Paper II — Q3
(a) (i) Write the structure of the product formed in the following reactions: (1) (2) (10 marks) (ii) Describe the synthesis of…
Write the structure of the product formed in the following reactions: (1) (2) 10 marks
Describe the synthesis of ketone (A) when only isobutanol is available as the starting material: 5 marks
Write the steps involved in the following conversion: 10 marks
Elucidate the structure of C and D in the following conversion: 5 marks
Provide the structure of missing reagents and intermediates in the following synthesis: [diagram] 10 marks
Write the structure of the product formed and the steps involved in the following reaction: [diagram] 10 marks
हिंदी में प्रश्न पढ़ें
निम्नलिखित अभिक्रियाओं में बनने वाले उत्पाद की संरचना लिखिए : (1) (2) 10 अंक
कीटोन (A) के संश्लेषण का वर्णन कीजिए जब केवल आइसोब्यूटेनॉल प्रारंभिक पदार्थ के रूप में उपलब्ध हो : 5 अंक
निम्नलिखित रूपांतरण में सम्मिलित चरणों को लिखिए : 10 अंक
निम्नलिखित रूपांतरण में C तथा D की संरचना को स्पष्ट कीजिए : 5 अंक
निम्नलिखित संश्लेषण में लुप्त अभिकर्मकों तथा मध्यवर्तियों की संरचना उपलब्ध कराइए : [diagram] 10 अंक
निम्नलिखित अभिक्रिया में बने उत्पाद की संरचना तथा सम्मिलित चरणों को लिखिए : [diagram] 10 अंक
The figure this question refers to, in words
The question paper is a scan and the diagram did not survive as text. This is the figure as read from the original page — every component, value and label — so the question can be worked from the text below.
(a) A chemical structure labeled 'A' is shown. It is a ketone with the structure: (CH3)2CH-CH2-C(=O)-CH(CH3)2. The molecule consists of a central carbonyl group (C=O) flanked by two isobutyl groups. The text above it asks to describe the synthesis of this ketone (A) when only isobutanol is available as the starting material.
(b) A reaction scheme involving an unknown compound C (molecular formula C8H12). Compound C reacts with 1) O3 and 2) HOAc to form a dicarboxylic acid: HOOC-(CH2)6-COOH (suberic acid). Compound C also reacts with H2/Pt to form an unknown compound D (molecular formula C8H16). The question asks to elucidate the structures of C and D.
(c) A reaction scheme starting from a central ketone molecule: 1-cyclohexylethanone (a cyclohexane ring attached to a carbonyl group, which is attached to a methyl group). The reaction splits into two paths. Path 1 (left): Reagents are 1) LDA/THF, 2) D2O, leading to product Q. Path 2 (right): Reagents are 1) Excess NaOH, 2) D2O, leading to product P.
(c(i)) A reaction scheme showing a multi-step synthesis. The starting material is 3-methyl-1-butanol (a primary alcohol with a methyl branch on the beta-carbon). An arrow labeled 'M' points to an intermediate aldehyde, 3-methylbutanal. A second arrow labeled 'N' points to an intermediate labeled 'O'. A third arrow labeled 'P' points to the final product, 1-amino-2-methyl-1-butanol (a primary amine with a hydroxyl group on the adjacent carbon and a methyl branch on the beta-carbon).
(c(ii)) A reaction scheme showing a bicyclic organic molecule. The structure is a norbornane derivative (bicyclo[2.2.1]heptane skeleton) with a ketone group (C=O) and a diazonium group (N2+) attached to the same carbon atom. An arrow points to the right with reaction conditions 'hv' (light) and 'MeOH' (methanol) written above and below the arrow, respectively. The product is indicated by a question mark.
