What is Benzene
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).
FAQs on Substituted Benzene Compounds: Nomenclature
1. What is the basic IUPAC rule for naming a monosubstituted benzene compound?
The name is formed by placing the name of the substituent as a prefix to the word 'benzene'. For example, if a chlorine atom (-Cl) is attached to the ring, the compound is named Chlorobenzene. Similarly, a nitro group (-NO₂) gives Nitrobenzene.
2. How are disubstituted benzene compounds named using ortho, meta, and para prefixes?
For disubstituted benzenes, the relative positions of the two substituents are indicated by prefixes:
- Ortho- (o-): Used for substituents at positions 1 and 2.
- Meta- (m-): Used for substituents at positions 1 and 3.
- Para- (p-): Used for substituents at positions 1 and 4.
For example, benzene with two bromine atoms at positions 1 and 4 is called p-Dibromobenzene.
3. What is the IUPAC name for a benzene ring with a -CHO group attached?
A benzene ring with an aldehyde (-CHO) group is commonly known as Benzaldehyde. This common name is accepted by IUPAC. Its systematic IUPAC name is Benzenecarbaldehyde. For the CBSE Class 11 and 12 syllabus, Benzaldehyde is the most frequently used name.
4. How is the numbering decided when a benzene ring has more than two substituents?
When a benzene ring has three or more substituents, their positions are indicated by numbering the carbon atoms of the ring. The numbering is done in a way that gives the lowest possible numbers (locants) to the substituents. The substituents are then listed in alphabetical order before the word 'benzene'. For example, a benzene ring with Cl at C1, Br at C2, and NO₂ at C4 is named 2-Bromo-1-chloro-4-nitrobenzene.
5. What is the difference in naming 1,2-dichlorobenzene and 1,3-dichlorobenzene?
The names indicate the different relative positions of the two chlorine atoms on the benzene ring.
- 1,2-Dichlorobenzene: The chlorine atoms are on adjacent carbons. This is also known as ortho-dichlorobenzene (o-dichlorobenzene).
- 1,3-Dichlorobenzene: The chlorine atoms are separated by one carbon atom. This is also known as meta-dichlorobenzene (m-dichlorobenzene).
6. When is a benzene ring named as a 'phenyl' group instead of the parent chain?
A benzene ring is treated as a substituent, called a phenyl group (C₆H₅-), when it is attached to an aliphatic chain that has a functional group or is longer than the ring. For example, in C₆H₅CH₂CH₂OH, the parent chain is ethanol, and the benzene ring is a substituent. The IUPAC name is 2-Phenylethanol.
7. What happens if a substituted benzene has multiple different functional groups? How is the principal functional group decided?
When multiple functional groups are present, one is chosen as the principal functional group based on a pre-defined priority order. This group gives the base name to the molecule (e.g., Phenol for -OH, Benzoic acid for -COOH). All other groups are treated as substituents and named as prefixes. For example, in a benzene ring with both -OH and -NO₂ groups, the -OH group has higher priority. If they are at positions 1 and 4, the compound is named 4-Nitrophenol, not 4-Hydroxynitrobenzene.
8. Why are common names like Toluene and Aniline still accepted in IUPAC nomenclature?
Common names like Toluene (methylbenzene), Aniline (aminobenzene), and Phenol (hydroxybenzene) are retained by IUPAC because of their widespread and historical use in chemical literature and industry. These names are often simpler and more convenient than their systematic IUPAC names. For many substituted compounds, these accepted common names are used as the parent name. For instance, a benzene ring with a -CH₃ group at C1 and a -NO₂ group at C2 is named 2-Nitrotoluene.
