How Chemical Reactions Occur

In writing the equation for a chemical reaction, we put the reactants on the left and the products on the right with an arrow between them. 

Chemisfs believe fhaf molecules reacf by colliding with each other. 

Some collisions are violent enough to break bonds, allowing the reactants to rearrange to form fhe producfs. For example, consider the reaction 

The idea that reactions occur during molecular collisions, which is called the collision model, explains many characteristics of chemical reactions. 

For example, it explains why a reaction proceeds faster if the concentrations of the reacting molecules are increased (higher concentrations lead to more collisions and therefore to more reaction events). The collision model also explains why reactions go faster at higher temperatures. 

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A nitro group behaves the same way m both reactions it attracts electrons Reaction is retarded when electrons flow from the aromatic ring to the attacking species (electrophilic aromatic substitution) Reaction is facilitated when electrons flow from the attacking species to the aromatic ring (nucleophilic aromatic substitution) By being aware of the connection between reactivity and substituent effects you will sharpen your appreciation of how chemical reactions occur... 

The emphasis in prior chapters has been on those aspects of reaction mechanisms that follow directly from the kinetics. No account of mechanisms is complete, however, without reference to complementary techniques. These approaches are less rigorous than kinetics per se, but nonetheless very valuable in leading one to an understanding of how chemical reactions occur. 

A mass spectrometer provides an example of a molecular beam, in this case a beam of molecular ions. Molecular beams are used in many studies of fundamental chemical interactions. In a high vacuum, a molecular beam allows chemists to study the reactions that take place through specifically designed types of collisions. For example, a crossed-beam experiment involves the intersection of two molecular beams of two different substances. The types of substances, molecular speeds, and orientations of the beams can be changed systematically to give detailed information about how chemical reactions occur at the molecular level. Chemists also have learned how to create molecular beams in which the molecules have very little energy of motion. These isolated, low-energy molecules are ideal for studies of fundamental molecular properties. 

The story of the ozone hole illustrates how important it is to learn the molecular details of chemical reactions. Some chemists use information about how reactions occur to design and synthesize useful new compounds. Others explore how to modify reaction conditions to minimize the cost of producing industrial chemicals. This chapter explores how chemical reactions occur at the molecular level. We show how to describe a reaction from the molecular perspective, introduce the basic principles that govern these processes, and describe some experimental methods used to study chemical reactions. 

These examples illustrate only a small part of the range of time scales that chemists and chemical engineers examine in their studies of chemical phenomena. By visualizing chemical transformations, chemical scientists obtain a deeper fundamental understanding of how chemical reactions occur and even develop the ability to create favorable conditions to control some of these reactions. 

Phase transition has many aspects in common with chemical interaction, but the latter is exceedingly more complicated. Models of phase transformation can therefore not provide much more than a crude indication of how chemical reactions occur. 

In the introduction to this chapter we discussed how chemical reactions occurred. Recall that before a reaction can occur there must be a collision between one reactant with the proper orientation at the reactive site of another reactant that transfers enough energy to provide the activation energy. However, many reactions do not take place in quite this simple a way. Many reactions proceed from reactants to products through a sequence of reactions. This sequence of reactions is called the reaction mechanism. For example, consider the reaction... 

We are now ready to build a model of how chemical reactions occur at the molecular level. Our model must account for the Arrhenius equation and reveal the significance of the Arrhenius parameters A and Ea. Reactions in the gas phase are conceptually simpler than those in solution, so we begin with them. 

We have seen, above, that computational chemistry can sometimes tell us with good reliability whether a molecule can exist. Another important application is to indicate how one molecule gets to be another how chemical reactions occur. Indeed, the prime architect of one of the most useful computational tools, the AMI method (Chapter 6), questioned whether the mechanism of any organic reaction was really known. [36] before the advent of computational chemistry ... 

In 1808 Gay-Lussac published his Law of Combining Volumes of Gases. He determined that when different gases reacted, they would always do so in small whole number ratios (e.g., two volumes of hydrogen would react with one volume of oxygen in forming H20). This was one of the greatest advancements of its time and helped form the basis for later atomic theory and how chemical reactions occur. 

The main characteristic of my research is inventing ways of solving problems. I am a frustrated inventor I like to make instruments and come up with new ways of doing things. I am very interested in how chemical reactions occur, collision by collision, in the gas phase. For some time, I have been in the detective business in looking at chemical reactions. The clues are provided by lasers, which not only measure how the molecules vibrate and rotate but also measure how fast they are traveling and how... 

King, E. L. (1964) How Chemical Reactions Occur, Benjamin, Menlo Park, CA. 

You will investigate a model describing how chemical reactions occur as a result of collisions. 

Once chemists gained an appreciation of the fundamental principles of bonding, a logical next step became the understanding of how chemical reactions occurred. Most... 

The second category of generalizations is the one of real interest because it, along with possible exceptions from typical behavior, provides insights into how chemical reactions occur. One of the safest such generalizations is that the development of resonance and the concomitant solvation of the carbanion invariably appear to lag behind other bond changes. These PNS effects thus typically lead to a lowering of k0. The possible reasons why reactions proceed in this fashion are discussed elsewhere (8, 37-40). These reasons include quantum mechanical (resonance) and entropy effects (solvation). 

First, it seems desirable, even prima facie, that we develop an intuition of how chemical reactions occur. For example, molecular orbital theory provides a fair amount of detailed intuition about the energetics of chemical reactions, i.e., when do we expect large activation barriers, when do we expect concerted reactions, what is the effect of an electrophilic substituent on reaction product distributions, etc. A similar intuition has not been available concerning the role of dynamics in chemical reactivity. It is reasonable, as a chemist, to ask how one could enhance energy transfer specifically into the reaction coordinate, thus to make the reaction more efficient and to produce better reaction yields with fewer byproducts. 

For the rest of Part 1 of this book, we turn our attention to the discussion of just how chemical reactions occur and, as a consequence, we adopt a more theoretical viewpoint. To begin such a discussion we extend the description of elementary reactions that we started in Section 2.1. You should recall from that section that an elementary reaction is one which takes place in a single step, does not involve the formation of any intermediate species, and which passes through a single transition state. 

The concept of atoms is a very useful one. It explains many important observations, such as why compounds always have the same composition (a specific compound always contains the same types and numbers of atoms) and how chemical reactions occur (they involve a rearrangement of atoms). 

We have shown how clusters provide an interesting and powerful medium for the study of how chemical reactions occur. We no longer view clusters as merely allowing us to link the study of collision dynamics in the gas phase and in solution. The study of reaction dynamics in clusters has opened up new possibilities and uncovered new ways of inducing chemical reactions. 

OBJECTIVE To understand the collision model of how chemical reactions occur. 

Imperfect as it is—as are all scientific theories— conventional transition-state theory is of importance in providing a conceptual framework with the aid of which much insight is gained into how chemical reactions occur. It is possible without even making any numerical calculations to make qualitative predictions of many important kinetic effects. So far no alternative treatment has provided any such insight. 

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