Important questions based on NCERT syllabus for Chapter 13 - Hydrocarbons:
Question-1: What are the necessary conditions for any system to be aromatic?
Solution: The necessary conditions for a molecule to be aromatic are:
It should have a single cyclic cloud of delocalised n-electrons above and below the plane of the molecule.
It should be planar. This is because complete delocalization of n-electrons is possible only if the ring is planar to allow cyclic overlap of p-orbitals.
It should contain Huckel number of electrons, i.e., (4n + 2) n-electrons where n = 0, 1, 2, 3 etc.
A molecule which does not satisfy any one or more of the above conditions is said to be non-aromatic.
Question-2: Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty?
Solution: Due to the presence of an electron cloud containing 6 n-electrons above and below the plane of the ring, benzene is a rich source of electrons. Consequently, it attracts the electrophiles (electron-deficient) reagents towards it and repels nucleophiles (electron- rich) reagents. As a result, benzene undergoes electrophilic substitution reactions easily and nucleophilic substitutions with difficulty.
Question-3: Out of benzene, m-dinitrobenzene and toluene which will undergo nitration most easily and why?
Solution: CH3 group is electron-donating while—NO2 group is electron-withdrawing. Therefore, maximum electron density will be in toluene, followed by benzene and least in m-dinitrobenzene. Therefore, the ease of nitration decreases in the order: toluene > benzene > m-dinitrobenzene.