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Benzene or substituted benzenes are organic chemical compounds belonging to the family of hydrocarbons. Benzene has a planar ring shaped structure with six carbon atoms joined with one another and one hydrogen each. It has three double bonds and three single bonds with a lone pair of electrons in its structure. The lone pair of electrons in the molecular rings moves from one carbon atom to another and with the replacement of a hydrogen atom by the functional group in the molecule form a new benzyl compound such as toluene, phenol, aniline etc.
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Key Terms: Benzene, Hydrocarbons, Atoms, Ethanol, Hydrogen, Electron, Single Bond, Benzyl compound, Carbon, Molecular rings
What is Benzene?
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Benzene compound is defined as an organic compound made up of six carbon and six hydrogen atoms joined with each other in a planar ring structure. Due to the presence of carbon and hydrogen atoms in the structure it belongs to the family of Hydrocarbons.
With the cyclic pi bonds between the carbon atoms it is categorised as an aromatic hydrocarbon. The chemical formula of the benzene compound is C6H6 and its molar mass is 78.114 g per mole. It is a colourless and highly flammable liquid with odour smell and a natural constituent of crude oil.
Benzene
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Structure of Benzene
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The benzene ring has six C-C bonds in which three C-C double bonds of length 135 pm and three C-C single bonds of length 147 pm. The shorter distance of C-C double bond is due to the delocalisation of electrons equally distributed between the carbon atoms. With the replacement of one or more hydrogen atoms by any functional group such as alcohol, aldehyde, ketone etc different types of Benzene compounds are formed. And during naming all these substituted benzene compounds, we can use the prefix benzene as the substituent name.
Chemical Structure of Benzene
Mono-Substituted Benzene Compounds
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When one of the functional groups is added to the benzene compounds which have a single substituent in its molecular structure can be named with adding the prefix benzene as the substituent name. Below we have some of the names of mono-substituted benzene compounds along with its common name:
- Anisole
The common name of Anisole compound is methoxybenzene and its molecular formula is C6H5OCH3.
Anisole
- Aniline
Aniline is a nitro compound whose IUPAC name is aminobenzene and formula C6H5NH2. In this compound a NH2 group is added in place of a hydrogen atom.
Aniline
- Toluene
A methyl group is added to the benzene to form a compound named methylbenzene and the formula is C6H5CH3.
Toluene
Di-Substituted Benzene Compounds
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In case, if there are two substituents attached to the molecular structure benzene compounds, then we can name the compound in such a way that each carbon atom gets the lowest possible number.
- 1,3 dinitrobenzene
It is a di-substituted benzene compound in which two nitro functional groups have replaced the two hydrogen atoms in the molecular structure. It is given the name 1,3 dinitrobenzene on the basis of lowest carbon atom, if we name it as 1,5-dinitrobenzene is incorrect.
1,3 dinitrobenzene
- 1,3-dichlorobenzene
In this compound, two hydrogen atoms are replaced by two chlorine atoms and named on the basis of the lowest carbon atom.
1,3-dichlorobenzene
Note: We can also name the aromatic compounds as ortho- (o), meta- (m), para- (p) referring to its positions as 1,2-; 1,3- and 1,4- respectively, if there exists more than one substituent, Therefore we can call 1,3-dichlorobenzene compound as meta dichlorobenzene.
Poly-Substituted Benzene Compounds
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Poly-Substituted Benzene Compounds indicate there are two or more functional groups attached in the benzene ring. However, if a base compound is attached in the benzene ring then it will be assigned as the number 1. After the base compound, the rest of the functional group is numbered on the basis of lowest carbon atoms. Some of the poly-substituted compounds are given below
- 1-Bromo-2,4-dinitrobenzene: This compound is named on the basis of lowest carbon atom
- 3-Chloro-2-nitroaniline: In this compound aniline is a base compound that's why it has been assigned the number 2 and chlorine is in number 3.
Things to Remember
- Ortho prefix is used to name the aromatic compounds having two adjacent substituents functional groups.
- Meta prefix is named to the aromatic compound in which the two substituent groups are separated by one carbon atom.
- Para prefix is used for the compounds which are attached opposite to each other in an aromatic compound.
- If the substituent has more than six carbon atoms in the benzene ring it can be named using the prefix phenyl on the name of alkane.
- If the substituent has less than six carbon atoms then the resultant alkyl will be added as a prefix with the ending letter replaced by -yl.
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Sample Questions
Ques. Describe resonance in benzene. (1 Mark)
Ans. It shows two resonating structures. Both the forms differ in the arrangement of electrons. It mainly differs only in the arrangement of electrons, the number of carbon atoms and hydrogen atoms in benzene remain the same. The double bonds present in benzene molecules rotate in resonating structures of benzene and provide stability to the compound.
Ques. What is benzene substitution? (1 Mark)
Ans. It is an Electrophilic substitution of benzene compound in which an electron rich species or electrophilic substitutes the hydrogen atom of benzene such as in nitration, sulfonation, halogenation, Friedel Crafts alkylation and acylation, etc.
Ques. Which is the most reactive in electrophilic substitution? (1 Mark)
Ans. The Methyl group attached to the toluene compound is more reactive towards the electrophilic substitution reaction. As the Methyl group is an activating group, it increases the reactivity of benzene towards electrophilic substitution.
Ques. What are the most common substituted benzene? (1 Mark)
Ans. Aniline or Aminobenzene (C6H5NH2), Benzoic acid (C6H5CO2H), Benzaldehyde (C6H5CHO) and Benzonitrile or Cyanobenzene (C6H5CN) are some of the most common substituted benzene compound.
Ques. Which is least reactive in electrophilic substitution? (1 Mark)
Ans. Pyridine is the least reactive molecule towards substitution because of the stabilised resonance of electrons withdrawing from the ring by the nitrogen atom. It deactivates the benzene ring for substitution.
Ques. Which is more reactive phenol or toluene? (1 Mark)
Ans. Phenol is more reactive as compared to the toluene compound because the methyl group attached in toluene increases the electron density through hyperconjugation and inductive effect whereas the resonance effect is more stronger than the hyperconjugation shown by the Phenol compound.
Ques. Why is OH a more activating group than OCH3? (1 Mark)
Ans. OCH3 is more electron withdrawing than the OH group due to its -I effects. Actually, it has two lone pairs of oxygen atoms in the molecule whereas in OH group, the hydrogen atom is comparatively much much smaller than oxygen, so it does not show any steric repulsions.
Ques. Does benzene compound show substitution reaction? (2 Marks)
Ans. Benzene is a planar atom having delocalized electrons above and beneath the plane of the ring and it is an electron-rich species. Therefore, it is exceptionally attractive to electron-deficient species i.e., electrophiles. In this way, it goes through electrophilic replacement substitution reactions very easily.
Ques. How do you name a substituted benzene? (2 Marks)
Ans. For the substituted benzene rings where the substituent having more than of six carbons, the benzene ring is noted by utilizing a phenyl prefix on the alkane name. For the substituent benzene rings where the substituent contains under six carbons, the alkyl chain is added as a prefix with the ending changed to - yl.
Ques. What are the physical properties of Benzene? (2 Marks)
Ans. Some of the physical properties of Benzene compounds are given below:
- It is a colourless fluid with distinct odour smell
- Highly flammable and volatile
- Soluble in organic solvent and insoluble in water
- Show Resonance
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