Electron-withdrawing group

Electron-withdrawing groups exert an "inductive" or "electron-pulling" effect on covalent bonds. The strength of the electron-withdrawing group is inversely proportional to the pKa of the carboxylic acid.{{Cite web |date=2015-09-01 |title=20.4: Substituent Effects on Acidity |url=https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(Morsch_et_al.)/20%3A_Carboxylic_Acids_and_Nitriles/20.04%3A_Substituent_Effects_on_Acidity |access-date=2023-12-07 |website=Chemistry LibreTexts |language=en}}

:File:Carboxylic acid with different electron-withdrawing groups.png

The inductive effect is cumulative: trichloroacetic acid is 1000× stronger than chloroacetic acid.

:File:Carboxylic acid with multiple electron-withdrawing groups.png

:The impact of the EWG on pKa decreases with distances from the carboxylic group.

File:PKa depending on the length of a carboxylic chain.png

For benzoic acids, the effect is quantified by the Hammett equation:

:$\backslash log\; \backslash frac\{K\}\{K\_0\}\; =\; \backslash sigma\backslash rho$

where

:$\{K\}\_0$ = Reference constant

:$\backslash sigma$ = Substituent constant

:$\backslash rho$ = Reaction rate constant

Electron-withdrawing groups tend to lower Lewis basicity.{{Citation |last1=Caputo |first1=Christopher B. |title=Non-conventional Lewis Acids and Bases in Frustrated Lewis Pair Chemistry |date=2015 |url=http://dx.doi.org/10.1007/430_2015_177 |work=The Chemical Bond III |pages=1–29 |access-date=2023-11-05 |place=Cham |publisher=Springer International Publishing |isbn=978-3-319-35145-2 |last2=Stephan |first2=Douglas W.|doi=10.1007/430_2015_177 |url-access=subscription }} EWGs enhance the Lewis acidity, making compounds more reactive as Lewis acids. For example, fluorine is a stronger electron-withdrawing substituent than methyl, resulting in an increased Lewis acidity of boron trifluoride relative to trimethylborane.

This effect of EWG has been quantified in many of ways. The Tolman electronic parameter is determined by the frequency of a C-O vibrational mode (ν(CO)) of the coordination complexes [LNi(CO)₃] (L = Lewis base).{{Cite journal | last1 = Tolman | first1 = C. A. | title = Steric effects of phosphorus ligands in organometallic chemistry and homogeneous catalysis | journal = Chem. Rev. | volume = 77 | issue = 3 | pages = 313–348 | date = 1977 | doi = 10.1021/cr60307a002}}

File:Ortho meta para.png

Electrophilic aromatic substitution is famously affected by EWGs. The effect is transmitted by inductive and resonance effects.{{March6th}} Benzene with an EWG typically undergoes electrophilic substitution at meta positions. Overall the rates are diminished. thus EWGs are called deactivating.{{cn|date=March 2024}}

When it comes to nucleophilic substitution reactions, electron-withdrawing groups are more prone to nucleophilic substitution. For example, chlorodinitrobenzene is far more susceptible to reactions displacing chloride compared to chlorobenzene.{{cite journal |author=J. F. Bunnett, R. M. Conner |year=1960 |title=2,4-Dinitroiodobenzene |journal=Organic Syntheses |volume=40 |page=34 |doi=10.15227/orgsyn.040.0034}}

In the context of electron transfer, these groups enhance the oxidizing power tendency of the attached species. For example,  Tetracyanoethylene serves as an oxidant due to its four cyano substituents, which are electron-withdrawing.{{cite journal |last1=Connelly |first1=Neil G. |last2=Geiger |first2=William E. |year=1996 |title=Chemical Redox Agents for Organometallic Chemistry |journal=Chemical Reviews |volume=96 |issue=2 |pages=877–910 |doi=10.1021/cr940053x |pmid=11848774}}

Oxidants with EWGs are stronger than the parent compound. Acetylferrocenium is 300 mV more oxidizing than ferrocene.

Electron-withdrawing groups are the opposite effect of electron-donating groups (EDGs). Both describe functional groups, however, electron-withdrawing groups pull electron density away from a molecule, whereas EDGs push electron density onto a substituent.{{Cite web |last=Hunt |first=Ian |date=2023-10-22 |title=Chapter 12: Reactions of Arenes. Electrophilic Aromatic Substitution |url=https://www.chem.ucalgary.ca/courses/350/Carey5th/Ch12/ch12-8b.html}}

• Electron-donating group

Category:Physical organic chemistry

Category:Chemical bonding