BPSC PGT Chemistry Important Questions: Fundamental Concepts in Organic Reaction Mechanism

1. If the total number of bonds between two atoms is 3 the total number of resonating structures is 2 what is the bond order?
a) 0.5
b) 1.5
c) 2.5
d) 3.5

Answer: b
Explanation: Bond order is calculated using the formula:

$Bond order$ $=$ $Total number of bonds between two atoms/ Total number of resonating structures$ $$

In this case,
Thus, bond order is directly related to resonance, as shown by this equation.

2. Free radicals are formed during homolytic fission.
a) true
b) false

Answer: a
Explanation: Homolytic fission splits a covalent bond evenly, giving one electron to each atom. This produces free radicals, which are neutral, highly reactive, and usually formed in non-polar molecules under sunlight or high temperature.

3. What is the hybridization of singlet carbene?
a) sp
b) sp3
c) sp3d
d) sp2

Answer: d
Explanation: In a singlet carbene, the carbon atom is sp² hybridized. One of the hybrid orbitals holds a pair of electrons, while the unhybridized orbitals remain vacant. Singlet carbenes have a bent structure and are less stable compared to triplet carbenes.

4. In a molecule, when displacement of electron pair is away from the group it is ______________ electromeric effect.
a) zero
b) negative
c) positive
d) no

Answer: c
Explanation: The electromeric effect is the temporary polarization in a molecule with multiple bonds when an attacking reagent approaches. In this effect, the pair of π-electrons is completely shifted to one of the atoms. If the electron pair is displaced away from the atom or group, it is called the +E effect; if it shifts towards the atom or group, it is called the –E effect.

5. Which of the following is false regarding the inductive effect?
a) temporary effect
b) propagates through a carbon chain
c) permanent effect
d) Groups having a higher electron affinity than hydrogen show the negative inductive effect

Answer: a
Explanation: The inductive effect is a permanent effect that transmits through the carbon chain. Atoms or groups with greater electron affinity than hydrogen show the –I effect (electron-withdrawing), while those with lower affinity show the +I effect (electron-releasing). It can be understood as the shifting of shared electron pairs in polar covalent bonds.

6. Nucleophilic reagents behave as ______________
a) water
b) Lewis base
c) lewis acid
d) salt

Answer: b
Explanation: Nucleophilic reagents are electron-rich species and act as Lewis bases. They attack electron-deficient centers. For species with the same nucleophilic site, nucleophilicity generally increases with basicity.

7. Carbocations bear a ______________ charge.
a) no
b) negative
c) positive
d) 0

Answer: c
Explanation: Carbocations are formed during hydrolysis and consist of a carbon atom carrying a positive charge. Being electron-deficient, they have only 6 electrons in their valence shell. Carbocations are planar in shape, sp² hybridized, and possess an empty p-orbital.

8. Baker Nathan effect is related to ______________
a) inductive effect
b) electromeric effect
c) hyperconjugation
d) resonance

Answer: c
Explanation: Hyperconjugation is the delocalization of σ-electrons from a C–H bond of an alkyl group directly attached to an unsaturated system or to an atom with an adjacent π-orbital. This phenomenon is also known as no-bond resonance or the Baker–Nathan effect.

9. Resonating structures are also known as ______________ forms.
a) canonical
b) inductor
c) electromeric
d) nucleophilic

Answer: a
Explanation: When a single structure cannot represent all the properties of a molecule, two or more structures are drawn to describe it. These are called resonating structures or canonical forms, and the actual molecule is considered a resonance hybrid of them.

10. What is obtained by thermolysis of azides?
a) free radicals
b) carbocation
c) arene
d) nitrene

Answer: d
Explanation: Nitrenes are generated by the thermolysis of azides. They are neutral, monovalent nitrogen species in which the nitrogen atom carries two unshared electrons along with a single bond to a monovalent atom or group. Nitrenes are highly reactive, similar to carbenes.