Chemistry 2024 Paper II 50 marks Explain

Q3

(a) (i) Predict the reaction mechanism and the major product formed when methyl cyanide is heated with dilute hydrochloric acid. (10 marks) (ii) Addition of bromine to cis-2-butene gives racemic-2,3-dibromobutane while trans-2-butene yields meso-2,3-dibromobutane. Justify. (5 marks) (b) (i) Explain the formation of enone in the following reaction : (5 marks) (ii) Classify the following sigmatropic rearrangement and comment if it is symmetry-allowed or symmetry-forbidden : (5 marks) (iii) Write the structure of the major product formed in the following reaction : (5 marks) (c) (i) Consider the following conversion : Show that the above conversion involves the formation of nitrene intermediate. Draw the orbital pictures of singlet and triplet states of nitrene. (10 marks) (ii) Account for the following : (1) Indole undergoes electrophilic substitution to give 3-substituted product, but not 2-substituted product. (2) Chlorobenzene does not undergo nucleophilic substitution reaction readily. (10 marks)

हिंदी में प्रश्न पढ़ें

(a) (i) मेथिल साइनाइड को तनु हाइड्रोक्लोरिक अम्ल के साथ गर्म करने पर बनने वाले मुख्य उत्पाद तथा क्रियाविधि का अनुमान लगाइए। (10 अंक) (ii) ब्रोमीन, सिस-2-ब्यूटीन के साथ योगज करके रेसिमिक-2,3-डाइब्रोमोब्यूटेन देता है, जबकि ट्रांस-2-ब्यूटीन के साथ मीसो-2,3-डाइब्रोमोब्यूटेन देता है। औचित्य सिद्ध कीजिए। (5 अंक) (b) (i) निम्नलिखित अभिक्रिया में ईनोन के बनने की व्याख्या कीजिए : (5 अंक) (ii) निम्नलिखित सिग्मानुवर्ती पुनर्विन्यास को वर्गीकृत कीजिए और यह टिप्पणी कीजिए कि यह सममिति-अनुमत है या सममिति-वर्जित : (5 अंक) (iii) निम्नलिखित अभिक्रिया में बनने वाले मुख्य उत्पाद की संरचना लिखिए : (5 अंक) (c) (i) निम्नलिखित रूपांतरण पर विचार कीजिए : दिखाइए कि उपर्युक्त रूपांतरण में मध्यवर्ती नाइट्रीन बनता है। नाइट्रीन की एकक व त्रिक अवस्थाओं के कक्षीय चित्र दर्शाइए। (10 अंक) (ii) निम्नलिखित के कारण दीजिए : (1) इंडोल, इलेक्ट्रोनार्गी प्रतिस्थापन द्वारा 3-प्रतिस्थापित उत्पाद देता है, लेकिन 2-प्रतिस्थापित उत्पाद नहीं। (2) क्लोरोबेंजीन की नाभिकरागी प्रतिस्थापन अभिक्रिया आसानी से नहीं होती है। (10 अंक)

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Directive word: Explain

This question asks you to explain. The directive word signals the depth of analysis expected, the structure of your answer, and the weight of evidence you must bring.

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How this answer will be evaluated

Approach

Begin with a brief introduction acknowledging the diversity of reaction mechanisms covered. For part (a), spend approximately 25% time on hydrolysis mechanism and 15% on bromine addition stereochemistry. For part (b), allocate 15% each across the three sub-parts covering enone formation, sigmatropic classification, and product prediction. For part (c), devote 30% to nitrene chemistry with orbital diagrams and 15% to indole/aryl halide reactivity explanations. Conclude with a synthesis statement on how mechanistic understanding enables predictive organic synthesis.

