(a) Ethanolic solution of compound I on irradiation leads to the formation of compounds II, III and IV. The resulting reaction mixture exhibits bands at 1787, 1740, 1715 and 1685 cm⁻¹ for νC=O. Assign these C=O stretching frequencies to the correspon

Question

(a) Ethanolic solution of compound I on irradiation leads to the formation of compounds II, III and IV. The resulting reaction mixture exhibits bands at 1787, 1740, 1715 and 1685 cm⁻¹ for νC=O. Assign these C=O stretching frequencies to the corresponding compounds giving reasons : (15 marks)

(b) (i) 2,2-Dimethyl cyclopropanone undergoes ring opening when attacked by methoxide ion and the product obtained possesses the following spectral data :
IR (ν, cm⁻¹) : 1740, 1160
¹H NMR (δ) : 3·6 (3H, s), 1·2 (9H, s)
Mass (m/z) : 116, 85, 59, 31
Deduce the structure of the product with reasons. Write down the structure of another possible product. (10 marks)

(ii) Arrange the following compounds in the order of increasing coupling constant values (J_{Ha-Hb}) :
I
II
III (5 marks)

(c) (i) 2-Chloro-2,3-dimethyl butane on dehydrohalogenation can lead to the formation of two products. Explain how the two can be distinguished using ¹H NMR and IR spectral data. (10 marks)

(ii) The mass spectral data of diethyl ether is as under :
m/z 74, m/z 59, m/z 45, m/z 31, m/z 29
Explain the fragmentation pattern. (5 marks)

(iii) Using the following data, calculate the bond length of HCl :
I = 2·70×10⁻⁴⁷ kg m²
1 a.m.u. = 1·661×10⁻²⁷ kg (5 marks)

UPSC Answer Check · Accepted Answer

This question demands explanation of spectroscopic assignments, structural elucidation, and calculations across five sub-parts. Allocate approximately 30% time/words to part (a) [15 marks] covering Norrish Type I/II photochemistry and carbonyl stretching frequency assignments; 20% to (b)(i) [10 marks] for ring-opening mechanism and spectral interpretation; 10% to (b)(ii) [5 marks] for dihedral angle-coupling constant relationship; 20% to (c)(i) [10 marks] for elimination product distinction; and 20% combined to (c)(ii)-(iii) [10 marks] for fragmentation pattern and bond length calculation. Structure as: brief introduction on spectroscopic principles, systematic treatment of each sub-part with structures and reasoning, and concluding summary.
- Part (a): Assignment of 1787 cm⁻¹ to strained ketone (II), 1740 cm⁻¹ to ester (III), 1715 cm⁻¹ to ketone (IV), and 1685 cm⁻¹ to α,β-unsaturated ketone with explanation of ring strain, conjugation, and hydrogen bonding effects on νC=O
- Part (b)(i): Structure elucidation as methyl 3,3-dimethylbutanoate from IR (ester C=O and C-O), NMR (OCH₃ singlet, t-butyl singlet), and MS (m/z 116 molecular ion, McLafferty rearrangement to m/z 85); alternative product as methyl 2,2-dimethylpropanoate from nucleophilic attack at less hindered carbon
- Part (b)(ii): Correct order of J(Ha-Hb) based on Karplus equation: I (anti-periplanar, ~16-18 Hz) > III (gauche, ~2-4 Hz) > II (near 90°, ~0-1 Hz) or equivalent based on given structures
- Part (c)(i): Distinction between 2,3-dimethyl-2-butene (more substituted, IR weak/absent =C-H stretch, NMR vinylic H absent) and 2,3-dimethyl-1-butene (less substituted, IR =C-H stretch ~3100 cm⁻¹, NMR two vinylic H signals, coupling pattern)
- Part (c)(ii): Fragmentation pattern of diethyl ether: m/z 74 (M⁺•), m/z 59 (α-cleavage losing CH₃•), m/z 45 (α-cleavage losing C₂H₅•), m/z 31 (CH₂=OH⁺ base peak), m/z 29 (C₂H₅⁺)
- Part (c)(iii): Calculation of reduced mass μ = (1×35.5)/(1+35.5) × 1.661×10⁻²⁷ kg, then bond length r = √(I/μ) = 1.27 Å or equivalent correct calculation with proper unit conversion

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	25%	12.5	Demonstrates flawless understanding of IR frequency-structure correlations (ring strain, conjugation, H-bonding), Karplus dihedral-coupling relationship, and mass spectrometry fragmentation rules; correctly identifies all photochemical products and elimination regioisomers	Shows basic understanding of spectroscopic principles but misassigns one frequency or confuses coupling constant trends; minor errors in identifying products or fragmentation pathways	Fundamental misconceptions about IR frequency trends (e.g., assigns higher frequency to conjugated system), incorrect application of Karplus equation, or major errors in mass spectral interpretation
Mechanism / equation	20%	10	Clearly writes Norrish Type I/II photochemical mechanisms for (a), nucleophilic ring-opening mechanism with regioselectivity for (b)(i), E1/E2 elimination with Zaitsev/Saytzeff preference for (c)(i), and McLafferty rearrangement for (c)(ii); all electron movements shown with curved arrows	Mentions mechanisms without detailed arrow-pushing or misses regioselectivity explanation; incomplete description of photochemical pathways	No mechanisms provided or fundamentally incorrect electron flow; confuses photochemical pathways with thermal reactions
Numerical accuracy	15%	7.5	Part (c)(iii): Correct reduced mass calculation (μ = 1.626×10⁻²⁷ kg), proper unit conversion, accurate bond length determination (r ≈ 1.27-1.29 Å); all values with appropriate significant figures	Correct formula but arithmetic error in reduced mass or unit conversion; final answer within 10% of correct value	Incorrect formula (e.g., uses atomic mass directly without reduced mass), wrong unit conversion, or calculation errors yielding unrealistic bond length (<0.5 Å or >3 Å)
Diagram / structure	25%	12.5	All structures clearly drawn with proper stereochemistry: compound I (likely a ketone with γ-hydrogen), II-IV for (a), two possible ester products for (b)(i) with correct connectivity, both alkene products for (c)(i), and clear depiction of dihedral angles for (b)(ii); labeled with IUPAC names	Structures drawn but with minor errors in connectivity or missing stereochemical details; some structures implied rather than explicitly shown	Missing critical structures, incorrect connectivity (e.g., wrong ring size), or illegible drawings; no attempt to show alternative products
Application context	15%	7.5	Effectively integrates spectroscopic data for definitive structural proof; explains why specific technique distinguishes isomers; relates to real-world analytical chemistry applications (pharmaceutical quality control, forensic analysis)	Makes superficial connections between data and structure without explaining diagnostic value; limited discussion of practical relevance	Lists spectral data without interpretation or fails to explain how techniques distinguish products; no appreciation of spectroscopy in chemical analysis

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