Chapter 6: Reactions of Alkenes: Addition Reactions Ch 6 contents
Radicals (review)
A radical is a species that contains unpaired electrons.
Typically formed by a homolytic bond cleavage as represented by the fishhook curly arrows:
fishhook arrows showing homolytic cleavage of Br2
Stability:
The general stability order of simple alkyl radicals is: (most stable) 3o > 2o > 1o > methyl (least stable)

radical stability order

This is because alkyl groups are weakly electron donating due to hyperconjugation and inductive effects.
Note that this is the same order as for carbocations. Resonance effects can further stabilize radicals when present.

Structure:
 

Alkyl radicals are sp2 hybridised, planar systems at the radical C centre. 
The p-orbital that is not utilised in the hybrids contains the single electron. 		 radical geometry

Reactivity:
As they have an incomplete octet, radicals are excellent electrophiles and react readily with nucleophiles.
Alternatively, loss of H. can generate a p bond

Rearrangements:
Unlike carbocations, radicals do not tend to undergo rearrangements

Reactions involving radicals: