Posted by This tutorial introduces radicals, and explains how single-headed curved arrows are used to draw radical mechanisms.
Introduction
Radicals are chemical species which have an unpaired valence electron on an atom. This is different from what we’ve seen for polar reactions, where electrons are always paired, either in lone pairs, or in covalent bonds. The chemical structure of a radical can be drawn by putting a single dot on the atom which has the unpaired electron. In intro organic chemistry, you’ll mainly see radicals on carbons and halogens (e.g., bromine, chlorine):
Notice that the unpaired electron on carbon and bromine in these examples does not affect the formal charge of these atoms. When a carbon atom has four bonds, it has a neutral charge – one electron from each bond belongs to that carbon. In a carbon radical which has three bonds, the unpaired electron belongs entirely to the carbon, and so the carbon still has four electrons. You can find background on formal charges in Formal Charges and Mechanisms (Part 1).
In Drawing Curved Arrows (Part 1), polar reactions and radical reactions were introduced. While the mechanisms of polar reactions involve the movement of pairs of electrons, the mechanisms of radical reactions involve the movement of individual electrons.
To draw the mechanism of a polar reaction, we use double-headed curved arrows, and each arrow represents the movement of pairs of electrons. As radical reactions involve the movement of individual electrons, we use single-headed curved arrows, and each arrow represents the movement of a single electron.
Drawing Curved Arrows for Radical Mechanisms
Like with polar mechanisms, there are only a few ways that curved arrows can be drawn for radical mechanisms. It’s worth revisiting how to draw curved arrows for polar mechanisms – in Drawing Curved Arrows (Part 1) – before reading on about radical mechanisms.
Curved arrows can start from a σ bond, showing the cleavage of that bond. Both of the atoms which were bonded together take one electron, and so both atoms are now radicals. This is known as homolytic bond cleavage, and is a major mechanism for making radicals in organic chemistry.
If a curved arrow starts from a radical, it can:
- Form a covalent bond with another radical. This is shown by drawing two curved arrows; one arrow starts from each radical, and both arrows end where the bond is formed.
- Form a covalent bond with another atom (which is not a radical), breaking a σ or π bond on that atom, and putting a new radical on the other end of the broken bond.
Notice that if a radical reacts with another radical, the product is not a radical. However, if a radical reacts with something that is not a radical, one of the products will still be a radical.
Summary
Radicals are chemical species which contain an unpaired electron. Radical mechanisms involve the movement of individual electrons, and are drawn using single-headed curved arrows.
Radicals can be made by homolytic bond cleavage. Radicals can make σ bonds with other radicals, and can make σ bonds with atoms in other molecules (resulting in cleavage of a σ or π bond on that atom).
Related
Drawing Curved Arrows (Part 1) – an introduction to drawing curved arrows to represent polar reaction mechanisms.
Periodic Chemistry 