4. For the iron reaction above, which undergoes a dissociative mechanism: A) How

Question

4. For the iron reaction above, which undergoes a dissociative mechanism:

A) How would changing the leaving group from Cl- to Br- affect the reaction rate of the reaction? Why?

B) How would doubling the concentration of the NH3 entering group affect the reaction rate of the reaction? Why?

C) How would the reaction rate change if the reaction were carried out in 0.1 M KCl solution compared to in pure water, assuming that the amounts of starting complex and NH3 are the same in both experiments? Why?

D) What do you predict for the sign and relative magnitude of S‡ and V‡ for the reaction?

E) Why is the sign for H‡ always positive in general?

Explanation / Answer

4.The above iron (ll) complex undergoes dissociative mechanism.

(A) In SN2 reaction,the rate depends upon the nature of the leaving group.In this reaction,leaving group leaves as -vely charged species.In the transition state.the -ve charge gets distributed over the nucleophile and the leaving group.Better the leaving group,more stable is the transition state.Thus -ve charge greately stabilised by the leaving group and rate of reaction increases.The stability of the anion is inversely proportional to its basicity.hence ,weakest bases are the best leaving groups in SN2 reaction.Bromide ion is good leaving group while chloride ion is poor leaving group.

In SN1 reaction ,the intermediate carbocation is formed by the removal of leaving group.Better the leaving group,easier is the formation of carbocation due to decrease in activation energy .This helps in increasing the reactivity of alkyl halide in SN1 reaction.

(B) These reactions proceed by a dissociation process in which bond-breaking between the leaving group and iron is much more than its bond-making with the entering group. This does not require that a five-coordinated reactive intermediate, which can survive several molecular collisions and discriminate between entering nucleophiles, be formed. Nor does it exclude the possibility of some attachment to the entering NH3 in the transition state.No intermediate is formed along the reaction pathway of sufficient stability to be selective in its reactions. This was done by allowing a ammonia solution of [Fe(H2O)5 Cl ]2+.

(c)

(D)Thermodynamic equation linking activation entropy, S‡ and activation volume,V‡ directly. However, similarities in S‡ and V‡ stem from the same source in both the thermodynamic and kinetic senses, so intuitively expects some degree of S‡ /V‡correlation. Much of the discussion surrounding the interpretation of V‡ has focused on possible correlation with S‡ ,Some scientists have reported good S‡ /V‡ correlations for aquation, isomerisation and racemisation reactions of metal complex.Few activation volumes have been reported for redox reactions between metal complexes and non-metal oxidants.

(E) Every reaction in which bonds are broken will necessarily have a higher energy transition state on the reaction path that must be traversed before products can form. This is true for both exothermic and endothermic reactions. In order for the reactants to reach this transition state, energy must be supplied from the surroundings and reactant molecules must orient themselves in a suitable fashion. The heat energy needed to raise the reactants to the transition state energy level is called the activation enthalpy, H‡. However, in these introductory discussions a distinction between enthalpy and "potential energy" is not made. As expected, the rate at which chemical reactions proceed is, in large part, inversely proportional to their activation enthalpies, and is dependent on the concentrations of the reactants.From the activation parametres,

G‡ = H‡ – TS‡

Elements that have solid standard states (e.g. carbon) present an even more complex bond dissociation energy challenge. Fortunately, it is possible to determine the bond dissociation energy of diatomic elements and compounds with precision by non-thermodynamic methods, and together with thermodynamic data such information permits a table of average bond energies to be assembled. These bond energies or bond dissociation enthalpies are always positive, since they represent the endothermic homolysis of a covalent bond. It must be emphasized that for the common covalent bonds found in polyatomic molecules (e.g. C-H and C-C) these are average dissociation enthalpies, in contrast to specific bond dissociation enthalpies determined for individual bonds in designated compounds.

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