Organic Chemistry Text Book (CHEM 3401 and 3402)
2.5 Curved Arrows
The Arrow Notation in Mechanisms
Since chemical reactions involve the breaking and making of bonds, a consideration of the movement of bonding ( and non-bonding ) valence shell electrons is essential to this understanding. It is now common practice to show the movement of electrons with curved arrows, and a sequence of equations depicting the consequences of such electron shifts is termed a mechanism. In general, two kinds of curved arrows are used in drawing mechanisms:
| A full head on the arrow indicates the movement or shift of an electron pair: |  |
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| A partial head (fishhook) on the arrow indicates the shift of a single electron: |  |
The use of these symbols in bond-breaking and bond-making reactions is illustrated below. If a covalent single bond is broken so that one electron of the shared pair remains with each fragment, as in the first example, this bond-breaking is called homolysis. If the bond breaks with both electrons of the shared pair remaining with one fragment, as in the second and third examples, this is called heterolysis.
| Bond-Breaking | Bond-Making | |
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Other Arrow Symbols
Chemists also use arrow symbols for other purposes, and it is essential to use them correctly.
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The Reaction Arrow |
The Equilibrium Arrow |
The Resonance Arrow |
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The following equations illustrate the proper use of these symbols:
