Integrated SequencePhysics Chemistry Organic Biology

Web Resources



  click if a link is broken



Special points of emphasis

Reactions of Alkanes

Reactions of Alkenes

Reactions of Conjugated Species

Halogenation (chlorination or bromination) of alkanes occurs by means of a free radical chain mechanism. As we discussed before, an important consideration of regioselectivity in this reaction is that the halide radical will preferentially attack to yield the most substituted alkyl radical. More highly substituted radicals are more stable. This is also true for free radical halogenation of alkenes and aromatic compounds with aliphatic side chains. Free radical halogenation prefers the allylic position, the position next to a double bond, and the benzyllic position, the first carbon in a side-chain off a benzene ring because both allow resonance of the radical, stabilizing it.

An important consideration to remember with free radical halogenation of alkenes at the allylic position is that this reaction may be competing with electrophilic addition of halogen directly to the double bond. Halogen molecules will preferentially react by electrophilic addition, except at high temperatures, where the free radical halogenation occurs with substitution for an allylic hydrogen.








The WikiPremed MCAT Course is a free comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum.

Please read our policies on privacy and shipping & returns.  Contact Us.
MCAT is a registered trademark of the Association of American Medical Colleges, which does not endorse the WikiPremed Course.


Creative Commons License
The work of WikiPremed is published under a Creative Commons Attribution Share Alike 3.0 License. There are elements of work here, such as a subset of the images in the archive from WikiPedia, that originated as GNU General Public License works, so take care to follow the unique stipulations of that license in printed reproductions. You can use the resources here for commercial or non-commercial purposes, but please give attribution and a link to the production credits and edit history of the resource. For the works here which began as my individual work, please attribute "John Wetzel, an author at wikipremed.com".