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Q9.20(iii):
How would you convert the following compounds into benzene? (iii) Hexane
Solution :
Step 1: Theoretical Foundation of Aromatization
The conversion of an acyclic alkane ($C_nH_{2n+2}$) into an aromatic hydrocarbon ($C_6H_6$) is achieved through a process known as dehydrocyclization or aromatization. For $n$-hexane ($CH_3(CH_2)_4CH_3$), this process involves two simultaneous reactions: the cyclization of the carbon chain and the dehydrogenation of the molecule [Per the principle of catalytic reforming in petrochemical industry].
Step 2: Reagents and Conditions
To facilitate the transformation of $n$-hexane to benzene, the alkane must be heated under high pressure in the presence of specific metallic catalysts. The standard industrial and laboratory conditions are as follows:
- Catalysts: Oxides of Vanadium ($V_2O_5$), Chromium ($Cr_2O_3$), or Molybdenum ($Mo_2O_3$) supported on Alumina ($Al_2O_3$).
- Temperature: Approximately $773 \, K$ ($500 \, ^\circ C$).
- Pressure: $10 - 20 \, atm$.
Step 3: Chemical Equation and Mechanism
The reaction proceeds via the removal of four molecules of hydrogen ($H_2$) to establish the resonance-stabilized aromatic ring system.
$CH_3-CH_2-CH_2-CH_2-CH_2-CH_3 \xrightarrow{Cr_2O_3/Al_2O_3, 773K} C_6H_6 + 4H_2$
[Justification: The enthalpy of the reaction is endothermic; high temperatures are required to overcome the activation energy barrier for the closure of the six-membered ring and subsequent loss of hydrogen atoms].
Visual Representation of Dehydrocyclization
Step 4: Summary of Atomic Rearrangement
| Species | Molecular Formula | Degree of Unsaturation |
|---|---|---|
| n-Hexane | $C_6H_{14}$ | 0 |
| Benzene | $C_6H_6$ | 4 |
The conversion increases the degree of unsaturation by 4, reflecting the loss of $4 \times H_2$ molecules to form the three double bonds and the closed-ring structure [Per the Huckel's Rule of aromaticity ($4n+2$ $\pi$ electrons, where $n=1$)].
Final Solution: $n$-Hexane is converted into benzene by heating it with $Cr_2O_3$, $V_2O_5$, or $Mo_2O_3$ supported over alumina at $773 \, K$ under high pressure, a process known as catalytic dehydrocyclization.
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
- Q9.21: 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
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