Key points expected

  • (a)(i) Acid hydrolysis of nitriles: nucleophilic addition-elimination mechanism via amide intermediate, final product acetic acid; conditions specify dilute HCl and heating
  • (a)(ii) Anti-addition of Br₂ to alkenes via cyclic bromonium ion; cis-alkene gives racemic mixture, trans-alkene gives meso compound due to stereospecific ring opening
  • (b)(i) Enone formation via aldol condensation or related mechanism (Robinson annulation or simple dehydration of β-hydroxy ketone)
  • (b)(ii) [3,3]-sigmatropic rearrangement (Claisen or Cope type) with Woodward-Hoffmann analysis showing symmetry-allowed thermal [4n+2] process
  • (b)(iii) Structure prediction requiring application of named reaction or pericyclic/selectivity principles
  • (c)(i) Nitrene generation from azide thermolysis or photolysis; singlet (paired electrons, sp², empty p-orbital) vs triplet (two unpaired electrons, sp, linear) orbital diagrams
  • (c)(ii)(1) Indole electrophilic substitution at C-3 due to preferred attack on pyrrole ring preserving benzene aromaticity; C-2 attack disrupts both rings' aromaticity
  • (c)(ii)(2) Chlorobenzene: sp² carbon, resonance stabilization of lone pair, partial double bond character, poor leaving group Cl⁻ without electron-withdrawing groups

Evaluation rubric

DimensionWeightMax marksExcellentAveragePoor
Concept correctness25%15Demonstrates precise understanding of nitrile hydrolysis, anti-addition stereochemistry, sigmatropic selection rules, nitrene spin states, and heterocyclic/aryl halide reactivity; correctly identifies [3,3]-sigmatropic nature and explains why C-3 substitution dominates in indoleShows basic familiarity with most concepts but confuses syn/anti addition, misidentifies sigmatropic order, or provides incomplete explanation for indole regioselectivityFundamental errors such as claiming syn-addition of bromine, misclassifying sigmatropic rearrangement, or stating chlorobenzene undergoes easy SN2 substitution
Mechanism / equation25%15Writes complete stepwise mechanisms for nitrile hydrolysis (addition of water, tetrahedral intermediate, amide formation, further hydrolysis), bromonium ion formation with proper curved arrows, and nitrene generation; includes all intermediates and electron flowShows major mechanistic steps but misses intermediates (e.g., omits amide in nitrile hydrolysis) or has minor arrow-pushing errors; mechanisms generally correct but incompleteMechanisms with major errors, missing crucial steps, incorrect arrow directions, or confusion between SN1/SN2/E1/E2 and addition mechanisms
Numerical accuracy10%6Correctly applies stereochemical descriptors (R/S, E/Z, meso/racemic), accurately counts electrons in pericyclic analysis, and properly assigns molecular orbital symmetries for Woodward-Hoffmann rulesMinor errors in stereochemical assignments or electron counting; generally correct but inconsistent application of systematic nomenclatureMajor errors in stereochemistry (confusing meso with racemic), incorrect electron counts for pericyclic reactions, or inability to apply selection rules
Diagram / structure25%15Clear, well-drawn structures showing stereochemistry (wedge/dash for bromonium products), orbital diagrams for singlet (sp² with empty p) and triplet (sp with two singly occupied p) nitrene with proper electron representation, and accurate resonance structures for chlorobenzeneStructures generally recognizable but stereochemistry poorly represented, orbital diagrams incomplete or lacking electron distinction, or missing resonance formsPoorly drawn structures, missing stereochemical detail essential for the question, no orbital diagrams, or chemically impossible representations
Application context15%9Connects mechanisms to synthetic utility (nitrile hydrolysis in pharmaceutical synthesis like ibuprofen production, stereospecific bromination in natural product synthesis, nitrene chemistry in C-H amination), and relates indole chemistry to biologically relevant compounds like tryptophanBrief mention of applications without elaboration; some connection to real-world relevance but not well-integrated with mechanistic explanationsNo application context provided; purely theoretical treatment without recognition of practical significance in industrial or biological chemistry

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