41. Which of the following reactions occurs via a free radical mechanism?
A. Halogenation of alkanes
B. Addition of HBr to propene
C. Nitration of benzene
D. Esterification of alcohols
Answer: A. Halogenation of alkanes
Explanation:
The halogenation of alkanes proceeds via a free radical mechanism involving the generation of halogen radicals and their propagation through the reaction.
42. Which of the following is the correct order of reactivity towards nucleophilic substitution (SN1)?
A. Methyl > Ethyl > Isopropyl > Tert-butyl
B. Tert-butyl > Isopropyl > Ethyl > Methyl
C. Ethyl > Methyl > Tert-butyl > Isopropyl
D. Methyl > Tert-butyl > Isopropyl > Ethyl
Answer: B. Tert-butyl > Isopropyl > Ethyl > Methyl
Explanation:
The stability of the carbocation formed determines the reactivity in the SN1 mechanism. Tertiary carbocations are more stable than secondary, followed by primary and methyl carbocations.
43. Which of the following undergoes nucleophilic addition?
A. Benzene
B. Propanal
C. Ethene
D. 1-Butyne
Answer: B. Propanal
Explanation:
Carbonyl compounds like aldehydes (propanal) undergo nucleophilic addition reactions because the carbonyl carbon is electrophilic and susceptible to nucleophilic attack.
44. Which of the following reactions is an example of a condensation reaction?
A. Dehydration of alcohols
B. Hydrolysis of esters
C. Aldol reaction
D. Hydrogenation of alkenes
Answer: C. Aldol reaction
Explanation:
The Aldol reaction is a condensation reaction where two aldehydes or ketones combine, with the elimination of a small molecule, often water.
45. Which of the following exhibits tautomerism?
A. Propanal
B. Propanone
C. Butanal
D. 2-Pentanone
Answer: B. Propanone
Explanation:
Keto-enol tautomerism occurs in carbonyl compounds, including ketones like propanone, where the keto form is in equilibrium with the enol form.
46. Which of the following compounds exhibits meso isomerism?
A. 2,3-Dibromobutane
B. 2,3-Dichloropentane
C. 2-Butene
D. Propanoic acid
Answer: A. 2,3-Dibromobutane
Explanation:
Meso compounds have a plane of symmetry and are optically inactive despite having chiral centers. 2,3-Dibromobutane has two chiral centers but a plane of symmetry, making it a meso compound.
47. Which of the following is the most stable carbocation?
A. Methyl carbocation (CH₃⁺)
B. Ethyl carbocation (CH₃CH₂⁺)
C. Isopropyl carbocation (CH₃CH⁺CH₃)
D. Tert-butyl carbocation [(CH₃)₃C⁺]
Answer: D. Tert-butyl carbocation [(CH₃)₃C⁺]
Explanation:
The stability of carbocations increases with the number of alkyl groups attached to the positively charged carbon atom due to hyperconjugation and the inductive effect. The tertiary carbocation (tert-butyl) is the most stable.
48. Which of the following reactions proceeds via a carbocation intermediate?
A. SN1 reaction
B. SN2 reaction
C. E2 reaction
D. Free radical halogenation
Answer: A. SN1 reaction
Explanation:
The SN1 mechanism involves a two-step process where the first step is the formation of a carbocation intermediate, followed by nucleophilic attack.
49. Which of the following compounds undergoes nucleophilic substitution the fastest via the SN2 mechanism?
A. Methyl bromide
B. Ethyl bromide
C. Isopropyl bromide
D. Tert-butyl bromide
Answer: A. Methyl bromide
Explanation:
The SN2 mechanism is favored by less steric hindrance. Methyl bromide, with no bulky groups attached to the carbon bearing the leaving group, undergoes SN2 substitution the fastest.
50. Which of the following reactions follows anti-Markovnikov’s rule?
A. Addition of HBr to propene in the presence of peroxide
B. Addition of HCl to ethene
C. Hydration of propene
D. Addition of HBr to ethene
Answer: A. Addition of HBr to propene in the presence of peroxide
Explanation:
According to the anti-Markovnikov rule, in the presence of peroxides, HBr adds to an alkene in such a way that the bromine atom attaches to the less substituted carbon.
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