21. Which of the following reactions is an example of an electrophilic addition?
A. Addition of HBr to propene
B. Nitration of benzene
C. Dehydration of alcohols
D. Esterification of carboxylic acids
Answer: A. Addition of HBr to propene
Explanation:
In the addition of HBr to propene, an electrophile (H⁺) adds to the carbon-carbon double bond, followed by nucleophilic attack by Br⁻. This is an example of electrophilic addition.
22. Which one of the following molecules can exhibit optical activity?
A. 2-Chloropropane
B. 2-Chlorobutane
C. 2-Chlorobutene
D. 1-Chlorobutane
Answer: B. 2-Chlorobutane
Explanation:
2-Chlorobutane has a chiral carbon (the second carbon), which is attached to four different groups, making it optically active.
23. Which one of the following reactions proceeds through a carbocation intermediate?
A. SN1 reaction
B. SN2 reaction
C. E2 reaction
D. Free radical substitution
Answer: A. SN1 reaction
Explanation:
The SN1 mechanism involves the formation of a carbocation intermediate in the rate-determining step.
24. In the reaction: CH₃CH₂CH₂Br + NaCN → CH₃CH₂CH₂CN, the mechanism followed is:
A. SN1
B. SN2
C. E1
D. E2
Answer: B. SN2
Explanation:
Primary alkyl halides, such as CH₃CH₂CH₂Br, typically undergo nucleophilic substitution via the SN2 mechanism, where the nucleophile (CN⁻) attacks the carbon bearing the leaving group from the opposite side.
25. Which of the following compounds will undergo nucleophilic substitution via an SN2 mechanism the slowest?
A. Methyl bromide
B. Ethyl bromide
C. Isopropyl bromide
D. Tert-butyl bromide
Answer: D. Tert-butyl bromide
Explanation:
The SN2 mechanism requires backside attack by the nucleophile, which is hindered in tertiary alkyl halides like tert-butyl bromide due to steric hindrance. Therefore, tert-butyl bromide reacts the slowest via the SN2 mechanism.
26. Which of the following reactions represents a nucleophilic addition mechanism?
A. Addition of HBr to ethene
B. Addition of CN⁻ to acetone
C. Bromination of benzene
D. Nitration of benzene
Answer: B. Addition of CN⁻ to acetone
Explanation:
Nucleophilic addition occurs in carbonyl compounds like acetone, where a nucleophile (CN⁻) adds to the electrophilic carbon of the carbonyl group.
27. Which of the following exhibits both geometrical and optical isomerism?
A. 2-Butanol
B. 2-Butene
C. 2-Chlorobutane
D. 3-Methylpent-2-ene
Answer: D. 3-Methylpent-2-ene
Explanation:
3-Methylpent-2-ene exhibits geometrical isomerism due to restricted rotation around the double bond, and optical isomerism due to the presence of a chiral center at the third carbon.
28. Which of the following is the major product when 2-bromopropane reacts with KOH in ethanol?
A. Propene
B. 1-Propanol
C. 2-Propanol
D. 2-Bromo-1-propene
Answer: A. Propene
Explanation:
In the presence of KOH in ethanol, elimination occurs via the E2 mechanism, and the major product is propene.
29. Which of the following represents an electrophile?
A. OH⁻
B. H₂O
C. NO₂⁺
D. CH₃O⁻
Answer: C. NO₂⁺
Explanation:
An electrophile is an electron-deficient species that can accept a pair of electrons. NO₂⁺ (nitronium ion) is an example of an electrophile as it is positively charged and electron-deficient.
30. Which of the following pairs of compounds are functional isomers?
A. Ethanol and Dimethyl ether
B. Propanal and Propanone
C. Butanal and Butanone
D. Ethanoic acid and Methyl methanoate
Answer: A. Ethanol and Dimethyl ether
Explanation:
Functional isomers have the same molecular formula but different functional groups. Ethanol (an alcohol) and dimethyl ether (an ether) have the same molecular formula (C₂H₆O) but different functional groups.
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