Skip to ContentGo to accessibility pageKeyboard shortcuts menu
OpenStax Logo
Organic Chemistry

6.6 Radical Reactions

Organic Chemistry6.6 Radical Reactions

6.6 • Radical Reactions

Radical reactions are much less common than polar reactions but are nevertheless important in some industrial processes and biological pathways. We’ll look at them in more detail in Sections 10.2 and 10.3 but will briefly see how they occur at this point.

A radical is highly reactive because it contains an atom with an odd number of electrons (usually seven) in its valence shell, rather than a noble-gas octet. The radical can achieve a valence-shell octet in several ways however. For instance, it might abstract an atom and one bonding electron from another reactant, leaving behind a new radical. The net result is a radical substitution reaction.

A radical reacts with single-bonded species AB to form single-bonded species Rad-A and a B radical.

Alternatively, a reactant radical might add to a double bond, taking one electron from the double bond and yielding a new radical. The net result is a radical addition reaction.

A radical species reacts with an alkene, adding that species at one carbon and leaving a radical at the other carbon.

An example of an industrially useful radical reaction is the reaction of chlorine with methane to yield chloromethane, which is used to manufacture the solvents dichloromethane (CH2Cl2) and chloroform (CHCl3). The process begins with irradiation of Cl2 with ultraviolet light to break the relatively weak ClCl bond of Cl2 and produce chlorine radicals (·Cl).

Lewis structure of chlorine molecule cleaving homolytically in the presence of light to form two chlorine atoms (radicals).

Chlorine radicals then react with methane by abstracting a hydrogen atom to give HCl and a methyl radical (·CH3) that reacts further with Cl2 to give chloromethane plus a new chlorine radical that cycles back and repeats the first step. Thus, once the sequence has started, it becomes a self-sustaining cycle of repeating steps (a) and (b), making the overall process a chain reaction.

Atomic (radical) chlorine reacts with methane to form hydrogen chloride and methyl radical. Chlorine molecule reacts with methyl radical to form chlorine atom (radical) and chloromethane.

As a biological example of a radical reaction, look at the synthesis of prostaglandins, a large class of molecules found in virtually all body tissues and fluids. A number of pharmaceuticals are based on or derived from prostaglandins, including medicines that induce labor during childbirth, reduce intraocular pressure in glaucoma, control bronchial asthma, and help treat congenital heart defects.

Prostaglandin biosynthesis is initiated by the abstraction of a hydrogen atom from arachidonic acid by an iron–oxygen radical, thereby generating a carbon radical in a substitution reaction. Don’t be intimidated by the size of the molecules; focus on the changes that occur in each step. (To help you do that, the unchanged part of the molecule is “ghosted,” with only the reactive part clearly visible.)

An oxygen radical species abstracts a hydrogen atom from arachidonic acid, leaving a carbon radical in the arachidonic acid.

Following the initial abstraction of a hydrogen atom, the carbon radical then reacts with O2 to give an oxygen radical, which reacts with a C═CC═C bond within the same molecule in an addition reaction. Several further transformations ultimately yield prostaglandin H2.

A radical addition reaction shows the formation of prostaglandin H 2 (P G H 2).
Problem 6-8
Radical chlorination of alkanes is not generally useful because mixtures of products often result when more than one kind of C−H bond is present in the substrate. Draw and name all monochloro substitution products C6H13Cl you might obtain by reaction of 2-methylpentane with Cl2.
Problem 6-9

Using curved fishhook arrows, propose a mechanism for the formation of the cyclopentane ring of prostaglandin H2.

Electron movement results in formation of cyclopentane ring in a prostaglandin H 2 radical.
Citation/Attribution

This book may not be used in the training of large language models or otherwise be ingested into large language models or generative AI offerings without OpenStax's permission.

Want to cite, share, or modify this book? This book uses the Creative Commons Attribution-NonCommercial-ShareAlike License and you must attribute OpenStax.

Attribution information
  • If you are redistributing all or part of this book in a print format, then you must include on every physical page the following attribution:
    Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter
  • If you are redistributing all or part of this book in a digital format, then you must include on every digital page view the following attribution:
    Access for free at https://openstax.org/books/organic-chemistry/pages/1-why-this-chapter
Citation information

© Aug 5, 2024 OpenStax. Textbook content produced by OpenStax is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike License . The OpenStax name, OpenStax logo, OpenStax book covers, OpenStax CNX name, and OpenStax CNX logo are not subject to the Creative Commons license and may not be reproduced without the prior and express written consent of Rice University.