electronic-displacement-effects MCQs for UPSC Prelims

26 practice questions on electronic-displacement-effects from the Organic Chemistry - Some Basic Principles and Techniques section of the UPSC Prelims syllabus. 26 come with a written explanation. Try the sample set below - the answer stays hidden until you ask for it.

9 Easy 10 Medium 7 Hard

Sample questions

Q1
medium

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.

  1. A Both A and R are true, and R is the correct explanation of A
  2. B Both A and R are true, but R is not the correct explanation of A
  3. C A is true, but R is false
  4. D A is false, but R is true
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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
easy

In the hyperconjugation effect, which of the following is true for the stability of alkenes?

  1. A Hyperconjugation has no effect on the stability of alkenes.
  2. B Hyperconjugation decreases the stability of alkenes.
  3. C Alkenes with fewer alpha-hydrogens are more stable due to hyperconjugation.
  4. D Alkenes 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.

Q3
easy

How many resonance structures can be drawn for the nitrate ion (NO3-)?

  1. A 3
  2. B 4
  3. C 5
  4. D 2
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Correct answer: A - 3

The nitrate ion (NO3-) has three resonance structures because the negative charge can be delocalized over three equivalent oxygen atoms. Misconceptions may arise from not considering equivalent structures, leading students to undercount.

Q4
medium

A student dissolves chloroacetic acid and acetic acid in water. Which solution will have a higher pH?

  1. A Cannot be determined without concentrations
  2. B Solution of acetic acid
  3. C Solution of chloroacetic acid
  4. D Both solutions will have the same pH
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Correct answer: B - Solution of acetic acid

Chloroacetic acid is stronger than acetic acid due to its electron-withdrawing -Cl group, which stabilizes the carboxylate ion. Thus, the solution of acetic acid will have a higher pH, as it is less acidic. Students may mistakenly think chloroacetic acid has a higher pH due to its chlorine content.

Q5
easy

Calculate the pKa value of acetic acid if the acidity constant (Ka) is given as 1.8 x 10^-5. Use the formula pKa = -log(Ka).

  1. A 3.74
  2. B 4.74
  3. C 5.74
  4. D 6.74
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Correct answer: B - 4.74

Using the formula pKa = -log(Ka), with Ka = 1.8 x 10^-5, the pKa is calculated as -log(1.8 x 10^-5) which is approximately 4.74. The distractors involve common errors in calculating logarithms or misapplying the pKa formula.

Q6
hard

Assertion (A): The electromeric effect is a temporary effect occurring in the presence of an attacking reagent. Reason (R): Electrons are permanently displaced within a molecule during the electromeric effect.

  1. A Both A and R are true, and R is the correct explanation of A.
  2. B Both A and R are true, but R is not the correct explanation of A.
  3. C A is true, but R is false.
  4. D A is false, but R is true.
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Correct answer: C - A is true, but R is false.

The electromeric effect is indeed a temporary effect that occurs only in the presence of an attacking reagent, making the assertion true. However, the reason is incorrect because the displacement of electrons is not permanent; it is temporary and reversible.

Q7
hard

Assertion (A): Hyperconjugation increases the stability of a carbocation. Reason (R): Hyperconjugation is the overlap of a lone pair on an atom next to the positive carbon with its empty $p$ orbital.

  1. A Both A and R true, R explains A
  2. B Both A and R true, R does not explain A
  3. C A true, R false
  4. D A false, R true
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Correct answer: C - A true, R false

A is true. R is false, because that description belongs to resonance, the $+M$ effect. Hyperconjugation uses the sigma electrons of a $\text{C-H}$ bond on an adjacent carbon, not a lone pair; those bonding electrons overlap with the empty $p$ orbital and spread the positive charge. This is why the number of alpha hydrogens, not the number of lone pairs, sets how much hyperconjugation is possible.

Q8
medium

In the context of resonance, which of the following factors primarily determines the stability of a resonance structure?

  1. A The number of unpaired electrons
  2. B The overall molecular weight of the compound
  3. C The presence of a complete octet in the structure
  4. D The presence of a positive charge on an electronegative atom
Show answer and explanation

Correct answer: C - The presence of a complete octet in the structure

The stability of a resonance structure is primarily determined by the presence of a complete octet for the atoms in the structure. Structures with a complete octet are more stable. A positive charge on an electronegative atom or unpaired electrons typically decrease stability, and molecular weight is not a direct factor in resonance stability.

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