Substituents, Effect of Substituents on Reactivity and Orientation of Mono Substituted Benzene Compounds Towards Electrophilic Substitution Reaction : Pharmaguideline

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Substituents, Effect of Substituents on Reactivity and Orientation of Mono Substituted Benzene Compounds Towards Electrophilic Substitution Reaction

The hydrogen atoms in benzene are all the same. A monosubstituted derivative of benzene can be made by replacing any hydrogen atom with a substituent. Di and poly substituted derivatives result when two or three substituents are replaced. Benzene can become ortho, meta, or para when it is di-substituted into its derivative. Depending on the substituents already present in the ring, the position of the incoming group is determined. In order to direct an incoming group, a group within the ring must possess directive influence. Benzenes already present with groups not only decide where further substitution takes place, but also affects the nucleus' reactivity to further electrophilic substitution reactions.

Effects of Substituents on Orientation

Monosubstituted Benzene can have a second substituent in any of the remaining five positions since it already contains one substituent. The ortho product is given by positions 2 and 6, while the meta product is given by positions 3 and 5. It’s position 4 which is unique and gives para product. Within each of the already present substituents, there are two ortho and two meta substitutions. Groups can be classified into two classes based on their directive influence:

Ortho-para directing groups - The incoming groups are directed to these positions by these groups. These include Alkyl(R), Halogens (-cl, -Br F I), Phenyl(-C6H6), Hydroxyl(-OH), and Amino(-NH2), etc.

Meta-directing groups - Incoming groups are directed to the meta position by this group. The following are examples: trialkyl ammonium ions (N+R3), cyano(-CN), nitro(NO2), aldehydes (-CHO), and carboxylic acids (-COOH).

Multilinked substituents, whether they contain two or more bonds, are called meta-directing substituents, while those with just one or two bonds but containing one of more electron pairs on the atom are called ortho- and para-directing substitutes.

Effects of substituents on reactivity - By interacting with the benzene π - electron, these electrons increase the electron density of ortho and para directing groups (with the exception of alkyl and phenyl). These activated rings can now undergo electrophilic substitution. Therefore, all ortho and para directing groups accept halogens as a signal.

Benzene ring groups may act as meta-directing groups due to the multiple bonds within the ring. Benzene's electron density decreases due to multiple bonds, which prevents it from undergoing further electrophilic substitution. The fact that they activate Benzene makes them called deactivating groups. Activating substituents are substances that activate the benzene ring for further electrophilic substitution, while deactivating substrates deactivate benzene rings for further substitution.
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