Hydrocarbons MCQs for NEET — Chemistry Questions with Answers

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.

Aromaticity requires the presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .). This is often referred to as Hückel Rule.

The NCERT text states: 'The nature of groups or substituents attached to benzene ring is responsible for activation or deactivation of the benzene ring towards further electrophilic substitution and also for orientation of the incoming group.' This directly supports option o3.

The NCERT text states: 'The delocalised $\pi$ electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localised between two carbon atoms. Therefore, presence of delocalised $\pi$ electrons in benzene makes it more stable than the hypothetical cyclohexatriene.' This confirms option o3.

The NCERT text states the characteristics of aromaticity: '(i) Planarity (ii) Complete delocalisation of the $\pi$ electrons in the ring (iii) Presence of (4n + 2) $\pi$ electrons in the ring where n is an integer (n = 0, 1, 2, . . .). This is often referred to as Hückel Rule.' The presence of a benzene ring is often associated with aromatic compounds, but not a universal rule for aromaticity itself, as 'non-benzenoids' can also be aromatic. Thus, option o4 is not a strict criterion.

The NCERT text specifies: 'In the trivial system of nomenclature the terms ortho (o), meta (m) and para (p) are used as prefixes to indicate the relative positions 1,2;1,3 and 1,4 respectively.' This matches option o2.