Hydrocarbons MCQs for NEET — Chemistry Questions with Answers

The NCERT text explains: 'Rotation around C=C bond is not free. It is restricted... This illustrates that the restricted rotation of atoms or groups around the doubly bonded carbon atoms gives rise to different geometries of such compounds. The stereoisomers of this type are called geometrical isomers.' This restricted rotation is a characteristic feature of the π-bond.

The context says, 'Structures I and II possess same molecular formula but differ in their boiling points and other properties... Structures I and II are isomers of butane... Since difference in properties is due to difference in their structures, they are known as structural isomers. It is also clear that structures I and III have continuous chain of carbon atoms but structures II, IV and V have a branched chain. Such structural isomers which differ in chain of carbon atoms are known as chain isomers.' Structure I ($CH_3-CH_2-CH_2-CH_3$) is n-butane (continuous chain) and Structure II ($CH_3-CH(CH_3)-CH_3$) is 2-methylpropane (branched chain), both are chain isomers of $C_4H_{10}$.

The NCERT text under 9.3 Alkenes states: 'If there is one double bond between two carbon atoms in alkenes, they must possess two hydrogen atoms less than alkanes. Hence, general formula for alkenes is $C_nH_{2n}$'.

The NCERT text states: 'The double bond is shorter in bond length (134 pm) than the C–C single bond (154 pm).' Therefore, a C-C single bond is longer than a C=C double bond.

The NCERT problem 9.9 provides examples of isomers for $C_5H_{10}$ including Pent-1-ene ($CH_2=CH-CH_2-CH_2-CH_3$) and Pent-2-ene ($CH_3-CH=CH-CH_2-CH_3$). These compounds have the same carbon chain but differ in the position of the double bond, which is a characteristic of position isomerism. The section on structural isomerism also defines: 'Position isomerism: When two or more compounds differ in the position of substituent atom or functional group on the carbon skeleton, they are called position isomers'.

The NCERT text defines position isomerism as compounds differing 'in the position of substituent atom or functional group on the carbon skeleton'. Propan-1-ol ($CH_3CH_2CH_2OH$) and Propan-2-ol ($CH_3CH(OH)CH_3$) have the same carbon skeleton and functional group (-OH), but the position of the -OH group is different. But-1-ene and 2-Methylprop-1-ene are chain isomers (or skeletal isomers). Diethyl ether and Butan-1-ol are functional group isomers. n-Butane and Isobutane are also chain isomers.

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).