[Chemistry Class Notes] on Substituted Benzene Compounds: Nomenclature Pdf for Exam

Benzene is defined as a hydrocarbon, having the chemical formula as C6H6. It contains six carbon atoms, which are joined in a ring and contain one hydrogen atom, which is attached to every carbon atom. By replacing either one or more of the hydrogen atoms with the functional group, we can avail many benzene compounds. And, while naming the substituted benzene compounds, we can prefix the substituent name to the word benzene.

Mono-Substituted Benzene Compounds

For the benzene compounds that consist of a single substituent, we simply prefix the substituent name to as benzene. A few examples, along with the common names, can be listed as follows:

Methylbenzene or Toluene

Benzene Compounds – Toluene

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Toluene

Hydroxybenzene or Phenol

Benzene Compounds – Phenol

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Phenol

Aminobenzene or Aniline

Benzene Compounds – Aniline

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Aniline

Ethylbenzene

Benzene Compounds

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Di-Substituted Benzene Compounds

When there exist two substituents in the compound, we can number every carbon atom in such a way that the substituents can be attached to the lowest possible numbered carbon atom.

• 1,3-dinitrobenzene is given as the name of the compound, which is given below. Naming it as 1,5-dinitrobenzene is not correct due to the reason, the carbon atom may not be numbered lowest.

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1,3-dichlorobenzene

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Also, when there exists more than one substituent, we can also name their positions as ortho- (o), meta- (m), para- (p). They also refer to the positions as 1,2-; 1,3- and 1,4- respectively. Therefore, we can name the 1,3-dichlorobenzene compound as m-dichlorobenzene.

Poly-Substituted Benzene Compounds

In the case of the poly-substituted compounds, if there exists a base compound, we can assign that as position 1. Then, we choose the next specific compound for numbering such that it will get the lowest number. Moreover, if there exists zero special or base group, we will list them in the alphabetical order by giving them the lowest numbers.

1-Bromo-2,4-dinitrobenzene

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For the following compound, aniline is the base compound. So, we name it as 3-Chloro-2-nitroaniline.

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Key Points

• For the substituted benzene rings type, in which the substituent contains more than six carbons, the benzene ring can be noted by using a phenyl prefix on the name of alkane.

• For the substituted benzene rings, in which the substituent has less than six carbons, the resultant alkyl chain can be added as a prefix with the ending changed to as -yl.

• Whereas, for the benzene rings having multiple substituents, the ring atoms can be numbered to minimize the substituent groups numbering; alternatively, as meta or para or ortho nomenclature (benzene nomenclature) is used for the di-substituted rings.

Terms

• orthoA prefix can be used to name the aromatic ring having two adjacent substituents.

• metaA prefix can be used to name an aromatic ring having two substituents separated by one carbon present on the ring.

• Whereas, paraA prefix can be used to name an aromatic ring having two substituents directly across from one another present on the ring.

Aromatic compounds are defined as the ring structures having unusual stability because of the delocalized pi-electron density, which can be shared between all of the carbon atoms present in the ring.

Historically, there exists a number of common names for aromatic structures. These particular names can be frequently used in favour of standardized IUPAC nomenclature (benzene nomenclature). For example, we can say that methylbenzene is often referred to as toluene, whereas dimethyl benzene is often referred to as xylene.

Naming the Aromatic Compounds

This type of nomenclature explains us through the IUPAC rules for benzene type of molecules and includes the common names for the substituted benzene.

Aromatic Compounds 

Aromatic Compounds having a Single Substituent

When there exists a single substituent on a benzene ring, and the substituent has either six or fewer carbons, then, the substituent is included as a prefix to the benzene compound. Alkyl groups can be named as per the alkane series convention, which is ending with -yl: methyl (for one carbon), ethyl (for two carbons), and propyl (for three carbons), and follows.

If the substituent has more than six carbons, the alkane portion is first named, and then, the aromatic ring portion can be added as a suffix. For example, an aromatic ring, which is bonded to an 8-carbon chain, would be the 1-phenyloctane, but not octyl benzene.

Aromatic Compounds Having Multiple Substituents

When there exist multiple substituents, the ring atoms can be numbered to minimize the numbers that are assigned to the positions, which are substituted.

The disubstituted benzene rings are named according to the relative positions of the substituents: the ortho– prefix can be used when the substituents occupy the adjacent positions on the ring (1,2), meta– prefix can be used when the substituents are separated by one ring position (1,3), and finally, para– prefix can be used when they are found on the opposite sides of the ring (1,4).