Pointers on Free Radical Reactions

by James

Here’s some pointers about free radical reactions

  • A free radical is a species with a single unpaired electron.
  • Free radical reactions are commonly initiated by light (hv), which promotes a single electron into an anti-bonding orbital, leading to homolytic cleavage.
  • “Homolytic” dissociation means “same-breaking”. It’s breaking a bond and each atom gets the same number of electrons (1).
  •  Since they are lacking in a single electron, you can consider free radicals “electron-deficient”. So free radicals are stabilized by neighbors that can donate electrons, such as alkyl groups.
  • Order of free radical stability: tertiary > secondary > primary
  • Note that a less reactive radical like Br• can only “afford” to break the weakest C-H bonds (tertiary, benzylic and allylic). We say it’s “selective”, for tertiary C-H bonds, but it’s similar to the way that most people are “selective” for buying Hyundais versus BMW’s: you can only buy what you can afford.

There are 3 steps in every free-radical reaction. Here it’s outlined for free radical chlorination of alkanes.

  • The first step: initiation, where the number of free radicals increases. This is where light (or heat, or peroxides in some cases) causes the homolytic dissociation. Note that at any given time the concentration of free radicals are small (this reaction doesn’t go to completion).
  • The second steps (there are two!).
  • Propagation step 1, where the radical removes hydrogen from the alkane giving the free radical. Note that the number of free radicals on each side of the equation is the same.
  • Propagation step 2, where the carbon radical then attacks Cl2, forming C-Cl and giving a new Cl radical.

A common mistake is to draw the second propagation step to form the C-Cl bond as between a carbon radical and a Cl radical. This is actually the last step, termination! Note that here the number of free radicals goes down from 2 in the starting material to 0 in the product.

Thanks for reading! James

PS – for a video I made on free radical chlorination (as well as the page in the Reaction Guide) – see here