Directive Influence of Functional Group in Mono Substituted Benzene

Mono-substituted benzene happens when one position in the ring is substituted by an atom, or a group of atoms, it is different from disubstituted benzene, where two positions are replaced. Benzene is an organic chemical compound which was first identified by English Michael Faraday. The word is derived from the term “gum benzoin" which indicates benzoin resin. It is basically seen as a carcinogen which poses a threat to health. Let us take a look at benzene, its functional groups, and their effects in order to get a better idea about the topic.

Key terms: Mono-substituted benzene, benzene ring, ortho and para-directing activators, Meta directing groups.

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What is Mono-substituted Benzene?

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Benzene, C₆H₆, is a popular compound in Chemistry, serving multiple uses in different fields. Mono-substituted benzene has been defined as, the case, where one of the pistons on the benzene ring is replaced or substituted with a distinct atom or a group of atoms, we call it as a monosubstituted benzene. When such forms of benzene face an electrophilic attack, in some cases reactivity of the benzene ring increases, which are called as activating groups. On the other hand, reactivity of benzene rings decreases in other cases, which are called deactivating groups. 

In the synthesis of mono-substituted benzene, important factors like functional group transformation, and synthetic planning have to be kept in mind. Considering the first factor, there are certain functional groups which can be added to an aromatic ring, following the process of electrophilic substitution. Benzene gives rise to new chemical compounds which are formed, after they have replaced one or more of the compounds hydrogen atoms with a separate functional group. Such benzene derivatives can be in the form of phenol, and toluene. 

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Effects of Substituents

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Orientation effect:

All substituents relating to benzene, bring about changes in the normal flow of the reaction, which is termed as the orientation effect. The three popular isomers, or disubstituted products ortho, para, and meta are present, which don’t have the same composition. It can also be said that the magnitude of them is different, and that all three are not formed in equal amounts. The substituents which are present on the benzene ring beforehand, their nature will define the position of the second substitution. All groups can be included in the two stages of ortho-para, and meta directing groups. 

Reactivity:

The substituents are also responsible for handling and determining the reactivity of the aromatic rings. Some unique substituents help in the activation of the ring, and end up making it more reactive than benzene itself. Whereas, some other constituents other than these, help in making the ring less reactive, than benzene. This is similar to that case of aromatic nitration, where -OH groups make the benzene ring more reactivated than the compound itself, whereas the nitro group does the opposite. 

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Types of Functional Groups

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The substituents of benzene can be classified into three important categories. These are, 

1. Ortho and para-directing activators

These groups are responsible for emitting electrons and thereby forming or setting the benzene ring in motion. The density of electrons at both ortho and para positions elevates, because of activation of the benzene ring. After having directed the incoming groups to both primary stages, there are many activating groups like –NH₂, –NHR, –NHCOCH₃, –OCH₃,–CH₃, –C₂H₅, which are better known by the term ortho-para directors. Considering the -OH group, it has quite the influence on the whole electrophilic attack. 

Because of the created resonance in the ring of benzene, overall density rises up as compared to the meta stage. So it is called an activation group. Therefore, halogens end up becoming such ortho-para directors. At this stage it doesn't matter that halogens are actual deactivators because of the -I effect. 

2. Meta directing groups

These groups are associated with the sole purpose of directing the coming groups into the meta stage. In simple terms, it means these groups take away electrons from the ring of benzene and neutralise their effect. The common examples are –NO₂, –CN, –CHO, –COR, –COOH, –COOR, and –SO₃H. Talking about NO₂ or nitrogen group, upon coming in contact, the overall density of the ring falls down, and so NO₂ acts as a deactivator. On top of that, electron density on the ortho and para stages is lesser as compared to that in meta position. It is because of this, that the two meta positions in electron density, which leads ultimately to substitution of meta. 

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Things To Remember 

• Monosubstituted benzene means a situation where one unique position in the ring of benzene is replaced by an atom, or many atoms together. Methylbenzene, C₆H₅CH₃ is a derivative of benzene and has only one constituent and is a common example of monosubstituted benzene. 
• Prolonged exposure to benzene has been considered as harmful, as it can lead to serious health issues of abnormalities in bone marrow, severe leukaemia, cancer, and even cardiovascular irregularities. The American Petroleum Institute termed it as human carcinogen. 
• The substituents of benzene have been divided into 3 broad categories. These are ortho, para, and meta. Ortho and para are responsible for activation of the benzene ring by giving up electrons. The common examples are -OH, and -NH₂. 
• There is the meta directing group which follows the opposite rule. These withdraw electrons, and deactivate the ring of benzene. The overall density of the benzene ring decreases here. -NO₂, and -COOH are major deactivators who help carry out this step.
• Ortho, para directing deactivating groups are also present which withdraw benzene ring, and neutralise it. They guide the incoming ortho group and para position. Overall density decreases here, because of halogens. Halogens have a strong negative effect here.