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Q9.23:
Out of benzene, $m$-dinitrobenzene and toluene which will undergo nitration most easily and why?
Solution :
Step 1: Analyzing the Electronic Effects of Substituents
Nitration is an electrophilic aromatic substitution reaction. The rate of the reaction is governed by the electron density of the benzene ring. Substituents that increase electron density (activating groups) facilitate the attack of the electrophile ($NO_2^+$), while substituents that decrease electron density (deactivating groups) hinder it.
- Benzene ($C_6H_6$): The reference compound with uniform electron density.
- Toluene ($C_6H_5CH_3$): The methyl group ($-CH_3$) is an electron-donating group. It exerts this effect via two mechanisms:
- Inductive Effect ($+I$ effect): The alkyl group releases electrons toward the ring.
- Hyperconjugation: The $\sigma$-electrons of the $C-H$ bonds in the methyl group delocalize into the $\pi$-system of the ring.
- m-Dinitrobenzene ($C_6H_4(NO_2)_2$): The nitro group ($-NO_2$) is a strong electron-withdrawing group via both:
- Negative Inductive Effect ($-I$): Electronegativity of nitrogen and oxygen pulls electrons away from the ring.
- Negative Mesomeric Effect ($-M$): Delocalization of $\pi$-electrons toward the $NO_2$ group reduces overall electron density.
Step 2: Comparative Reactivity Profile
| Compound | Substituent Effect | Ring Activation Status |
|---|---|---|
| Toluene | $+I$ and Hyperconjugation | Activated (More reactive than benzene) |
| Benzene | None (Reference) | Neutral |
| m-Dinitrobenzene | $-I$ and $-M$ | Strongly Deactivated (Less reactive than benzene) |
Step 3: Visual Representation of Substituent Effects
Step 4: Conclusion
The reactivity order for electrophilic nitration is dictated by the availability of electron density for the incoming electrophile. Since the methyl group in toluene increases electron density through hyperconjugation and inductive effects, it activates the ring. Conversely, the two nitro groups in m-dinitrobenzene strongly withdraw electron density, deactivating the ring.
Final Solution: Toluene will undergo nitration most easily because the methyl group is an electron-donating group that increases the electron density of the benzene ring, making it more susceptible to electrophilic attack compared to benzene and m-dinitrobenzene.
More Questions from Class 11 Chemistry Hydrocarbons EXERCISES
- Q9.1: How do you account for the formation of ethane during chlorination of methane ?
- Q9.10: Why is benzene extra ordinarily stable though it contains three double bonds?
- Q9.11: What are the necessary conditions for any system to be aromatic?
- Q9.12(i): Explain why the following systems are not aromatic? (i)
- Q9.12(ii): Explain why the following systems are not aromatic? (ii)
- Q9.12(iii): Explain why the following systems are not aromatic? (iii)
- Q9.13(i): How will you convert benzene into (i) $p$-nitrobromobenzene
- Q9.13(ii): How will you convert benzene into (ii) $m$-nitrochlorobenzene
- Q9.13(iii): How will you convert benzene into (iii) $p$-nitrotoluene
- Q9.13(iv): How will you convert benzene into (iv) acetophenone?
- Q9.14: In the alkane $H_3C-CH_2-C(CH_3)_2-CH_2-CH(CH_3)_2$, identify $1^{\circ},2^{\circ},3^{\circ}$ carbon atoms and give the number of H atoms bonded to each one of these.
- Q9.15: What effect does branching of an alkane chain has on its boiling point?
- Q9.16: Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism.
- Q9.17: Write down the products of ozonolysis of 1,2-dimethylbenzene ($o$-xylene). How does the result support Kekulé structure for benzene?
- Q9.18: Arrange benzene, $n$-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour.
- Q9.19: Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?
- Q9.2(a): Write IUPAC names of the following compounds : (a) $CH_3CH=C(CH_3)_2$
- Q9.2(b): Write IUPAC names of the following compounds : (b) $CH_2=CH-C \equiv C-CH_3$
- Q9.2(c): Write IUPAC names of the following compounds : (c)
- Q9.2(d): Write IUPAC names of the following compounds : (d)
- Q9.2(e): Write IUPAC names of the following compounds : (e)
- Q9.2(f): Write IUPAC names of the following compounds : (f)
- Q9.2(g): Write IUPAC names of the following compounds : (g)
- Q9.20(i): How would you convert the following compounds into benzene? (i) Ethyne
- Q9.20(ii): How would you convert the following compounds into benzene? (ii) Ethene
- Q9.20(iii): How would you convert the following compounds into benzene? (iii) Hexane
- Q9.21: Write structures of all the alkenes which on hydrogenation give 2-methylbutane.
- Q9.22(a): Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, $E^+$ (a) Chlorobenzene, 2,4-dinitrochlorobenzene, $p$-nitrochlorobenzene
- Q9.22(b): Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, $E^+$ (b) Toluene, $p-H_3C-C_6H_4-NO_2$, $p-O_2N-C_6H_4-NO_2$.
- Q9.24: Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene.
- Q9.25: Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example.
- Q9.3(a): For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated : (a) $C_4H_8$ (one double bond)
- Q9.3(b): For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated : (b) $C_5H_8$ (one triple bond)
- Q9.4(i): Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (i) Pent-2-ene
- Q9.4(ii): Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (ii) 3,4-Dimethylhept-3-ene
- Q9.4(iii): Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (iii) 2-Ethylbut-1-ene
- Q9.4(iv): Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (iv) 1-Phenylbut-1-ene
- Q9.5: An alkene ‘$A$’ on ozonolysis gives a mixture of ethanal and pentan-3-one. Write structure and IUPAC name of ‘$A$’.
- Q9.6: An alkene ‘$A$’ contains three $C-C$, eight $C-H$ $\sigma$ bonds and one $C-C$ $\pi$ bond. ‘$A$’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of ‘$A$’.
- Q9.7: Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene?
- Q9.8(i): Write chemical equations for combustion reaction of the following hydrocarbons: (i) Butane
- Q9.8(ii): Write chemical equations for combustion reaction of the following hydrocarbons: (ii) Pentene
- Q9.8(iii): Write chemical equations for combustion reaction of the following hydrocarbons: (iii) Hexyne
- Q9.8(iv): Write chemical equations for combustion reaction of the following hydrocarbons: (iv) Toluene
- Q9.9: Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why?
CBSE Solutions for Class 11 Chemistry Hydrocarbons
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