JEE Chemistry Organic Chemistry Some Basic Principles and Techniques MCQs Set 06

Answer: (c) SO\(_3\)

Explanation:
1. SO\(_3\) has an electron-deficient sulfur and accepts an electron pair, so it is an electrophile; the others have lone pairs and act as nucleophiles.

Teacher's Note:
1. Key fact to remember: SO\(_3\) has an electron-deficient sulfur and accepts an electron pair, so it is an electrophile; the others have lone pairs and act as nucleophiles.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (a) NH\(_2^-\)

Explanation:
1. The amide ion has lone pairs and a negative charge, so it donates electrons. NO\(_2^+\) and :CCl\(_2\) are electrophiles, and the methyl radical is neutral and electron-deficient.

Teacher's Note:
1. Key fact to remember: The amide ion has lone pairs and a negative charge, so it donates electrons. NO\(_2^+\) and :CCl\(_2\) are electrophiles, and the methyl radical is neutral and electron-deficient.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) Carbocation

Explanation:
1. A carbocation carbon has three bonds and no lone pair, i.e. a sextet (three pairs). A carbanion has eight electrons and a free radical seven.

Teacher's Note:
1. Key fact to remember: A carbocation carbon has three bonds and no lone pair, i.e. a sextet (three pairs). A carbanion has eight electrons and a free radical seven.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (b) base as well as a nucleophile

Explanation:
1. It has a lone pair that can accept a proton (base) or attack an electron-poor centre (nucleophile).

Teacher's Note:
1. Key fact to remember: It has a lone pair that can accept a proton (base) or attack an electron-poor centre (nucleophile).
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (b) \( ^-\)CH\(_2\)NO\(_2\)

Explanation:
1. The nitro group stabilises the negative charge by its -I and -M effects. The alkyl groups in the other options destabilise carbanions (+I effect), making them less stable than CH\(_3^-\).

Teacher's Note:
1. Key fact to remember: The nitro group stabilises the negative charge by its -I and -M effects. The alkyl groups in the other options destabilise carbanions (+I effect), making them less stable than CH\(_3^-\).
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) -NH\(_3^+\) > -NO\(_2\) > -CN

Explanation:
1. A group with a full positive charge withdraws electrons most strongly, then the nitro group (N bearing two electronegative oxygens), then the cyano group.

Teacher's Note:
1. Key fact to remember: A group with a full positive charge withdraws electrons most strongly, then the nitro group (N bearing two electronegative oxygens), then the cyano group.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (a) HC\(\equiv\)C\(^-\)

Explanation:
1. Stability of a carbanion increases with the s-character of the carbon bearing the charge: sp (50%) > sp\(^2\) (33%) > sp\(^3\) (25%).

Teacher's Note:
1. Key fact to remember: Stability of a carbanion increases with the s-character of the carbon bearing the charge: sp (50%) > sp\(^2\) (33%) > sp\(^3\) (25%).
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (c) III > II > I

Explanation:
1. Greater s-character stabilises the negative charge: sp (III) > sp\(^2\) (II) > sp\(^3\) (I).

Teacher's Note:
1. Key fact to remember: Greater s-character stabilises the negative charge: sp (III) > sp\(^2\) (II) > sp\(^3\) (I).
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) CH\(_3\)COOH

Explanation:
1. The highest pK\(_a\) belongs to the weakest acid. Acetic acid is weaker than formic acid (no electron-donating methyl in HCOOH) and than the halo-substituted acids.

Teacher's Note:
1. Key fact to remember: The highest pK\(_a\) belongs to the weakest acid. Acetic acid is weaker than formic acid (no electron-donating methyl in HCOOH) and than the halo-substituted acids.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (a) Trifluoroacetic acid

Explanation:
1. Three fluorine atoms exert a very strong -I effect and stabilise the carboxylate ion most.

Teacher's Note:
1. Key fact to remember: Three fluorine atoms exert a very strong -I effect and stabilise the carboxylate ion most.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (c) -CH\(_3\)

Explanation:
1. Alkyl groups push electrons through the sigma bond. -OH, -OCH\(_3\) and -Cl withdraw electrons by the inductive effect.

Teacher's Note:
1. Key fact to remember: Alkyl groups push electrons through the sigma bond. -OH, -OCH\(_3\) and -Cl withdraw electrons by the inductive effect.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (b) CH\(_3\)CH\(_2\)CCl\(_2\)COOH

Explanation:
1. Two Cl atoms on the carbon next to COOH give the strongest -I effect, and the effect falls rapidly with distance. So this is the strongest acid.

Teacher's Note:
1. Key fact to remember: Two Cl atoms on the carbon next to COOH give the strongest -I effect, and the effect falls rapidly with distance. So this is the strongest acid.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (a) CH\(_3\)CH\(_2\)CH(Cl)COOH

Explanation:
1. The chlorine on the alpha carbon exerts a strong -I effect, giving pK\(_a\) about 2.8, lower than that of formic acid (3.75), the chloro acid with Cl on the gamma carbon and acetic acid.

Teacher's Note:
1. Key fact to remember: The chlorine on the alpha carbon exerts a strong -I effect, giving pK\(_a\) about 2.8, lower than that of formic acid (3.75), the chloro acid with Cl on the gamma carbon and acetic acid.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (c) HCOOH

Explanation:
1. The alkyl groups of the other acids push electrons and destabilise the carboxylate ion, making them weaker than acetic acid; formic acid has no alkyl group.

