Dr, Lowe - Substitution Reactions

Question 1

The Facts:

Answer 1

Rate laws, rate constants

Question 2

Hypothesis:

Answer 2

the mechanism (fits the facts)

Question 3

Kinetics describe…

Answer 3

how fast the equilibrium is established

Question 4

Thermodynamics gives…

Answer 4

no information on rate

Question 5

The rate limiting step is…

Answer 5

the slowest step of a multistep reaction

Question 6

Flooding Technique:

Answer 6

Simplifies data analysis for rate equations with multiple reactants

Question 7

How would you go about performing the flooding technique for A+B –> products

Answer 7

• Make [B]&raquo_space; [A] (ten fold or higher)
• this makes the rate =kobs[A]
• kobs = k[B]

Question 8

What happens as a result of the flooding technique

Answer 8

• Pseudo 1st order conditions
• concentration of one reactant changes significantly over time (the on no in excess)
• Other concentration is constant throughout the reaction and can be treated as part of the rate constant.

Question 9

If kobs = k2[B] what graph would you plot and what would the graph show?

Answer 9

• kobs vs. [B] graph
• Slope would be linear
• Slope = k2
  Intercept = 0

Question 10

Name the two types of Substitution Reactions

Answer 10

Dissociative

Associative

Question 11

Define Dissociative Substitution

Answer 11

Dissociation of X to yield a 5 coordinate intermediate. M-X bond breakage is the RDS.
It is independent of ligand [Y]
Rate = k1[ML5X]

Question 12

How would you prove a reaction undergoes the Dissociative mechanism?

Answer 12

Plot kobs v [ligand] graph
should be linear/constant
line: kobs = k1

Question 13

delta H and delta S values of a dissociative mechanism?

Answer 13

H = large + positive
S = positive

Question 14

Define Associate Substitution

Answer 14

Collision of ML5X with Y yields a 7-coordinate intermediate.
Rate depends on [ML5X] and [Y]
Cleavage of the bond is fast step
Rate = k1[ML5X][Y]

Question 15

How would you prove a reaction undergoes the Associative mechanism?

Answer 15

• Use flooding technique and make [Y]&raquo_space; [ML5X]
• kobs = k1[Y]
• kobs dependent on [Y]
• Plot kobs vs. [Y] graph
• Slope = k1
• intercept = 0

Question 16

delta H and delta S values of an associative mechanism?

Answer 16

H = positive + small (< D)
S = < 0

Question 17

Associative and Dissociative

Answer 17

Most metal complex substitution pathways bond formation between the metal and entering group is thought to be CONCURRENT with bond cleavage between the metal and leaving group.

Question 18

What is the name of a mechanism that involves both Dissociative and Associative mechanisms?

Answer 18

Interchange (I) mechanism

Question 19

What are the two types of interchange mechanisms?

Answer 19

Dissociative interchange (Id) - bond breaking dominates bond formation.

Associative interchange (Ia) - bond formation dominates bond breaking

Question 20

What are the dependences of the entering group with Id and Ia?

Answer 20

Ia shows dependence on entering group.

Id shows only a small dependence on the entering group.

Question 21

What type of process is the interchange mechanism known as?

Answer 21

It is known as a concerted process.

All the intermediate species have the same coordination number as the starting complex.

Question 22

How would you plot an Associative-Dissociative Graph?

Answer 22

kobs vs. L
intercept = k1
slope = k2
kobs = k1 + k2[L]
Slope is proportional to nucleophilicity

Question 23

Factors affecting k1 and k2 pathway

Answer 23

Slope (k2) varies with ligand.

Intercept (k1) is independent of ligand but varies with coordination power of solvent.

k1 is dominant in polar solvents.

Question 24

What is the name of the process where strongly coordinating solvents affect k1?

Answer 24

Solvent Participation

Question 25

What is the difference between the Limiting Dissociative mechanism and the Dissociative mechanism?

Answer 25

An extra reversible step (k-1)

Question 26

What does the limiting dissociative graph look like?

Answer 26

Exponential increase and plateaus off.

Question 27

What is the equation for kobs in the LD mechanism? (non-linear)

Answer 27

Kobs = k1k2[Y]/k-1[X]+k2[Y]

Question 28

How does amount of [Y] affect the rate?

Answer 28

Low [Y] = kobs dependent on [Y] (k2<< k-1) - like A High [Y] = kobs independent on [Y] (k2 >>k-1) - like D Strong nucleophiles reach klim at low [ ] Weak nucleophiles reach klim slowly.

Question 29

Inverted form of kobs equation?

Answer 29

1/kobs = 1/k1 + k-1[X]/k1k2[Y]

Question 30

What does the inverted graph look like?

Answer 30

Linearised Steep slope = weak intercept = 1/k1

Question 31

How can rate coefficient be affected?

Answer 31

By varying temperature and pressure.

Question 32

How would you go about performing temperature variation?

Answer 32

Arrhenius equation + graph

Question 33

How would you go about performing pressure variation?

Answer 33

ln(kp/kp=0) = -V/RT x P ln(kp/kp=0) vs. P graph gradient = -V/RT

Question 34

Comments of delta V

Answer 34

V = positive (S) V = negative (A) tends to be more accurate than S

Question 35

Ligand Substitution on Alkyl Complexes

Answer 35

Similar to Dlim but not k-1[X] term (only k-1)

Question 36

kobs for ligan substitution on alkyl complexes?

Answer 36

kobs = k1k2[L]/k-1k2[L]