The compound A, acetal of acetaldehyde, on reaction with B, under given reaction conditions, yields C and the final product D :

(i) Write down the structures of the products C and D. Propose a mechanism with suitable explanation.

(ii) Write down th

Question

The compound A, acetal of acetaldehyde, on reaction with B, under given reaction conditions, yields C and the final product D :

(i) Write down the structures of the products C and D. Propose a mechanism with suitable explanation.

(ii) Write down the product(s) if ortho-ester E is used instead of the compound A under the similar reaction conditions :

UPSC Answer Check · Accepted Answer

The directive 'explain' demands clear reasoning with mechanistic detail. Structure your answer as: (i) Identify A as acetaldehyde dimethyl acetal (1,1-dimethoxyethane) and B as a Grignard reagent or nucleophile; propose acid-catalyzed cleavage leading to hemiacetal intermediate C and final alcohol D after nucleophilic addition (~60% effort). (ii) For ortho-ester E (triethyl orthoacetate or similar), show analogous cleavage yielding different product distribution due to three OR groups (~40% effort). Use curved arrows throughout and label all intermediates.
- For (i): Correct identification of A as acetaldehyde dimethyl acetal (CH₃CH(OCH₃)₂) and recognition that B is a Grignard reagent (RMgX) or organolithium reagent acting under acidic workup conditions
- For (i): Structure of C as the hemiacetal intermediate (CH₃CH(OH)(OCH₃)) formed after protonation and loss of one methoxy group; structure of D as the tertiary alcohol (CH₃CH(OH)R) from nucleophilic addition
- For (i): Detailed acid-catalyzed mechanism showing protonation of acetal oxygen, departure of methanol as leaving group, formation of oxonium ion, nucleophilic attack by R⁻ from Grignard reagent, and acidic workup
- For (ii): Recognition that ortho-ester E (e.g., triethyl orthoacetate CH₃C(OCH₂CH₃)₃) undergoes similar acid-catalyzed cleavage but produces ethyl ester and ethanol as products instead of hemiacetal/alcohol pathway
- For (ii): Explanation that ortho-esters have three alkoxy groups leading to different reactivity—two equivalents of ethanol eliminated to form ester rather than stable hemiacetal intermediate
- Comparative insight: Both reactions demonstrate protection/deprotection strategy in organic synthesis; acetals protect aldehydes while ortho-esters serve as acyl anion equivalents or protecting groups for carboxylic acids

Dimension	Weight	Max marks	Excellent	Average	Poor
Concept correctness	20%	4	Correctly identifies A as acetaldehyde dimethyl acetal, recognizes B as Grignard/organometallic reagent, distinguishes hemiacetal (C) from final alcohol (D), and correctly identifies ortho-ester E reactivity with three alkoxy groups; no conceptual confusion between acetal and ketal or ortho-ester chemistry	Correctly identifies most structures but confuses hemiacetal with acetal or misidentifies ortho-ester behavior; minor errors in recognizing the role of B or the nature of intermediates	Fundamental errors such as identifying A as a ketal, confusing acetal hydrolysis with reduction, or treating ortho-ester identical to acetal; demonstrates lack of understanding of protecting group chemistry
Mechanism / equation	25%	5	Complete stepwise mechanism for (i) with all intermediates (protonated acetal, oxonium ion, tetrahedral intermediate) and curved arrows; for (ii) shows ortho-ester protonation, sequential ethanol loss, and ester formation; explains regioselectivity and driving forces	Mechanism mostly correct but missing key intermediates or electron flow details; ortho-ester mechanism incomplete or confused with acetal pathway; some steps implied rather than shown	Mechanism absent or fundamentally flawed; incorrect arrow pushing, wrong leaving groups, or impossible intermediates; no distinction between the two reaction pathways
Numerical accuracy	10%	2	Correct stoichiometry for both reactions: 1:1:1:1 ratio for acetal cleavage and nucleophilic addition; recognizes two equivalents of ethanol from ortho-ester; accurate molecular formulas if implied	Minor stoichiometric errors or missing coefficients; correct products but unbalanced equations	Major stoichiometric errors or completely absent quantitative treatment; incorrect product ratios
Diagram / structure	25%	5	Clear, unambiguous structural drawings of A, B (general or specific), C, D, and E with correct connectivity; stereochemistry shown where relevant; neat skeletal or condensed structures with all functional groups labeled; separate clear diagrams for both parts	Structures mostly correct but some ambiguity in drawing (e.g., missing hydrogens, unclear connectivity); one structure incorrect or missing; ortho-ester structure poorly represented	Structures largely incorrect or missing; confused line-angle drawings; impossible bonding patterns; no visual distinction between acetal and ortho-ester architectures
Application context	20%	4	Explicitly connects to synthetic utility: acetals as carbonyl protecting groups in multi-step synthesis (e.g., steroid synthesis, natural product synthesis like pheromones); ortho-esters as masked acyl anions or for ester protection; mentions Indian context such as application in pharmaceutical synthesis (e.g., anti-malarials, anti-cancer drugs developed by Indian research institutions)	Brief mention of protecting group utility without specific examples; generic reference to organic synthesis without Indian or contemporary relevance	No application context provided; fails to recognize synthetic significance; irrelevant or incorrect applications cited

Q3

Directive word: Explain

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