Kinetics, Reactivity, Mechanisms

Question 1

Arrhenius Eqn.

Answer 1

k = Ae^(-E_a/RT)
E_a is activation energy
R is universal gas constant
k is rate constant

Question 2

Transition State Theory

Answer 2

-assumes an equilibrium between the reactants and activated complex (not true, but is assumed)

Question 3

Magnitude of rate constants

Answer 3

-fastest rxn has a rate constant of k~10^13 s^-1 (largest rate constant possible)

Question 4

Hammond Postulate

Answer 4

-two consecutive states of similar energy will be structurally similar
-this holds when the energies are relatively close
EXOTHERMIC - product-like TS
ENDOTHERMIC - reactant-like TS

Question 5

Hammond Postulate for Intermediates

Answer 5

• less stable intermediates = less stable TS

- an intermediate must exist for one bond vibration, otherwise it is TS (>10^-13 s)

Question 6

Marcus theory

Answer 6

Treats rxns with intersecting parabolas
-intersection of curves = transition state
-the magnitude of the barrier determines the rate
Barrier comprised of two parts: thermodynamic part and intrinsic part
-stronger bonds have a thinner Morse potential

Question 7

Using Marcus Theory

Answer 7

• parabolas adjusted to give the correct DG*
• the intersection represents the TS, allows for calculation of DG(double dagger)
• increasing exothermicity eventually predicted to result in slower rxns (parabola moves down, eventually slows rxn down

Question 8

Self exchange reaction

Answer 8

• swapping a substituent for the same substituent (overall DG* = 0)
  ie. CH3Br + Br- —-> CH3Br + Br-

Question 9

Reactivity vs. Selectivity

Answer 9

selectivity: one pdt formed over another
- highly exothermic rxn displays little selectivity, TS closely resembles reactant
- endothermic rxn with pdt like TS are more selective

Question 10

Curtin-Hammett principle

Answer 10

• pdt ratio determined by activation energy differences, not pdt stability or ratio of intermediates
• relative stability TS sufficient to predict major pdt, dont need to consider pdt stability

Question 11

Principle of microscopic reversibility

Answer 11

rxn path is the same in fwd direction and reverse direction

-same TS followed in either direction

Question 12

Kinetic vs. Thermodynamic control

Answer 12

Kinetic: product ratio determined by relative DG(double dagger)
Thermodynamic: pdt ratio determined by relative DG*
-requires that products can equilibrate under rxn conditions (rxn must be reversible)

Question 13

O’Ferrall Jencks plots

Answer 13

• use Hammond+Anti-Hammond effect to determine change in TS when the reactants are changed
• consider the vector sum of Hammond and anti-Hammond effects for new TS location
• consider major bond breaking/formation

Question 14

Hammond Effect

Answer 14

TS shifts along diagonal towards the higher energy corner

Question 15

Anti-Hammond effect

Answer 15

TS shifts perpendicular to diagonal towards lower energy corner