Model answer
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(a) (i) Write the structure of the product formed in the following reactions:
1. 2-Bromocyclohexanone with Methoxide The reaction of 2-bromocyclohexanone with methoxide (MeO⁻) proceeds via the Favorskii rearrangement. The base abstracts the acidic proton at C-6 to form an enolate, which undergoes intramolecular displacement of the bromide ion to form a bicyclic cyclopropanone intermediate. Methoxide then adds to the cyclopropanone carbonyl, and the tetrahedral intermediate cleaves a ring C–C bond to generate the more stable carbanion. Protonation yields the ring-contracted product, methyl cyclopentanecarboxylate. This process is distinct from a simple S_N2 substitution on the α-bromo ketone.
2. cis-Pent-2-ene with Lwowski Reagent The reaction of *cis*-pent-2-ene with N-(4-nitrobenzenesulfonyloxy)urethane and NEt₃ is a Lwowski aziridination. The base induces α-elimination to generate an ethoxycarbonylnitrene (:NCO₂Et). This singlet nitrene adds to the alkene stereospecifically, retaining the cis geometry of the starting material. The product is ***cis*-2-ethyl-3-methyl-N-(ethoxycarbonyl)aziridine**. The use of excess reagent ensures that aziridination is favored over competing C–H insertion reactions.
(a) (ii) Describe the synthesis of ketone (A) when only isobutanol is available as the starting material:
The target ketone (A) is 2,6-dimethylheptan-4-one (diisobutyl ketone). Since both halves of the molecule are isobutyl groups, the synthesis begins by converting isobutanol into two different derivatives. First, a portion of isobutanol is converted to isobutyl bromide using PBr₃, which is then treated with magnesium in dry ether to form isobutylmagnesium bromide. A second portion of isobutyl bromide is treated with NaCN in DMSO to yield 3-methylbutanenitrile. The Grignard reagent is added to the nitrile to form an imine salt, which upon hydrolysis with H₃O⁺ yields the target ketone. This route is preferred over using acid chlorides, as the Grignard reagent stops at the ketone stage when reacting with a nitrile.
(b) (i) Write the steps involved in the following conversion:
The conversion of *tert*-butyl chloroacetate and ethyl acrylate to a cyclopropane-1,2-dicarboxylate is a Michael-Initiated Ring Closure (MIRC). First, t-BuOK deprotonates the α-carbon of *tert*-butyl chloroacetate to generate a nucleophilic enolate. This enolate undergoes a Michael addition to the β-carbon of ethyl acrylate, forming a new carbon-carbon bond and a stabilized enolate. The resulting enolate then performs an intramolecular 3-exo-tet displacement of the chloride ion. This ring closure produces the cyclopropane ring, yielding a mixed *tert*-butyl/ethyl cyclopropane-1,2-dicarboxylate. The reaction typically favors the trans isomer due to steric minimization during the ring closure.
(b) (ii) Elucidate the structure of C and D in the following conversion:
The starting material C has the molecular formula C₈H₁₂. Ozonolysis of C followed by treatment with acetic acid (HOAc) yields a single diacid product, HO₂C(CH₂)₆CO₂H. C₈H₁₂ has three degrees of unsaturation (one ring plus a C≡C), and oxidative cleavage of a triple bond gives two –COOH groups in one molecule, so C must be a cyclic alkyne. The eight-carbon chain in the product confirms that C is cyclooctyne. The second pathway involves the reduction of C using H₂/Pt. This catalytic hydrogenation saturates the triple bond, converting the alkyne into the corresponding alkane. Therefore, D is cyclooctane (C₈H₁₆). A cyclic diene of the same formula, such as 1,5-cyclooctadiene, would have given two molecules of succinic acid, which is inconsistent with the single C₈ diacid obtained.
(c) (i) Provide the structure of missing reagents and intermediates in the following synthesis:
The synthesis begins with the oxidation of 3-methylbutan-1-ol to the corresponding aldehyde, 3-methylbutanal. Reagent M is a mild oxidant such as PCC (pyridinium chlorochromate) or DMP. The aldehyde then reacts with reagent N, which is NaCN in the presence of a weak acid (or TMSCN), to form the cyanohydrin intermediate O. The structure of O is 2-hydroxy-4-methylpentanenitrile, (CH₃)₂CHCH₂CH(OH)CN. Finally, reagent P is a reducing agent such as LiAlH₄ or H₂/Ni, which reduces the nitrile group to a primary amine. The final product is 1-amino-4-methylpentan-2-ol. This sequence effectively extends the carbon chain by one unit while introducing an amino group.
