ChE 553 Lecture 17 Prediction of Mechanisms 1

Objectives Develop methods to predict mechanisms Apply the ideas for a simple reaction 2

You Already Learned About the Mechanisms of Reactions in Organic Chemistry Organic view of mechanisms – things to memorize Masel view of mechanisms – things to calculate 3

Key: Activation Barriers Control Mechanisms Reaction goes by the pathway that has the lowest activation barrier between reactants and products –Catalytic cycles used to lower barriers 4

The Idea of Computing a Mechanism 5 1) write down all possible reactions 2) Use rules to make sure no important reactions are missing. 3) Use rules to eliminate excess reactions.

General Rules for Mechanisms All commercially important mechanisms are basically the same !! Step 1  Create reactive species Step 2  Catalytic cycle to pump out product Step 3  Reactive species lost: 6

Example: H 2 + Br 2  2HBr 7 Figure 5.1 A cycle for HBr formation via reaction (5.3).

Initiation-Propagation Mechanisms Initiation step: create reactive species Transfer step: convert initial radical into a more reactive species Propagation step: go around cycle to produce product Termination step: destroy radicals 8

Consider: H 2 + Br 2  2HBr 9 Figure 5.1 A cycle for HBr formation via reaction (5.3).

Discussion Problem: The reaction CH 3 CH 3  CH 2 CH 2 + H 2 Goes By the Following Mechanism 10 Label each step as being a a) initiation b) propagation c) termination d) transfer.

Examples of Initiation Propagation Mechanisms 11

General Approach to Finding a Mechanism Guess or predict all of the species that are likely to form during the reaction. Write down all of the possible reaction of those species (only include 7 generic types of reactions). Use various rules to pare down the list to manageable of steps. 12

Rules for Initiation Propagation Reactions There must be at least one initiation reaction The propagation reactions must occur in a cycle where radicals react with the reactants to form new radicals and then the new radicals react to from the original radicals again All of the steps in the catalytic cycle must have low barriers There should be at least on termination reaction where two radicals combine to yield stable species 13

Example: The Reaction CH 3 CH 3  H 2 C=CH 2 +H 2 Obeys the Following Mechanism: 14 Verify that it follows the rules

Step 1: Make a Diagram of the Reaction Similar to That in Figure

Step 2: Identify the Initiation Step, the Transfer Step, the Propagation Steps, the Termination Steps 16

Step 2: Continued b) Reaction 1 – initiation Reaction 2 – chain transfer Reaction 3 – propagation (  - hydrogen elimination) Step 4 – propagation (hydrogen transfer) Reaction 5 - termination 17

The Mechanisms Does the mechanisms follow the rules? There is an initiation step (step 1) There is a catalytic cycle (steps 3 and 4) There is a termination step (step 5) Still need to verify that the activation barriers are low enough 18

Empirical Rules for Activation Barriers Practical reactions Ea<0.15 Tinitiation reactionsSet minimum T Ea<0.05 T catalytic For Reaction cycle Ea<0.07 TTransfer reactions and side reactions 19

Methods to Estimate E a Polanyi relationship 20 Blowers Masel Equation when (10.63) (10.65)

Intrinsic Barriers and Transfer. Coefficients for Different Types of Neutral Species 21

Next: Estimate the Activation Barriers Consider 22

Next: CH 3 +CH 3 CH 3  CH 4 +CH 2 CH 3 23 This is an atom transfer reaction. From Table 5.4

CH 2 CH 3 +X  CH 3 CH 2 +H+X 24

HCH 3 CH 3  H 2 +CH 2 CH 3 25

CH 3 + CH 3 +X  CH 3 CH 3 +X 26

Next: Calculate Temperature to Meet Constants 27

Solution For initiation 28 For propagation

Example 5.B Consider the Following Alternate Mechanism for Ethylene Production from Ethane 29 a)Does this mechanism follow all of the rule at 810K? b)Is this mechanism more or less likely than the mechanism in example 5.A?

Solution 30

HCH 3 CH 3  CH 4 + CH 3 31

This Reaction is a Reaction in the Catalytic Cycle 32

Which Mechanism is Better 33 Ea=8.9 Ea=37.4

Summary Today derived a series of rules for reactions Must be an initiation reaction Must have a catalytic cycle Should have termination Barriers low enough Next time: Use rules to predict mechanisms. 34