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Introduction of Reaction Mechanisms

Mechanisms of Organic Reactions

A complete explanation of the changes in bonding and structure that take place in the course of a reaction and sequence of this type of events is called the reaction mechanism. A representation of plausible electron reorganization should be included in a reaction mechanism, and identification of any intermediate species that may be formed as the reaction progresses.

These characteristics are described in the following sections.

1.      The Arrow Notation in Mechanisms

Chemical reactions include the breaking and making of bonds, a consideration of movement of bonding (and non-bonding) valence shell electrons is important to this understanding. Now with curved arrows it is a general practice to display the movement of electrons and sequence of the equations depicting the consequences of this type of electron shifts is termed a mechanism. In general there are two types of curved arrows are used in drawing mechanisms:

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The explanation of the use of these symbols in bond-breaking and bond-making reactions is demonstrated below. If a covalent single bond is broken so that one electron of the shared pair remains with each fragment, like described in the first case, this bond-breaking is called homolysis. The heterolysis is if bond breaks with both electrons of the shared pair remaining with one fragment, as in second and third cases.

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Other Arrow Symbols

For other purposes the Chemists also use arrow symbols, and it is essential to use them correctly.

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2. Reactive Intermediates

As shown above, generally the products of bond breaking are not stable, and cannot be isolated for prolonged study. These types of species are referred to as reactive intermediates and are believed to be transient intermediates in several reactions.

General structures and names of four such types of intermediates are given below.

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The pair of extensively used terms related to the Lewis acid-base notation has to be introduced here.

Nucleophile:   Atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile (or Lewis acid).

Electrophile:   Electron deficient atom, the ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.

After understanding of these definitions, it's clear that carbanions are nucleophiles and carbocations (called carbonium ions in the older literature) are electrophiles. The Carbenes have only a valence shell sextet of electrons and are therefore electron deficient. In this sense they are the electrophiles, but non-bonding electron pair also gives carbenes nucleophilic character. Like a rule, electrophilic character dominates carbene reactivity. Carbon radicals have seven valence electrons and may be considered the electron deficient; however, they do not in the general bond to nucleophilic electron pairs so their chemistry exhibits unique variations from that of conventional electrophiles. The Radical intermediates are often called free radicals.

The significance of nucleophile or electrophile terms appears from the fact that many organic reactions involve at some stage the bonding of a nucleophile to an electrophile, a process that usually leads to a stable intermediate or product. Reactions of this kind of are sometimes called ionic reactions, because the ionic products or reactants are often involved.

When considering the stereochemistry of reactions the shapes ideally assumed by these intermediates become important, where they play a role. Simple tetravalent compound like methane, CH4, has a tetrahedral configuration. Carbocations have only three bonds to charge bearing carbon, so it adopts planar trigonal configuration. In shape carbanions are pyramidal, but these species invert swiftly at room temperature, passing through the higher energy planar form in which the electron pair occupies a p-orbital. In the configuration radicals are intermediate, difference of energy between pyramidal and planar forms being very small. Since 3 points determine a plane, shape of the carbenes must be planar; though the valence electron distribution varies.

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