Reaction Mechanisms

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A reaction mechanism is an detailed explanation of "how" a chemical reaction is thought to occur. While mechanisms can be disproven, it is very difficult to prove that a mechanism is correct. Even so, a basic understanding of a few of the more common mechanisms allows one to understand (and in some cases predict) a wide range of chemical reactions.

Curved Arrow Formalism

The curved arrow formalism is a means of illustrating reaction mechanisms. The key feature of this approach is an emphasis on electron flow. The course of a chemical reaction is shown by arrows that start on an electron pair (either a lone pair or a bonding pair) and end on the atom or bond where the electrons end up in the product. Put another way, the tail of the arrow starts on the electron pair that reacts and the arrow head ends on the atom or bond to show the formation of either a new lone pair or a new chemical (covalent) bond. This formalism is used to illustrate each of the mechanisms shown below.

Curved Arrow Formalism

Illustrative Reaction Mechanisms

Alkene Addition - Hydrohalogenation

Dehydration of Alcohols

Hemiacetal Formation

Esterification of Carboxylic Acids

Aldol Condensation