Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT text lists 'Nucleophilic substitution' and 'Elimination reactions' as two main categories of reactions for haloalkanes, often occurring simultaneously or in competition depending on reaction conditions (e.g., strength of base/nucleophile, temperature, solvent).

The NCERT states, 'When a haloalkane with $\beta$-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from $\beta$-carbon and a halogen atom from the $\alpha$-carbon atom.' This process leads to the formation of a carbon-carbon double bond, thus producing an alkene.

2-chloropropane has $\beta$-hydrogens on both adjacent methyl groups. Heating with alcoholic KOH will cause an elimination reaction (dehydrohalogenation), removing a chlorine atom and a hydrogen atom from an adjacent carbon, resulting in the formation of propene.

While both aqueous and alcoholic KOH can act as a base/nucleophile, alcoholic KOH is a stronger base and a weaker nucleophile due to the steric hindrance of the alkoxide ion formed in alcohol, favoring elimination (E2) over substitution (SN2). Aqueous KOH, being a strong nucleophile, would primarily lead to nucleophilic substitution (formation of alcohol).

In an elimination reaction, the $\alpha$-carbon (bearing the halogen) and the $\beta$-carbon (losing a hydrogen) both change their hybridization state. The $\alpha$-carbon, initially sp3 hybridized, becomes part of a carbon-carbon double bond, thus transitioning to sp2 hybridization.

The NCERT text explicitly states, 'When a haloalkane with $\beta$-hydrogen atom is heated with alcoholic solution of potassium hydroxide, there is elimination of hydrogen atom from $\beta$-carbon and a halogen atom from the $\alpha$-carbon atom.' This confirms the necessity of a $\beta$-hydrogen. They typically form alkenes, can be unimolecular (E1) or bimolecular (E2), and are favored by higher temperatures and strong bases.

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.