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Exploring the secret of reaction rates: How do electron attracting groups affect the reaction of aromatic rings?
In organic chemistry, the electrophilic Aromatic Substitution (EAS) reaction of aromatic rings is a very important process. In this process, the existing substituents on the aromatic ring will significantly affect the reaction rate and the regioselectivity of the product. This article will delve into how electron donating groups (EDGs) and electron withdrawing groups (EWGs) affect the electron density of aromatic rings, thereby changing the kinetics and product distribution of reactions.
The electron donating group makes the aromatic ring more nucleophilic, thus promoting the electronucleophilic substitution reaction, while the electron withdrawing group does the opposite and reduces the reactivity of the aromatic ring.
Electron donating groups, also known as electron releasing groups (ERGs), donate electron density to aromatic rings through resonance effects or induction effects. These groups make the π system of the aromatic ring more nucleophilic, thereby increasing its likelihood of participating in electronucleophilic substitution reactions. At this time, the aromatic ring is more likely to react with the electrophile, so this type of group is also called an activated group. In addition, these groups often direct substitution reactions at the ortho and para positions. In 1892, the Crum Brown–Gibson rule first described these selectivities, that is, the effect of substitution of EDGs and EWGs on aromatic rings.
EDGs usually promote electronucleophilic substitution reactions at the ortho and para positions, while EWGs tend to direct reactions toward the meta position.
In contrast, electron attracting groups remove electron density from the π system of aromatic rings, reducing their reactivity. Under the influence of these groups, the nucleophilicity of the aromatic ring is significantly reduced, weakening its ability to participate in electronucleophilic substitution reactions. Therefore, such groups are called deactivating groups. Especially under the influence of EWGs, substitution reactions tend to be concentrated at the meta position, while the chances of reaction with the ortho and para positions are significantly reduced. These electron-attracting groups can be divided into weak deactivating groups and strong deactivating groups. Weak deactivating groups can sometimes also cause reactions at the ortho and para positions, but significantly reduce the reactivity relative to the meta position.
Strong deactivating groups often preferentially direct meta reactions rather than ortho or para reactions.
Discussing activating groups, we found that most activating groups fall into the category of resonance electron donors (+M). Although many of these groups also have a deactivating effect (-I) through induction to some extent, the resonance effect of the electron donation is generally stronger. This phenomenon does not apply to certain halogens (such as chlorine, bromine and iodine), whose resonance effects significantly affect the chemical reactivity of aromatic rings. For example, although fluorine has a deactivating effect, its reaction rate at the para position often exceeds 1, which makes it regarded as an activating group at this position. This purely kinetic consideration will help us understand the effect of various substituents on aromatic rings undergoing EAS reactions.
Fluorine is an exception, as it reacts at the para position at a higher rate than other substituents, causing it to show an activating effect.
Deactivating groups, such as nitroso groups, sulfate esters and different types of carboxylic acids, will produce strong electron attraction on carboxyl groups such as oxazolone. These structures are caused by electropositive atoms attached directly to the aromatic ring. Although these groups together contribute to the resonance effect, they strongly slow down the reaction rate due to their electronegative shift in the aromatic ring. Furthermore, the participation of these groups makes the aromatic ring extremely electron-poor.
The reaction rate of aromatic rings containing electron-withdrawing groups is much lower than that of aromatic rings without these groups.
In comparison, although structures with amino groups, alcohol groups and ether groups (such as benzoin) have some induction effect (-I), the resonance effect (+M) often overwhelms this effect, resulting in They are still considered electron donating groups (EDGs) and are highly reactive in the ortho and para positions. Especially under basic conditions, the ethoxylation reaction of phenols is significantly accelerated because the mononegative oxygen causes the molecule to donate more electrons to the reaction.
However, the influence of different substituents on the reaction site is not unidirectional, and the interaction between them often triggers changes in reaction selectivity. When two or more substituents are already present on an aromatic ring, the position of the third substituent can often be predicted. Their presence will make the compound have strong symmetry or strengthen the effect of certain substituents, further affecting the reactivity of the aromatic ring.
As we further understand the fundamentals of these reactions, we are left wondering how future organic chemistry reactions will continue to be affected by these electronic effects, and how exploring novel substituents may redefine our understanding of chemical reactions. ?
