: is a poor L.G.
reversible attack
irreversible attack
relatively low transition state energy
relatively high transition state energy
If pKa difference is more than 10 units