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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
page 2 of 2
The curved arrow (see Figure 3) formalism is such an important tool that entire books have been devoted to it. The point of the curved, double-headed arrow is to show the movement of an electron pair. They start at the tail and end up at the head. This is important for following how mechanisms work.
The Curved Arrow (single headed)
The single-headed (or “single-barbed”) arrow, shown in Figure 4, depicts the movement of a single electron. It is useful in discussions of radical chemistry mechanisms. Also, it is identical in all other respects to the double-headed arrow.
The Broken Arrow
This arrow (see Figure 5) is used to show reactions that don’t work. For example, fluorine is a bad leaving group in nucleophilic substitutions and quinine can definitely not be synthesized from the oxidation of aniline.
The Retrosynthesis Arrow
The open arrow here (see Figure 6) doesn’t actually show a “reaction,” per se, but instead more of a mental exercise. The retrosynthetic arrow is meant to depict the process of breaking down a complex molecule to simpler starting materials. This is useful as a planning device to highlight a key strategy used for building a molecule.
* Excerpt is taken with permission from Master Organic Chemistry's "The 8 Types of Arrows in Organic Chemistry, Explained."