12.6 Reaction Mechanisms Discuss #11 Can you predict a rate law based on the reaction? If so, why? If not, why not? What issues do you encounter?

12.6 Reaction Mechanisms Collisions of reactants are necessary for reaction to occur. In many reactions, it is too difficult for all of the reactants to collide properly in one step. Most reactions occur in a series of steps Reaction Mechanism = step by step description of how reaction occurs Mechanism is a detailed pathway for a reaction

Mechanisms Practice: #50

Mechanism Concepts Usually have several possible mechanisms that can be proposed for a reaction. Mechanisms identify the elementary reactions (one-step reactions) that add to give the overall reaction. *Elementary steps must add up to give the balanced chemical equation.

Elementary Steps Three types of elementary reactions Unimolecular  One reacting molecule A  C + D Bimolecular  Two reacting molecules 2A  C + D A + B  C + D Termolecular  Three reacting molecules 2A + B  C + D Molecularity: the number of reactant molecules involved in an elementary reaction greater molecularity possible, but is rare

Elementary Steps: Whiteboard #49

Mechanisms Reactions may by elementary: CH3NC  CH3CN Rate = k[CH3NC] O3 + NO  O2 + NO2 Rate = k[O3][NO] What evidence would we need to prove these are elementary reactions???

Elementary Reactions For elementary (single step) reactions, coefficient from balanced equation gives the reaction order Reaction Rate Law Rxn order (reactants) Rxn Order (overall) A  C + D Rate = k[A] 1st A + B  C + D Rate = k[A][B] 2nd 2A  C + D Rate = k[A]2

So, Single or Multi- Step? Many rate laws are complex, so the overall reaction would not likely be an elementary reaction. These have multistep mechanisms. Each step in the mechanism is a proposed elementary (1 step) process. Reactions must add, as in Hess’s law.

Group Work 1 Reaction: Cl2 + CHCl3  HCl + CCl4 The following is a proposed mechanism: Cl2 2Cl fast Cl + CHCl3  HCl + CCl3 slow Cl + CCl3  CCl4 fast Is the mechanism valid? Support w/ evidence. Write the rate law based on the proposed mechanism.

Multistep Mechanisms One step in multistep mechanism is slower than the others -- called the rate-determining step. The proposed mechanism must be consistent with the rate law and other experimental evidence Consider two possible mechanisms for the substitution reaction of a halogenoalkane: CH3Cl + OH-  CH3OH + Cl-

Possible Mechanisms

SN2 Mechanism Substitution reactions can occur with several different mechanisms. One possibility, called the SN2 mechanism, involves the simultaneous bonding of both the entering and the leaving group to the central atom.

SN1 Mechanism Substitution reactions can occur with several different mechanisms. One possibility, called the SN1 mechanism, involves the breaking of the bond to the leaving group before making a bond to the entering group. An intermediate of lower coordination number is formed.

Multistep Mechanisms Consider another example: Tl3+ + 2Fe2+  Tl+ + 2Fe3+ Experimental Rate Law Rate = k[Tl3+][Fe2+]2[Fe3+]-1

Multistep Mechanisms Tl3+ + 2Fe2+  Tl+ + 2Fe3+ Proposed Mechanism: Tl3+ + Fe2+ ⇌ Tl2+ + Fe3+ fast Tl2+ + Fe2+  Tl+ + Fe3+ slow (rate det.) Rate = ?

Rate = k[Tl3+][Fe2+]2[Fe3+]-1 Rate Law written based on slow step (must be compared to experimentally determined rate law) Intermediate: substance produced in one step and consumed in a later step (Tl2+) Intermediates can NEVER appear in the rate law.

Group Work 1 S2O82- + I-  2SO42- + I+ slow I+ + I-  I2 fast Write the overall reaction. Which step is rate determining? What is the intermediate? S2O82- + 2I-  2SO42- + I2 1st step; I+

Mechanism Summary What makes a good mechanism? Sum of the elementary steps gives the overall balanced equation. The rate law based on the mechanism slow step is consistent with the experimental rate law. No reaction intermediate(s) included in the rate law.