Substitution Reactions
2.2. Mechanisms of Substitution Reactions
A reaction mechanism is a detailed process which involves all the step by step elementary reactions which culminates the observed overall reaction.[5] Langford and Gray[6] attempted to classify the inorganic reactions based on the concepts of stoichiometric mechanism and intimate mechanisms.[2] The stoichiometric mechanism can be classified in one of the three forms described below.[2, 7-9]
a. Limiting dissociative (D)- intermediate with lower coordination number.
The D term replaces the older SN1 term.
b. Limiting associative (A)- intermediate with higher coordination number.
The A term replaces the older SN2 term.
c. Interchange (I)- no observable intermediate forms here.
Since the activation mechanism is related to the mode of activation step, the stoichiometric interchange mechanism is further classified into its intimate mechanism. They are:[1, 2]
a. Dissociatively activated (ID) - there is no direct interaction between the reactive centre and the entering group in the transition state
b. Associatively activated (IA) – there is bonding between the incoming group and the reactive centre in the transition state.
In intimate mechanism the bond-breaking and the bond-formation take place in a pre- formed aggregate. The incoming group, Y, and the leaving group, X, are interchanged in the inner and the outer coordination sphere of the metal centre. Therefore, the rate is independent of the incoming nucleophile and the relationship from dissociative to associative can be represented as in Figure 2.1.[1, 10]
R + X
RX
1 transition state
R X
Y 1 transition state within a preformed
aggregate
Y - -R - -X
RX + Y YR + X 1 transition state TS
2 transition states and 1 intermediate
YRX
RX + Y YR + X
Dissociative activation Associative activation
D ID IA A
Potential Energy
Figure 2. 1 The relationship between the mechanism of substitution and its energy profile and the classifications of Langford-Gray and Hughes-Ingold.[1, 10]
33 2.2.1. Dissociative Mechanism
In this mechanism, the R—X bond breaks (Figure 2.1) completely before incoming group attaches to the metal centre. This results into an intermediate with lower coordination number which is assumed to live long enough allowing the intermediate to discriminate the potential ligands in the surrounding medium before it reacts with the entering group.[11] The leaving group moves from the coordination shell to the solvation. This results in the favouring of solvent attack as the solvent is present in large access. However, the entering group dominates if it is present in a large excess. The formation of the product will therefore occur while the leaving group is in close proximity to the intermediate. Thus the rate of reaction depends on the nature of the leaving group and is not sensitive to the nature and the concentration of the incoming group.[8]
2.2.2. Dissociatively Activated Interchange Mechanism
In this reaction mechanism the leaving group is moving from the inner coordination sphere to the outer coordination sphere while the entering group moves from the outer coordination sphere to the inner coordination sphere. The metal-leaving group bond gets weakened before the entering group tightly binds to the metal centre. In this mechanism, if there is a reagent whose concentration is much less than that of the solvent which is already in the inner coordination sphere when the dissociation takes place, then the probability of the solvent attaching to the reactive metal centre is higher.
2.2.3. Associative Mechanism
In this mechanism the R—Y bond formation (Figure 2.1) takes place before the R—X bond breaks resulting in an increase in the coordination number around the metal centre. Here the bond making transition state is more distinct than the bond breaking transition state. When the entering group, Y, and the leaving group, X, are chemically identical, the bond making and the bond breaking transition states have the same energy. But, when there is a net chemical change, one will be at a higher energy level and the more stable transition state will have a deeper potential energy well. A typical energy profile diagram for the associative mechanism is shown in Figure 2.2.[10, 11]
34
Figure 2. 2 Energy profiles for the A mechanism for the substitution, showing the relationship between the intermediate and the bond-breaking transition states: (a) the bond breaking transiting state at higher energy (b) the bond-making transition state at higher energy.[10, 11]
In an associative mechanism bond making is an important step for the transition states. For a reaction that goes to completion: [2]
AX + Y ka
k-a X Y kb
AY + X (2. 4)
A
The steady-state approximations can be used if the concentration of the intermediate is small and constant[9, 12] and thus assumed to be negligible so that:
] ][
[ ] ][
] [ [ ] [
2 AX Y
k Y k AX k
k k dt
AX d dt AX d
b a
b
a (2.5)
where k2 = (ka kb)/(k-a + kb).
When kb << k-a, the second-order rate constant simplifies to k2 = (kakb)/k-a and hence the second-order rate constant not only depends on breaking the bond with the leaving group, but also is sensitive to the nature of the incoming group, Y.[2] This is indicative of the relative stability of the intermediate with the higher coordination number (ka/k-a). When kb >> k-a the second-order rate constant, k2 is equal to ka which is the case of rate-limiting bond formation.[2]
(a) (b) (b)
35
2.2.4. Associatively Activated Interchange Mechanism
In this mechanism the reaction rate is more dependent on the nature of the entering group since there is a bond formation between the entering group and the reactive centre in the transition state. The leaving group leaves the reactive centre only once the incoming group fully binds to the reactive centre.[13]