Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT text explains, 'The sp$^2$ hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp$^3$-hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is 177pm while in haloarene is 169 pm.'

The context states, 'C— Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.'

Allylic halides are defined as compounds 'in which the halogen atom is bonded to an sp$^3$-hybridised carbon atom adjacent to carbon-carbon double bond (C=C)'.

Although not explicitly stated as a formula within the provided snippets, bond polarity (which contributes to dipole moment) is directly related to electronegativity difference, and dipole moment itself is a product of charge separation and bond length ($μ = q imes d$). The NCERT text mentions 'some typical bond lengths, bond enthalpies and dipole moments are given in Table 6.2', implying these are related properties.

The NCERT text states, 'Instability of phenyl cation: In case of haloarenes, the phenyl cation formed as a result of self-ionisation will not be stabilised by resonance and therefore, S$_N$1 mechanism is ruled out.' This contributes to their lower reactivity in nucleophilic substitution.

While the provided text doesn't directly state the trend for bond enthalpy, it mentions 'Some typical bond lengths, bond enthalpies and dipole moments are given in Table 6.2.' Generally, as bond length increases (due to increasing atomic size), bond strength (and thus bond enthalpy) decreases. For example, C-F bonds are typically stronger than C-I bonds. This is a common chemical principle that a NEET student is expected to know to interpret tabulated data.

According to the NCERT text, 'Further, 2'-OH group present at every nucleotide in RNA is a reactive group and makes RNA labile and easily degradable.' Conversely, DNA contains 2-deoxyribose, lacking this reactive 2'-OH group, which contributes to its chemical stability.

The summary states: 'While DNA stores genetic information, RNA mostly helps in transfer and expression of information.' Also, 'DNA is the chemical basis of heredity and have the coded message for proteins to be synthesised in the cell.'

The text states: 'RNA can directly code for the synthesis of proteins, hence can easily express the characters. DNA, however, is dependent on RNA for synthesis of proteins.'

The NCERT text explicitly states: 'In fact, the presence of thymine at the place of uracil also confers additional stability to DNA.'