Organic Chemistry - Some Basic Principles and Techniques MCQs for UPSC Prelims
118 practice questions covering Organic Chemistry - Some Basic Principles and Techniques, organised into 2 topics.
118 questions include a written explanation.
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Assertion (A): Aniline undergoes electrophilic substitution much faster than benzene. Reason (R): The lone pair on the nitrogen of the $\text{-NH}_2$ group is delocalised into the ring, raising the electron density at the ortho and para positions.
ABoth A and R are true, and R is the correct explanation of A
BBoth A and R are true, but R is not the correct explanation of A
CA is true, but R is false
DA is false, but R is true
Show answer and explanation
Correct answer: A - Both A and R are true, and R is the correct explanation of A
Both statements are true and the reason explains the assertion. The nitrogen lone pair is donated into the ring by a $+M$ effect, which raises the ring's electron density and makes it a much better target for an electrophile. The extra density gathers at the ortho and para carbons, which is also why aniline yields mainly those products.
Q2
medium
Assertion (A): Cyclopentane is classified as an alicyclic compound.
Reason (R): Alicyclic compounds are cyclic compounds that contain a benzene ring.
ABoth A and R are true, and R explains A.
BBoth A and R are true, but R does not explain A.
CA is true, but R is false.
DA is false, but R is true.
Show answer and explanation
Correct answer: C - A is true, but R is false.
A is true: cyclopentane is a ring of carbon atoms with no aromatic character, which is what alicyclic means. R is false and describes aromatic compounds instead. Alicyclic compounds are rings that behave much like open chain aliphatic compounds, whereas a ring system built round benzene, such as toluene or naphthalene, is aromatic and not alicyclic.
Q3
medium
A chemist accidentally mixed an alcohol and a ketone. Identify the principal functional group using IUPAC priority rules between an alcohol (-OH) and a ketone (>C=O) when they are both present in a compound.
AAlcohol
BDepends on the number of functional groups present
CBoth have equal priority
DKetone
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Correct answer: D - Ketone
IUPAC seniority places the ketone above the alcohol, so where both are present the carbonyl is the principal characteristic group and the hydroxyl is cited as a hydroxy prefix. The alcohol therefore cannot be the principal group here. The hierarchy is fixed, so the two do not share equal priority, and it does not depend on how many groups of each kind the molecule carries.
Q4
easy
In the hyperconjugation effect, which of the following is true for the stability of alkenes?
AHyperconjugation has no effect on the stability of alkenes.
BHyperconjugation decreases the stability of alkenes.
CAlkenes with fewer alpha-hydrogens are more stable due to hyperconjugation.
DAlkenes with more alpha-hydrogens are more stable due to hyperconjugation.
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Correct answer: D - Alkenes with more alpha-hydrogens are more stable due to hyperconjugation.
Alkenes with more alpha-hydrogens are more stable because hyperconjugation allows the delocalization of electrons which increases the stability of the molecule. Fewer alpha-hydrogens reduce this effect and thus decrease stability.
Q5
medium
A hydrocarbon has the IUPAC name 4-ethyl-1,4-dimethylcyclohexane. Which of the following statements is true about its structure?
AIt has a six-membered ring with one ethyl group on the fourth carbon and two methyl groups on different carbons.
BThe parent chain is hexane.
CIt has a six-membered ring with two methyl groups on the first carbon.
DIt has a six-membered ring with one ethyl and two methyl groups on the fourth carbon.
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Correct answer: A - It has a six-membered ring with one ethyl group on the fourth carbon and two methyl groups on different carbons.
The correct statement is that the compound has a six-membered cyclohexane ring with an ethyl group on the fourth carbon and two methyl groups on different carbons (one on the first carbon and one on the fourth). 'The parent chain is hexane' ignores the cyclo form, and the other options misplace substituents on the ring.
Q6
easy
Which of the following pairs represents chain isomers?
Abut-1-ene and but-2-ene
Bn-butane and isobutane
Cethanol and dimethyl ether
Dacetaldehyde and acetone
Show answer and explanation
Correct answer: B - n-butane and isobutane
n-butane and isobutane are chain isomers because they differ in the branching of their carbon chains. But-1-ene and but-2-ene are position isomers, ethanol and dimethyl ether are functional isomers, and acetaldehyde and acetone are also functional isomers.
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