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Comparatively Speaking: Arrow Symbols in Organic Chemistry*
By: Anthony J. O'Lenick Jr., Siltech LLC; and James Ashenhurst, PhD,
Posted: September 25, 2012
There are eight different types of arrows encountered in organic chemistry. Here is a little guide to them.
The Forward Arrow
This arrow, shown in Figure 1, is otherwise known as the “reaction arrow.” The purpose of this arrow is to show action. “BH3, then NaOH/H2O2″ might not seem like “action,” but alkene's double bond is being ripped asunder in order to form new bonds to boron and hydrogen. With the addition of H2O2, the bond to boron is replaced with one to oxygen. That is a eventful day in the life of an alkene. There isn’t any hard and fast rule about what is supposed to go above or below the arrow, although reagents tend to go above and solvents tend to go below. You’ll often see a sequence of reactions placed over the arrow and numbered “1) , 2), 3)," etc. These represent individual steps that could often be shown with individual arrows of their own, but they are placed in series here over the arrow to save space.
The Equilibrium Arrow
This arrow, shown in Figure 1, shows a reaction that is reversible, usually in the context where the reversibility is being highlighted (such as in a reaction mechanism). To further highlight the position of an equilibrium, occasionally one of the arrows is longer than the other, showing that the equilibrium favors the starting materials or products.
The Resonance arrow.
Not to be confused with the equilibrium arrow, this double-headed arrow (see Figure 2) shows two (or more) species that are resonance structures of each other. They differ in the arrangements of their electrons and nothing else. Although it’s a separate discussion, it’s important to note that the molecule *does not* shuffle back and forth between these forms, but instead the “true picture” of the molecule is a combination or hybrid of these structures.
The Dashed Arrow
This arrow, shown in Figure 2, is often used to show a speculative or theoretical transformation, where conditions might have yet to be discovered. Alternatively, in a test situation, it’s a way of visually depicting the question, “How would you do this?”