Teacher's Note:
1. Key fact to remember: The alkyl groups of the other acids push electrons and destabilise the carboxylate ion, making them weaker than acetic acid; formic acid has no alkyl group.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (b) CCl\(_3\)COOH

Explanation:
1. Three chlorine atoms on the alpha carbon produce the largest -I effect, making it the strongest acid and giving it the lowest pK\(_a\).

Teacher's Note:
1. Key fact to remember: Three chlorine atoms on the alpha carbon produce the largest -I effect, making it the strongest acid and giving it the lowest pK\(_a\).
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) p-methoxybenzyl cation, \( ^+ \)CH\(_2\)C\(_6\)H\(_4\)OCH\(_3\)

Explanation:
1. The methoxy group in the para position donates electrons by resonance (+M), delocalising the positive charge onto oxygen. Cl and NO\(_2\) are electron-withdrawing in these cations.

Teacher's Note:
1. Key fact to remember: The methoxy group in the para position donates electrons by resonance (+M), delocalising the positive charge onto oxygen. Cl and NO\(_2\) are electron-withdrawing in these cations.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (c) p-nitrobenzoic acid

Explanation:
1. Carboxylic acids are stronger than phenol, and the nitro group withdraws electrons by -I and -M effects, stabilising the carboxylate ion further.

Teacher's Note:
1. Key fact to remember: Carboxylic acids are stronger than phenol, and the nitro group withdraws electrons by -I and -M effects, stabilising the carboxylate ion further.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (a) \( ^-\)CH\(_2\)-\( ^+ \)CH-Cl (negative charge on one carbon, positive on the other)

Explanation:
1. The structure with opposite charges on adjacent carbons has neither an octet at the positive carbon nor a favourable charge on the electronegative atom. The structure with + on chlorine is the contributor that makes the C-Cl bond shorter.

Teacher's Note:
1. Key fact to remember: The structure with opposite charges on adjacent carbons has neither an octet at the positive carbon nor a favourable charge on the electronegative atom. The structure with + on chlorine is the contributor that makes the C-Cl bond shorter.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) benzene

Explanation:
1. Benzene has a fully delocalised cyclic system of six pi electrons and has the highest resonance energy (about 150 kJ/mol).

Teacher's Note:
1. Key fact to remember: Benzene has a fully delocalised cyclic system of six pi electrons and has the highest resonance energy (about 150 kJ/mol).
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (b) CH\(_2\)=CH-Cl

Explanation:
1. In vinyl chloride the lone pair of Cl delocalises into the double bond, giving the C-Cl bond partial double bond character, and the carbon is also sp\(^2\), so the bond is shortest.

Teacher's Note:
1. Key fact to remember: In vinyl chloride the lone pair of Cl delocalises into the double bond, giving the C-Cl bond partial double bond character, and the carbon is also sp\(^2\), so the bond is shortest.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (b) d < b < c < a

Explanation:
1. The ethyl cation has no resonance, so it is the least stable. The benzyl cation is stabilised by resonance, p-methyl adds +I and hyperconjugation, and p-methoxy adds strong +M.

Teacher's Note:
1. Key fact to remember: The ethyl cation has no resonance, so it is the least stable. The benzyl cation is stabilised by resonance, p-methyl adds +I and hyperconjugation, and p-methoxy adds strong +M.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) benzylamine, C\(_6\)H\(_5\)CH\(_2\)NH\(_2\)

Explanation:
1. In benzylamine the lone pair on nitrogen is not in conjugation with the ring, so it is more available than in the anilines.

Teacher's Note:
1. Key fact to remember: In benzylamine the lone pair on nitrogen is not in conjugation with the ring, so it is more available than in the anilines.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (a) IV > I > III > II

Explanation:
1. The +I and hyperconjugation of CH\(_3\) increases the electron density on N. The -M groups withdraw electron density: nitro more strongly than CHO, so p-nitroaniline is the weakest.

Teacher's Note:
1. Key fact to remember: The +I and hyperconjugation of CH\(_3\) increases the electron density on N. The -M groups withdraw electron density: nitro more strongly than CHO, so p-nitroaniline is the weakest.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (c) p-nitrobenzyl carbanion

Explanation:
1. The nitro group in the para position delocalises the negative charge by -M and -I effects. A methoxy group destabilises a carbanion.

Teacher's Note:
1. Key fact to remember: The nitro group in the para position delocalises the negative charge by -M and -I effects. A methoxy group destabilises a carbanion.
2. Revise this point with the matching definition or example in the chapter notes.

Answer: (d) tropylium cation (cycloheptatrienyl cation)

Explanation:
1. The tropylium cation is aromatic (planar, cyclic, 6 pi electrons) and is exceptionally stable; the cyclopentadienyl cation is antiaromatic (4 pi electrons).

Teacher's Note:
1. Key fact to remember: The tropylium cation is aromatic (planar, cyclic, 6 pi electrons) and is exceptionally stable; the cyclopentadienyl cation is antiaromatic (4 pi electrons).
2. Revise this point with the matching definition or example in the chapter notes.