What is a reaction mechanism

Areaction mechanism describes in detail how a chemical reaction occurs. It describes which bonds break and which new ones form, as well as the order and relative rates of the various bond-breaking and bond-forming steps. If the reaction takes place in solution, the reaction mechanism describes the role of the solvent if the reaction involves a catalyst, the reaction mechanism describes the role of the catalyst. 

To understand the relationship between a chemical reaction and energy, think of a chemical bond as a spring. As a spring is stretched from its resting position, its energy increases. As 

An energy diagram for a one-step reaction between C and A—B.The dashed lines in the transition state indicate that the new C—A bond is partially formed and the A—B bond is partially broken.The energy of the reactants is higher than that of the products—the reaction is exothermic. 

A transition state is the point on the reaction coordinate at which the energy is at a maximum. At the transition state, sufficient energy has become concentrated in the proper bonds so that bonds in the reactants break. As they break, energy is redistributed and new bonds form, giving products. Once the transition state is reached, the reaction proceeds to give products, with the release of energy. 

A reaction in which the energy of the products is lower than the energy of the reactants a reaction in which heat is liberated. Endothermic reaction A reaction in which the energy of the products is higher than the energy of the reactants a reaction in which heat is absorbed. 

The term reaction mechanism is part of the everyday language of chemists, yet it conveys different things to different people. The IUPAC Gold book (www.goldbook.iupac.org) defines the mechanism of a reaction as A detailed description of the process leading from the reactants to the products of a reaction, including a characterization as complete as possible of the composition, structure, energy and other properties of reaction intermediates, products and transition states. An acceptable mechanism of a specified reaction (and there may be a number of such alternative mechanisms not excluded by the evidence) must be consistent with the reaction stoichiometry, the rate law and with all other available experimental data, such as the stereochemical course of the reaction. On the basis of Occam s razor (Section 3.7.4), one should always choose the simplest mechanism that is consistent with all available evidence. 

A truly mechanistic (in the sense of classical mechanics) description of a molecule s reaction is in fact prohibited by Heisenberg s uncertainty relations (Equation 2.1). Some reaction mechanisms of small molecules in the gas phase have been elucidated in the utmost detail, that is, reaction rate constants have been determined for individual rotational and vibrational quantum states of the reactant. We take a more modest view a reaction mechanism is the step-by-step sequence of elementary processes and reaction intermediates by which overall chemical change occurs. 

The mechanism of a photoreaction should ideally include a detailed characterization of the primary events as outlined by the classification of photochemical reaction pathways in Section 2.3 the lifetimes of the excited states that are involved in the reaction path, the quantum yields and hence the rate constants of all relevant photophysical and photochemical processes, in addition to the information about the structure and fate of any reactive intermediates, their lifetimes and reactivities. 

Photochemistry of Organic Compounds From Concepts to Practice Petr Klein and Jakob Wirz 2009 P. KISn and J. Wirz. ISBN 978-1-405-19088-6 

Knowledge of reaction mechanisms serves at least two purposes apart from the intellectual satisfaction of understanding . First, it provides a classification that reduces the myriad of chemical reactions of individual molecules to a more manageable set and thereby allows prediction by analogy. Second, it enables chemists to design reaction conditions in order to improve yields, the rate of product formation or selectivity of a reaction, to suppress undesired reactivity or to divert a reaction to form new products. 

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These results being quite untypical for zeolites give rise to a number of fundamental questions i) what makes the zeolite to function as an active catalyst ii) what makes N2O to function as a selective oxidant iii) what is the reaction mechanism. We shall shortly discuss the situation with these issues because of their importance for our further consideration. 

What is the compound resulting from heating 1 -(o-methoxyphenyl)per-fluoropropene with hydrobromic and acetic acid The reaction product contains phenolic and carboxylic hydroxyls. What is a probable mechanism of the reaction ... 

For many of us, what we require is expressed as a reaction mechanism or as a statement of how physicochemical factors determine activity and/or product selectivity. What constitutes a reaction mechanism will be discussed later on. What is however so unsatisfactory about some of what one reads in the literature is that either no mechanistic analysis is attempted at all, or that the conclusions drawn often rest on a very insubstantial base of experimental observation magnificent edifices of theoretical interpretation are sometimes supported by the flimsiest foundation of fact, and ignore either deliberately or accidentally much information from elsewhere that is germane to the argument. I particularly dislike those papers that devote an inordinate amount of space to the physical characterisation of catalysts and only a little to their catalytic properties. Obtaining information in excess of that required to answer the questions posed is a waste of time and effort it is a work of supererogation Full characterisation should be reserved for catalysts... 

