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Physical organic chemists like Christopher Ingold studied electrophilic aromatic substitution processes kinetically in the mid-twentieth century. Since benzene works as a nucleophile during electrophilic substitution, substituents that increase benzene’s electron density may speed up the reaction. The process can be slowed down by substituents that render the benzene moor electron-poor.
The orientation effect is defined as a direction that is determined by the type of the first substituent. The already present substituent can either activate or deactivate the benzene ring, increasing or decreasing the rate of subsequent substitution. This paper will discuss the directive effect of benzene and mono substitution along with a group of para directing.
What is the directive effect?
Ortho or para directing groups are electron-releasing groups that guide the entering group to ortho and para locations, where the electron density is higher. At certain positions, the aromatic ring becomes reactive.
Whenever mono substituted benzene is electrophilically attacked, the rate of the reaction and attack site differs depending on the functional group connected to it. Some groups are called activating groups because they enhance the reactivity of the benzene ring, while others are regarded as deactivating groups because they lower the reactivity.
We classify these groups into two categories based on how they influence the entering electrophile’s attack direction. Ortho-para directors enhance electron density in the “ortho” or “para” positions, whereas meta directors enhance electron density in the “meta” region.
What is the directive effect of benzene?
There are two things to keep in mind when substituted benzene compounds go through electrophilic substitution processes like those described above.
The first is the relative reactivity of chemical benzene. Experiments have shown that the substitutions on a benzene ring affect reactivity significantly. The frequency of electrophilic aromatic substitution is increased by ten thousand when hydroxy and methoxy substituents are added.
On the other hand, a nitro substituent reduces the ring’s sensitivity by about a million times. As assessed by molecule dipole moments, the electron giving or electron removing the effect of the substituents may be attributed to the stimulation or inactivation of the benzene ring for electrophilic substitution.
Each of the two substituent groups on a benzene ring affects subsequent substituents. The aggregate of the individual impacts of these substituents can more or less predict ring activation or deactivation. The direction of the established groups and their unique directive effect of benzene determine where a new substituent is inserted.
The directive effect of benzene around the benzene ring impacts the regiochemistry of the action and the reaction speed. They have complete control over the placement of the new substituent in the product. Remember that a new substitution can bind to the benzene ring in three locations, depending on the initial substituent. Substitution can happen at five different places all around the ring, but symmetry links two of them together. The connections meta–, ortho–, and para– can define isomerism in disubstituted benzenes. The first substituent can be found in two ortho– locations and two meta–positions.
What is known as monosubstituted benzene?
The benzene ring has double-bonded carbon atoms. Therefore, they can have three substitutions with another ion on alternate carbons. When the benzene ring has only one substitution on any of the six C-atoms, we regard it as the monosubstituted benzene.
The combination of two effects can explain the influence of a substituent just on the reaction of a benzene ring.
The first one is the substituent’s inductive impact. Except for metals and carbon, most elements have a far higher electronegativity than hydrogen. As a result, substituents containing nitrogen, oxygen, or halogen atoms that establish sigma bonds to an aromatic ring deactivate the ring by inductive electron removal.
The second impact occurs when mono substituted benzene is conjugated to the aromatic ring. Unlike the inductive shift, this conjugative interaction promotes electron pair transfer or withdrawal to or from the benzene ring. Suppose the element connected to the ring has one or even more non-bonding outer shell electrons pairs, such as nitrogen, oxygen, and halogens. In that case, electrons can flow into the aromatic ring via p– conjugation or resonance. The voltage concentration in the benzene ring is largest at ortho or para directing groups to the substituent in both cases.
Electron transfer by resonance leads to the inductive effect of nitrogen and oxide activating groups, and these compounds have extraordinary reactivity in electrophilic processes. Even though halogen atoms possess non-bonding valence pairs of electrons that participate in p-conjugation, their powerful inductive impact dominates, making molecules like chlorobenzene less reactive than benzene.
Conclusion
Electron releasing groups called ortho and para guidance groups steer the entering unit to ortho and para places with higher electron density. Electrophilic substitution happens largely at these sites as a result. The aromatic ring gets reactive at particular points. The meta state has a reduced electron density, making it less reactive. But on the other hand, electron-withdrawing compounds are meta-directing.
