Electron-withdrawing group

In chemistry, an electron-withdrawing group (EWG) is a substituent that has some of the following kinetic and thermodynamic implications:

• with regards to electron transfer, electron-withdrawing groups enhance the oxidizing power tendency of the appended species. Tetracyanoethylene is an oxidant because the alkene is appended to four cyano substituents, which are electron-withdrawing.^[1]
• with regards to acid-base reactions, acids with electron-withdrawing groups species have low acid dissociation constants. For EWG's attached to benzene, this effect is described by the Hammett equation, which allows EWGs to be discussed quantitatively.^[2]
• with regards to nucleophilic substitution reactions, electron-withdrawing groups are susceptible to attack by weak nucleophiles. For example, compared to chlorobenzene, chlorodinitrobenzene is susceptible to reactions that displace chloride.^[3]
• electron-withdrawing substituents enhance the Lewis acidity. Relative to methyl, fluorine is a strong EWG. It follows that boron trifluoride is a stronger Lewis acid than is trimethylborane.

References

1. Connelly, Neil G.; Geiger, William E. (1996). "Chemical Redox Agents for Organometallic Chemistry". Chemical Reviews. 96 (2): 877–910. doi:10.1021/cr940053x. PMID 11848774.
2. Smith, Michael B.; March, Jerry (2007), Advanced Organic Chemistry: Reactions, Mechanisms, and Structure (6th ed.), New York: Wiley-Interscience, ISBN 978-0-471-72091-1
3. J. F. Bunnett, R. M. Conner (1960). "2,4-Dinitroiodobenzene". Organic Syntheses. 40: 34. doi:10.15227/orgsyn.040.0034.