Electron-withdrawing group
An electron-withdrawing group (EWG) is a group or an atom which attracts electron density towards itself and away from other adjacent atoms.[1] An electron-withdrawing substituent has the following chemical 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.[2]
- With regards to acid-base reactions, acids with electron-withdrawing groups species have high acid dissociation constants. For EWG's attached to benzoic acids, this effect is described by the Hammett equation, which allows EWGs to be discussed quantitatively.[1]
- 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.
- Electron-withdrawing substituents often lower the Lewis basicity. [4]
See also
References
- 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
- 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.
- J. F. Bunnett, R. M. Conner (1960). "2,4-Dinitroiodobenzene". Organic Syntheses. 40: 34. doi:10.15227/orgsyn.040.0034.
- "6.4.2: All other things being equal, electron withdrawing groups tend to make Lewis acids stronger and bases weaker while electron donating groups tend to make Lewis bases stronger and acids weaker". Chemistry LibreTexts. 2019-07-05. Retrieved 2023-10-04.
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