Kinetics of reaction in solution

Solvent effects Solvation of reactant Catalysis Intermolecular reaction (ions, polar reactant) Diffusion (solvent viscosity) Hydrogen bond Change mechanism

1) Solvent effects on rate constants CH3I + Cl  CH3Cl + I solvent HCONH2 HCONHCH3 HCON(CH3)2 k/ dm3 mol-1 s-1 510-5 1.4 10-4 0.4 The reaction rate depends strongly on the solvent used.

Solvent effect on activation energy Solvent A standard Solvent B

For evaluation of solvent effect

Solvents 102k / dm3mol-1s-1 Hexane 0.00018 Benzene 0.0058 chlorobenzene 0.023 nitrobenzene 70.1 If the polarity of product is higher than that of reactants, higher polar solvent benefits the reaction.

2) Catalysis of water

3) Effect of ionic strength-primary/kinetics salt effect For ionic reactions in solution, activity coefficients must be taken into consideration. Therefore, the apparent rate constant depends on the ionic strength of the solution. For dilute solution

0.0 -0.2 -0.4 0.2 0.4 0.6 0.1 0.3 Effect of ionic strength on the rate of reaction between two ions. +4: Co(NH3)5Br2+ + Hg2+ +2: S2O82- + I +1: [NO2NCO2C2H5] + OH 0: CH3COOC2H5 + OH -1: H2O2 + H+ + Br -2: Co(NH3)5Br2+ + OH A substantial amount of inert salt (supporting electrolyte) was added to keep the ionic strength and therefore the activity coefficient of the solution essentially constant.

4) Cage effect Once reactant molecules meet in one solvent cage through diffusion (an encounter), they will be trapped in the cage for a relatively long time during which they collide repeatedly with each other (collision), providing more chance to react. Within a solvent cage (for 1~100 ns), two reactant molecules can collide 10 ~105 times. Does viscosity of solvent affect collision in the solvent cage? Solvent cage: capture reactant molecules Diffuse in or out, from or to other solvent cages. Experimental evidence: photolysis of CH3NNCH3 and CD3NNCD3 in gaseous phase and in solution. Given by Lyon and Levy in 1961.

The process can be represented as A + B → {AB} → products in which the {AB} term represents the caged reactants including the encounter pair and the activated complex. 10-12 ~ 10-10 s, collides for 10 ~ 105 times. Ere > Ediff. Reaction control. Ediff. > Ere, diffusion control.

5) Diffusion-controlled reactions

Discussion: If reaction is much slow than diffusion, activation control or kinetics control If diffusion is much slow than reaction, diffusion control

For activation control Resembles gaseous reaction, no effect of solvent. media gas CS2 C6H6 C2H5OH 106 k/ dm3 mol-1 s-1 6 10 20 Solvent effect on reaction between species of low polarity is weak.

For diffusion control Fickian first law Flux D diffusion coefficient with unit of m2s-1. For spherical particle, Einstein-Storks equation For second-order reaction Integration yields

For second-order reaction f is electrostatic factor. Diffusion-Limited Rate Constants, Slow reactions

For diffusion, Ea  10 kJmol-1 If rA = rB For diffusion, Ea  10 kJmol-1

4．Reactions in Solution 4.1 General Properties of Reactions in Solutions 4.2 Diffusion-Limited Rate Constants, Slow reactions 4.3 Effect of Ionic Strength on Reactions between Ions 4.4 Linear Free-Energy Relationships J.H. Espenson, Chemical Kinetics and Reaction Mechanisms, 2nd Ed (McGraw-Hill, New York, 1995)

Stochastic and Dynamic Views of Chemical Reaction Kinetics in Solutions Author: A. Kuznetsov ISBN-10: 2-88074-379-6 ISBN-13: 978-2-88074-379-6 Published in: January 1999