Tree

"Pushing Electrons;"
Representing Reaction Mechanisms

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The mechanism of a reaction is a step-by-step description of how that reaction is thought to occur. In microscopic detail, chemical reactions involve multiple steps, including encounter equilibria, bond-making and bond-breaking, diffusion steps and re-hybridization ("heavy atom rearrangement"). In undergraduate organic chemistry, mechanisms are generally simplified to include only bond-making and breaking steps and "curved arrows" are generally utilized to indicate the flow of electrons within these steps. Although these simplified mechanisms are only approximations, they are useful since they allow a clearer understanding of how a given reaction proceeds, and allow reactions to be organized by mechanisms, making the study of organic chemistry simpler and more logical.
The process of "pushing electrons" using curved arrows requires a few simple conventions:
    • Movement of a pair of electrons is shown using an arrow with a standard double-barbed arrowhead.
    • Movement of a single electron (a radical reactions) is shown using an arrow with only a single barb.
    • The arrow is drawn from the middle of the electron pair (or single electron) which is moving, to the atom accepting the electron(s).
Thus, homolytic cleavage of a carbon-bromine bond (a radical process) is shown below in (a) and the heterolytic cleavage is shown in (b).
 
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The heterolytic cleavage (b) leads to the formation of a pair of ions, which is indicated in the mechanism by the positive and negative charges. Two examples of this type of heterolytic cleavage reaction which are discussed in elementary organic chemistry are the E1 and SN1 reaction mechanisms. Both of these reactions are unimolecular (the "1") meaning that only one of the reactant molecules is present in the rate-limiting transition state. The prefixes on these general mechanisms refer to elimination ("E") and substitution, nucleophilic ("SN")....