Nature of mechanism is predicted by the following factor

By -
Nature of mechanism is predicted by the following factor:
1)Effect of charge on the complex :
It has been found that the divalent monochloro complexes react about 100 times lower than the monovalent dichloro complexes. Since a decrease in rate is observed as the charge of the complex increases a dissociation mechanism seems to be operative. Thus the acid hydrolysis of
(Co(NH3)4Cl2) can be shown as follows-


The acid hydrolysis of divalent complexes like (Co(NH3)4Cl(H2O)) also takes place in two step


2) Chelation effect:
When ammonia molecule in (Co(NH3)5Cl) are replaced partially or completely by poly amine like -en, -trien,-dien, -tetraen etc the rate of acid hydrolysis is decreased .As a result chelation generally the Co-N bond distance should be decreased and more charge should be transferred to the metal ion which result in enhencement of the rate of the hydrolysis reaction but experimentaly it has been found that the rate decrease some other factor must be responsible  for rate decrease.
       Chelated complex both in ground state and in the transition state is solvated.
Again the chelated complex is larger in size. The larger the size of the ion less its salvation energy will be and hence less easily it will be formed and thus the stability  of the trasition state decrease. In this way we can conclude that the rate of hydrolysis is slowed down by chelation because of reduce stability of the transition state Due to less efficient salvation .How ever the chelation can't the predict the mechanism.only it may be said that ionic bond Breaking  in the Transition state is more important.

3)Effect of substitution ethylenediamine:
When H-atoms on C Or N atom of ethylenediamine group of trans(Co(en)2Cl2) are replaced by the alkyl group like -CH3,-C2H5 etc the ligand becames more bulky.thus it favour dissociation mechanism as the Transition state become five coordinated.
        Due to steric hinderence the possibility  of association mechanism is decreased due to substitution in ethylene diamine.

4)Effect of leaving group:
The rate of acid hydrolysis of (Co(NH3)5X) corresponding to the replacement of X with water molecule depends on the nature of X because the bond Breaking step is important  in rate determining step.
        The reactive of X group decreases  the order
HCO3>NO3>I>Br>Cl>SO4>F>CH3COO>SCN>NO2
   This order correspond to the order of the decrease in thermodynamic stability  of the complex formed with this group this favour the dissociation  mechansim

Popular posts from this blog

Organic Photochemistry Part I

By -
Image
Organic Photochemistry Part I Learning Objectives:   In this unit we shall learn about the  i) Basic Principles of Photochemistry. ii). Several photophysical processes . iii). Photochemical reactions of a few classes of organic compounds. INTRODUCTION Classical Chemistry : It involves the reactions of molecules in their ground electronic states. Photochemistry : It involves the reactions of molecules in excited electronic states.  Organic photochemistry is the study of reactions that occur through excited states of organic molecules produced by the absorption of electromagnetic radiation in the UV-Vis region. Classical Chemistry & Photochemistry There are a few advantages of photochemistry over classical chemistry. 1.In photochemistry selective activation of individual reactant in a mixture of number of reactant is possible. 2.There is an activation energy requirement for every chemical reaction. However if the activation energy is very high, the desired reaction ca...
Read more

Interface-Lippmann equation

By -
Image
  Thermodynamics explaination of electrified interface-Lippmann equation 1.Consider electrode/ electrolyte interface as closed system where no matter leaves or enter in it, then    dU =TdS-W TdS=Q= Heat produced/supplied reversibly W= Work carried out reversibly.  2.Again consider it as an open system, matter can exchange between and surrounding .Let us introduce a mol of i species , the chemical potential on the system is μi  dni is the change of the number of moles, then work done = - μidni dμ= TdS-W-∑μidni ∑μidni= work done required to expel dni of species i The work done in the metal/solution interface (MI/S) are due to (a) volume expansion PdV (b) increase of surface area dA , the corresponding increase of interfacial tension ν. dA 3. Changing the external source of electricity for altering the charge on the metal by dq' M is  MI∆S φ.dq'M MI∆S φ=V, MI∆S φ= interfacial tension, V=applied potential dU=TdS-PdV- νdA - MI∆S φ.dq'M - ∑μidni…….(1) Each ter...
Read more

BUTLER- VOLMER EQUATION

By -
Image
  BUTLER- VOLMER EQUATION: Dependence of electrochemical reaction rate on overpotential. The electrode/solution current may be cathodic or anodic according to the flow of electrons which can measure the sign of overpotential i.e deviation from the equilibrium condition the two partial current densities are equal in magnitude and opposite in direction.i.e      η(overpotential) = ∆φ-∆φe , ∆φ = potential difference observed by adjusting the outside potential source for the change in current flow.     ∆φe = potential difference at equilibrium There will be polarization i.e potential variation from equilibrium     icath = ianod , at equilibrium. If icath = ianod = i 0  r1 = rate of M n+ formation (i cath)  r2=rate of M formation ( i anod)   i 0 = exchange current density                  icath > ianod          r2>r1, if E0 for (M n+ /M) at equilibriu...
Read more