How Organic Reactions Occur Mechanisms

Rearrangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product  

An example of a rearrangement reaction is the conversion of the alkene 1-butene into its constitutional isomer 2-butene by treatment with an acid catalyst  

Problem 5.1 Classify each of the following reactions as an addition, elimination, substitution, or 

Having looked at the kinds of reactions that take place, let s now sec 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. It describes which bonds are broken and in what order, which bonds are formed and in what order, and what the relative rates of the steps are. A complete mechanism must also account for all reactants used, all products formed, and the amount of each. 

All chemical reactions involve bond breaking and bond making. When two molecules come together, react, and yield products, specific bonds in the reactant molecules are broken, and specific bonds in the product molecules are formed. Fundamentally, there are two ways in which a covalent 

Rearrangement reactions occur when a single reactant undergoes a 

This single reactant... A- B. .. gives this isomeric product. 

Rearrangement reactions occur when a single reactant undergoes a reorganization of bonds and atoms to yield an isomeric product. An example is the conversion of dihydroxyacetone phosphate into its constitutional isomer glyceraldehyde 3-phosphate, a step in the glycolysis pathway by which carbohydrates are metabolized. 

We ll develop the point in more detail later, but you might note for now that the movement of one electron in the symmetrical process is indicated using a half-headed, or fishhook, arrow (a), whereas the movement of two 

Unsymmetrical bond-breaking (polar) two bonding electrons stay with one product. 

Just as there are two ways in which a bond can break, there are two ways in which a covalent two-electron bond can form. A bond can form in an electronically symmetrical way if one electron is donated to the new bond by each reactant or in an unsymmetrical way if both bonding electrons are donated by one reactant. 

Symmetrical bond-making (radical) one bonding electron is donated by each reactant. 

In addition to polar and radical reactions, there is a third, less commonly encountered process called a pericyclic reaction. Rather than explain pcricyclic reactions now, though, wee ll look at them more carefully in Chapter 30. 

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