Vidaara.orgClass 11 · Chemistry
CodeVID-C11-12-T2-01
Assignment — Electronic Effects & Reaction Mechanisms
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
A carbanion is a species with carbon bearing:
- A.a positive charge
- B.a negative charge and a lone pair
- C.an unpaired electron
- D.no charge
2.
Which group shows a $−I$ (electron-withdrawing) effect?
- A.$−CH_3$
- B.$−C_2H_5$
- C.$−NO_2$
- D.$−CH(CH_3)_2$
3.
Free-radical stability order is:
- A.$1^\circ > 2^\circ > 3^\circ$
- B.$3^\circ > 2^\circ > 1^\circ$
- C.$2^\circ > 3^\circ > 1^\circ$
- D.all equal
4.
Dehydration of ethanol to ethene is an example of:
- A.addition
- B.substitution
- C.elimination
- D.rearrangement
5.
The electromeric effect is:
- A.permanent
- B.temporary, reagent-induced
- C.a $\sigma$ effect
- D.seen only in alkanes
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Distinguish between a nucleophile and an electrophile with one example each.
7.
What is hyperconjugation? How does it affect carbocation stability?
8.
Differentiate inductive and resonance effects.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Arrange in increasing stability and justify: $CH_3^+$, $(CH_3)_2CH^+$, $(CH_3)_3C^+$.
10.
Classify each reaction: (i) $CH_3CH_2OH \xrightarrow{conc.\ H_2SO_4} CH_2=CH_2$; (ii) $CH_2=CH_2 + HBr \rightarrow CH_3CH_2Br$; (iii) $CH_4 + Cl_2 \xrightarrow{h\nu} CH_3Cl$.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Explain the four electronic displacement effects (inductive, resonance, electromeric, hyperconjugation), stating for each whether it is permanent or temporary.
Answer Key
Section A — Multiple Choice Questions
- (B) a negative charge and a lone pair
- (C) $−NO_2$
- (B) $3^\circ > 2^\circ > 1^\circ$
- (C) elimination
- (B) temporary, reagent-induced
Section B — Short Answer (2 marks)
- A nucleophile is electron-rich and donates an electron pair (e.g. $OH^-$); an electrophile is electron-deficient and accepts an electron pair (e.g. $NO_2^+$).
- Hyperconjugation is delocalisation of $\sigma(C−H)$ electrons of an alkyl group into an adjacent empty $p$/$\pi$ orbital. More $\alpha$-hydrogens means more hyperconjugation and greater carbocation stability.
- The inductive effect is a permanent polarisation of $\sigma$-bonds transmitted through the chain and falling off with distance; the resonance (mesomeric) effect is delocalisation of $\pi$/lone-pair electrons over a conjugated system and is generally stronger.
Section C — Short Answer (3 marks)
- $CH_3^+ < (CH_3)_2CH^+ < (CH_3)_3C^+$. Stability rises with the number of electron-releasing alkyl groups (+I) and $\alpha$-C−H bonds available for hyperconjugation, which is greatest for the tertiary cation.
- (i) elimination (dehydration); (ii) addition (across the double bond); (iii) substitution (free-radical).
Section D — Long Answer (5 marks)
- Inductive (I): permanent polarisation of $\sigma$-bonds by an electronegative atom, transmitted through the chain and decreasing with distance (+I for alkyl, −I for $−NO_2$, $−Cl$). Resonance/mesomeric (M): permanent delocalisation of $\pi$/lone-pair electrons over a conjugated system, represented by resonance structures; usually the strongest effect. Electromeric (E): a temporary, complete shift of a $\pi$-electron pair to one atom only in the presence of an attacking reagent, vanishing when the reagent is removed. Hyperconjugation: permanent delocalisation of $\sigma(C−H)$ electrons of an alkyl group into an adjacent empty $p$/$\pi$ orbital (no-bond resonance), increasing stability with more $\alpha$-hydrogens.
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