What is the reaction mechanism of the Pd-catalyzed formation of PPEs and PAEs While there has not been a full force mechanistic investigation of the Pd-catalyzed formation of PAEs, a working model is at hand (Scheme 6.3 and Scheme 6.4). If a Pd compound is used in the reaction, it has to be reduced into the catalytically active zerovalent species. In step I, an alkyne is transformed into a cuprate by the cocatalyst Cul in the presence of an amine. Then L2PdX2 reacts under transmetallation with the cuprate to give a dialkynylated tetrahedral intermediate that reductively eliminates to furnish a diyne byproduct... 

Describe how the Chapman cycle is a reaction mechanism. What is the molecularity of each reaction in the Chapman cycle ... 

How might this substitution actually take place on the molecular level Or, to restate the question, what is the reaction mechanism—that is, the sequence of molecular-level steps involved in the reaction At first blush, there are at least two major possibilities the dissociative (D) mechanism and the associative A) mechanism. 

The first necessary clarification must be what is a reaction Chemists describe a full reaction mechanism with the familiar arrow convention for electron pair redistribution over a linked chain or cycle of atoms as in ... 

When written in this way it is clear what is happening. The mechanisms of these reactions are probably similar, despite the different p values. The distinction is that in Reaction 10 the substituent X is on the substrate, its usual location but in Reaction 15 the substituent changes have been made on the reagent. Thus, electron-withdrawing substituents on the benzoyl chloride render the carbonyl carbon more positive and more susceptible to nucleophilic attack, whereas electron-donating substituents on the aniline increase the electron density on nitrogen, also facilitating nucleophilic attack. The mechanism may be an addition-elimination via a tetrahedral intermediate ... 

Having looked at the kinds of reactions that take place, let s now see how reactions occur. An overall description of how a reaction occurs is called a reaction mechanism. A mechanism describes in detail exactly what takes place at each stage of a chemical transformation—which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates of the steps ure A complete mcchuMism must 3lso account for 2 1 resctcints used 3jid all products formed. 

Fatty acids are synthesized by a nrultistep route that starts with acetate. The first step is a reaction between protein-bound acetyl and malonyl units to give a protein-bound 3-ketobutyryl unit. Show the mechanism, and tell what kind of reaction is occurring. 

Beginning students are sometimes led to believe that writing a chemical equation is a simple, mechanical process. Nothing could be further from the truth. One point that seems obvious is often overlooked. You cannot write an equation unless you know what happens in the reaction that it represents. All the reactants and all the products must be identified. Moreover, you must know their formulas and physical states. 

Hie possibility that a particle with energy Jess than the barrier height can penetrate is a quantum-mechanical phenomenon known as the tunnel effect. A number of examples are known in physics and chemistry. The problem illustrated here with a rectangular barrier was used by Eyring to estimate the rates of chemical reactions. ft forms the basis of what is known as the absolute reaction-rate theory. Another, more recent example is the inversion of the ammonia molecule, which was exploited in the ammonia maser - the fbiemnner of the laser (see Section 9.4,1). 

What is the significance of the parameter fi = (k2C BLDAf5 / kL in the choice and the mechanism of operation of a reactor for carrying out a second-order reaction, rate constant k2, between a gas A and a second reactant B of concentration CBL in a liquid In this expression, DA is the diffusivity of A in the liquid and kL is the liquid-film mass transfer coefficient. What is the reaction factor and how is it related to /l ... 

Kinetics as a consequence of a reaction mechanism. The deduction of the kinetics from a proposed reaction mechanism generally consists in a reasonably straightforward transformation, where all the mechanistic details are eliminated until only the net gas-phase reaction and its rate remains. This approach may be used to investigate if a proposed mechanism consistent, what the reaction rate is and if it is consistent with available experimental data. 

What is the total order n of a given reaction What are the most common reaction orders in environmental organic chemistry Does the total order of a given reaction tell you anything about the reaction mechanism What is a pseudo-nth-order rate constant ... 

Explain in words what a nucleophilic substitution reaction is. At what kind of atoms do such reactions primarily occur What is(are) the mechanism(s) and the corresponding rate law(s) of such reactions ... 

It should be evident that with a little practice you can very quickly, efficiently, and infallibly balance the most complicated electron-transfer equations. It is a straightforward mechanical process. This statement is true IF you know what the products of oxidation and reduction are. The most difficult situation that exists for balancing equations is the one characterized by the following request "Write a balanced ionic equation for the reaction, if any, that occurs when you mix A and B. You know the potential reactants because they are given, but that is all. 

Much of what comes later in this book will be concerned with what we know, or can find out, about the mechanisms of such reactions —a reaction mechanism being the actual sequence of events by which the reactants become converted to the products. Such information is of extraordinary value in defining and understanding the range of applicability of given reactions for practical preparations of desired compounds. 

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