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Q9.19:
Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?
Solution :
Step 1: Analysis of Benzene's Electronic Configuration
Benzene ($C_6H_6$) is a cyclic, planar molecule where each carbon atom is $sp^2$ hybridized. The six carbon atoms form a ring, with each carbon contributing one $p$-orbital containing one electron to a delocalized $\pi$-electron cloud. [Per Hückel’s Rule for aromaticity ($4n+2$ $\pi$-electrons), where $n=1$, benzene possesses a stable, delocalized electron-rich cloud above and below the plane of the ring].
Step 2: Justification for Electrophilic Substitution ($S_EAr$)
The $\pi$-electron cloud of benzene is highly electron-dense, making the ring inherently nucleophilic. Electrophiles ($E^+$), which are electron-deficient species, are strongly attracted to this region of high electron density.
- Step A (Formation of $\sigma$-complex): The electrophile attacks the $\pi$-system to form a carbocation intermediate (Arenium ion), stabilized by resonance. [By the Principle of Electrophilic Aromatic Substitution].
- Step B (Restoration of Aromaticity): The loss of a proton from the $sp^3$ hybridized carbon restores the aromaticity of the ring, which provides the thermodynamic driving force for the reaction.
Step 3: Justification for Resistance to Nucleophilic Substitution ($S_NAr$)
Nucleophiles are electron-rich species ($Nu^-$ or $Nu:^-$). The interaction between a nucleophile and the benzene ring is inhibited by two primary factors:
- Electronic Repulsion: Since the benzene ring is itself electron-rich, it exerts a strong electrostatic repulsion toward incoming nucleophiles.
- Stability of the $\pi$-system: For a nucleophilic substitution to occur, the nucleophile would need to attack the aromatic ring, disrupting the stable delocalized $\pi$-system. The activation energy ($E_a$) required to break the aromatic stabilization energy (approx. $150\ kJ/mol$) is prohibitively high under standard conditions. [Per the Law of Conservation of Energy and Aromatic Stability Constraints].
Step 4: Comparative Summary
| Feature | Electrophilic Substitution | Nucleophilic Substitution |
|---|---|---|
| Attacking Species | Electrophile ($E^+$) | Nucleophile ($Nu^-$) |
| Interaction | Attraction to electron-rich $\pi$-cloud | Repulsion by electron-rich $\pi$-cloud |
| Feasibility | High (occurs under mild conditions) | Low (requires drastic conditions/withdrawing groups) |
Final Solution: Benzene undergoes electrophilic substitution easily because its electron-rich $\pi$-cloud acts as a donor to electron-deficient electrophiles, restoring aromaticity in the product. It resists nucleophilic substitution because of electrostatic repulsion between the incoming nucleophile and the $\pi$-electron cloud, as well as the high energy penalty associated with disrupting the stable aromatic sextet.
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.
- Q9.2(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) $CH_3(CH_2)_4 CH (CH_2)_3 CH_3$ with a $CH_2–CH(CH_3)_2$ group attached to the CH.
- Q9.2(g): Write IUPAC names of the following compounds : (g) $CH_3 – CH = CH – CH_2 – CH = CH – CH – CH_2 – CH = CH_2$ with a $C_2H_5$ group attached to the third CH from the left.
- 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, $p-H_3C – C_6H_4 – NO_2$, $p-O_2N – C_6H_4 – NO_2$.
- Q9.23: Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why?
- 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
Chapters in CBSE - Class 11 Chemistry
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