Electron Movement

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Chapter: Organic Chemistry : Curved-Arrow Notation

In a very simple sense, most organic reactions are accomplished merely by the movements of electrons.


ELECTRON MOVEMENT

In a very simple sense, most organic reactions are accomplished merely by the movements of electrons. Since molecules are composed of atoms held together by bonds and covalent bonds are merely shared pairs of electrons, the conversion of one molecule to another by changes in chemical bonds between reactants and products can be described simply as changes in electron pairs that are shared between the various nuclei in the reactants. While this is a gross oversimplifica-tion, it nevertheless provides us with a very important tool with which to keep track of bonding changes that occur during the transformation of one molecule into another.

We keep track of electron pairs by noting changes in their location in molecules by means of curved arrows. The curved arrow depicts movement of an electron pair from the tail of the arrow to the head of the arrow. For example, in the reaction of a proton with water to produce the hydronium ion, a new bond is formed from oxygen to hydrogen. The electrons in that bond start out as a lone pair on oxygen and are donated to and shared with the proton. We can easily show this electron movement with a curved arrow from the lone pair on oxygen to the proton.


In a similar sense the Lewis acid – base reaction between ammonia and boron trifluoride can be depicted by a curved arrow from the lone pair of electrons on nitrogen to the boron atom. This electron movement creates a new bond between nitrogen and boron, but the curved-arrow notation clearly illustrates that the electron pair of that bond is supplied by the nitrogen. An additional consequence is that the nitrogen atom gains a formal positive charge and the boron atom gains a formal negative charge.


Before further applications of electron movement using curved-arrow notation are presented, it is important to recognize just why it is such a useful tool for describing reactions. First it permits us to keep track of valence shell electrons during a chemical reaction and thus serves as a method for electronic bookkeep-ing. Second it shows how changes in bonding result from changes in electron distribution. Third it can show likely mechanisms for chemical reactions in terms of the breaking and making of chemical bonds.

For curved-arrow notation to be used correctly, however, the structural and bonding principles which we have already learned must be adhered to. Thus donor – acceptor properties, oxidation states, hybridization and the octet structure of atoms, normal valences, formal charges, reactive intermediates, and so on, must all be taken into account as curved-arrow notation is used to track changes in electron distribution that occur during chemical reactions. Within the frame-work of these principles, however, curved-arrow notation can be a powerful and effective tool for depicting bonding changes during chemical reactions. To do this, it is necessary to first understand the general processes of bond formation and bond cleavage that are commonly encountered.

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