User:MarissaGrosz/Electron-withdrawing group

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An electron-withdrawing group (EWG) is a group or atom that has the ability to draw electron density toward itself and away from other adjacent atoms. This electron density transfer is often done by resonance or inductive effects. Electron-withdrawing groups have significant impacts on various chemical processes. 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 attachment to four cyano substituents, which are electron-withdrawing groups. In acid-base reactions, acids with electron-withdrawing groups exhibit high acid dissociation constants, and this effect can be quantitatively described using the Hammett equation, particularly in the case of benzoic acids. When it comes to nucleophilic substitution reactions, electron-withdrawing groups are more prone to attack by weak nucleophiles. For example, chlorodinitrobenzene is susceptible to reactions displacing chloride compared to chlorobenzene. Additionally, electron-withdrawing substituents 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. Electron-withdrawing groups also tend to reduce Lewis basicity.

In polymer solar cells, the incorporation of the electron-withdrawing group DPQ, transformed into DPQCF3F, enhances open-circuit voltage, linking with electron-donating compounds to create quinoxaline-based polymers. Meanwhile, adding an electron-withdrawing group to indoline dyes improves their oxidation potential, enhancing light-to-electricity conversion and boosting solar cell performance.

Electron-withdrawing groups are very different than electron-donating groups. Both are functional groups, however, electron-withdrawing groups have the ability to pull electron density away from a molecule, and electron-donating groups have the ability to push electron density onto a molecule.

Applications

Photovoltaic applications

In the realm of polymer solar cells, the incorporation of an electron-withdrawing group, 2,3-diphenylquinoxaline (DPQ), takes on a significant role. DPQ is transformed into its electron-withdrawing derivative, DPQCF₃F, containing multiple CF₃ and F groups. This modified molecule is then linked with the electron-donating compounds, indacenodithiophene (IDT) and indacenodithieno [3,2-b]thiophene (IDTT), to create quinoxaline-based conjugated polymers. The electron-withdrawing nature of DPQCF₃F elevates the open-circuit voltage within polymer solar cells. The synthesis of these conjugated polymers is achieved through a Stille-coupling reaction.

Adding an electron-withdrawing group to indoline dyes boosts their oxidation potential (E_ox). This increase improves the efficiency of converting light to electricity (IPCE), resulting in higher current output (J_sc). This enhancement is key for better solar cell performance.

Effect on other compounds

Effects on Acidity

Fluoroacetate anion stabilized by electron withdrawing inductive effect of fluorine

The presence of an electron-withdrawing group (EWG), exerts an "inductive" or "electron-pulling" effect on covalent bonds. In the fluoroacetate anion, the electronegative fluorine atom pulls electron density towards itself, leading to a partial positive charge on the carbon atom. This positive charge stabilizes the carboxylate anion, increasing the acidity of the corresponding carboxylic acid. In this context, fluorine acts as an electron-withdrawing group.

The strength of the electron-withdrawing group is inversely proportional to the pKa of the carboxylic acid.

The inductive effect is cumulative.

The acidity decreases with longer distances and more sigma bonds between electron-withdrawing and carboxylic groups.

Effect on a Benzene Ring

Substituents on a benzene ring impact the placement of additional substituents in electrophilic aromatic substitution. This is determined by inductive and resonance effects, which are related to electronegativity and can result in either meta or ortho-para directing properties.

As resonance forms for ortho-para substitutions are less stable, the major product is meta.

The rate of this second substitution is lower than that of the original benzene, thus EWGs are called deactivating.

Comparison with electron-donating groups

An electron-donating group (EDG) is a functional group that donates electrons to a molecule, enhancing its reactivity and nucleophilicity. When an EDG is present, it can influence a molecule in a couple of ways. It boosts the molecule's acidity by stabilizing the negative charge on its conjugate base through electron delocalization. Additionally, EDGs can increase the molecule's basicity by offering lone pairs of electrons capable of accepting protons.

On the other hand, an electron-withdrawing group (EWG), is a term used to describe a functional group that has the ability to pull electrons away from a molecule. This action makes the molecule less reactive and reduces its nucleophilicity. When an EWG is present in a molecule, it can reduce the molecule's acidity by causing instability in the conjugate base due to the dispersal of negative charge. Additionally, EWGs can lower the molecule's basicity by drawing away lone pairs of electrons that would otherwise be available for accepting protons.