NEET Practice Questions (MCQs) with Answers & Solutions

The text contrasts them: 'Mendel’s studies mainly described those traits that have distinct alternate forms... But if you look around you will find that there are many traits which are not so distinct in their occurrence and are spread across a gradient.' This gradient is characteristic of polygenic inheritance.

According to the NCERT text, the conditions for aromaticity are: (i) Planarity, (ii) Complete delocalisation of the π electrons in the ring, and (iii) Presence of (4n + 2) π electrons (Hückel Rule). While aromatic compounds typically have double bonds, 'presence of at least one double bond' is not a specific criterion for aromaticity as per Hückel's rule; rather, it refers to the complete delocalization of π electrons.

X-Ray diffraction data indicates that all six C—C bond lengths in benzene are of the same order (139 pm) which is intermediate between C—C single bond (154 pm) and C—C double bond (133 pm).

The absence of pure double bond in benzene accounts for the reluctance of benzene to show addition reactions under normal conditions, thus explaining the unusual behaviour of benzene. The delocalised π electron cloud is attracted more strongly by the nuclei of the carbon atoms, making it more stable and favoring substitution over addition reactions.

The six π electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. 9.6 (a) or (b).

Benzene is a hybrid of various resonating structures. The two structures, A and B given by Kekulé are the main contributing structures. The resonance structures (canonical structures or contributing structures) are hypothetical and individually do not represent any real molecule. They contribute to the actual structure in proportion to their stability.

All the six carbon atoms in benzene are $sp^2$ hybridized. Two $sp^2$ hybrid orbitals of each carbon atom overlap with $sp^2$ hybrid orbitals of adjacent carbon atoms to form six C—C sigma bonds which are in the hexagonal plane.

The Kekulé structure indicates the possibility of two isomeric 1, 2-dibromobenzenes, where in one, bromine atoms are attached to doubly bonded carbon atoms and in the other, to singly bonded carbons. This would imply two types of C-C bond lengths. However, X-ray data indicates that all six C—C bond lengths are the same (139 pm), intermediate between single and double bonds. This problem was addressed by Kekulé's concept of oscillating double bonds and later by resonance.

This can be represented in the form of two doughnuts (rings) of electron clouds [Fig. 9.7 (d)], one above and one below the plane of the hexagonal ring.

Aromatic hydrocarbons, despite having unsaturation, undergo mainly electrophilic substitution reactions. These undergo addition reactions only under special conditions. This 'unusual stability' (mentioned in the context) is a hallmark of aromatic compounds.