Editing Alkene (section)

=== Allylic substitution ===
The presence of a C=C π bond in unsaturated hydrocarbons weakens the dissociation energy of the [[allylic]] C−H bonds. Thus, these groupings are susceptible to [[free radical substitution]] at these C-H sites as well as addition reactions at the C=C site.  In the presence of [[radical initiator]]s, allylic C-H bonds can be halogenated.<ref>{{cite journal |doi=10.15227/orgsyn.073.0240 |title=1,3,5-Cyclooctatriene |journal=Organic Syntheses |date=1996 |volume=73 |page=240|first1=Masaji|last1=Oda|first2=Takeshi|last2=Kawase|first3=Hiroyuki|last3= Kurata }}</ref> The presence of two C=C bonds flanking one methylene, i.e., doubly allylic, results in particularly weak HC-H bonds.  The high reactivity of these situations is the basis for certain free radical reactions, manifested in the chemistry of [[drying oil]]s.