(c) (ii) Write the structure of the product formed and the steps involved in the following reaction:
The reaction of a α-diazoketone with methanol under photochemical conditions (hν) is a Wolff rearrangement. The first step involves the photochemical loss of nitrogen gas (N₂) to generate a ketocarbene intermediate. This carbene is unstable and undergoes a 1,2-shift (migration of the adjacent carbon group) to form a ketene. In this specific case, the migration results in a ring contraction of the bicyclic system. The ketene is then trapped by the nucleophilic methanol, which adds across the C=C double bond of the ketene. The final product is the ring-contracted methyl ester. This reaction is a powerful method for homologating carboxylic acids or performing ring contractions in complex polycyclic frameworks.
How this answer will be evaluated
What earns full marks: All parts answered with correct structures, mechanisms, and reasoning. Named reactions identified. Reagents and conditions specified.
Evaluation rubric
Each sub-part is marked on its own, against the marks and word limit printed on the paper.
- (a)(i) Draw the structures of the products for the two given reactions. 10 marks
explain— definition/context → points in order → small example → short close
Must cover
- (1) 2-bromocyclohexanone with methoxide: Favorskii rearrangement; enolate at C-6, intramolecular displacement of bromide to the bicyclic cyclopropanone, methoxide opens it to the more stable carbanion; product methyl cyclopentanecarboxylate (ring contraction)
- (2) cis-pent-2-ene with N-(4-nitrobenzenesulfonyloxy)urethane and NEt3 (Lwowski reagent): base generates ethoxycarbonylnitrene (:N–CO2Et), which adds to the alkene stereospecifically as the singlet; product cis-2-ethyl-3-methyl-N-(ethoxycarbonyl)aziridine, cis geometry retained
- Mechanism sketched for both: cyclopropanone intermediate; nitrene formation by α-elimination
- State why the excess reagent is used (nitrene also inserts into C–H; excess drives aziridination)
Loses marks
- Writing a β-keto ester or a nitro compound
- Losing the cis relationship in the aziridine
- Simple SN2 by methoxide on the α-bromo ketone
Earns more
- Alternative for (1): the semibenzilic mechanism where enolisation is blocked, and why it is not needed here
- Note the singlet/triplet nitrene distinction and what a triplet would give (loss of stereochemistry)
Extra mark
- Named reaction identification
- Reagent and conditions
- (a)(ii) Describe the synthesis of ketone (A) from isobutanol. 5 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- Target A is 2,6-dimethylheptan-4-one (diisobutyl ketone), a symmetrical ketone with two isobutyl groups, so both halves come from isobutanol
- Isobutanol → isobutyl bromide (PBr3 or HBr) → isobutylmagnesium bromide (Mg, dry ether)
- Isobutyl bromide → 3-methylbutanenitrile (NaCN, DMSO); the Grignard adds to the nitrile, and hydrolysis of the imine salt gives the ketone
- Alternative accepted: isobutyl bromide → NaCN → hydrolysis to 3-methylbutanoic acid → acid chloride → with the Grignard via the cadmium/Gilman reagent, or the thioester/Weinreb amide; or isobutanol → isobutyraldehyde → Grignard → secondary alcohol → oxidation gives a different (wrong) skeleton, so must be avoided
Loses marks
- A Grignard with formaldehyde (gives the wrong chain length)
- Routes that yield diisopropyl ketone or 2,6-dimethylheptan-3-one
- Reagents named without conditions
Earns more
- Explain why Grignard + nitrile stops at the ketone (imine salt) while Grignard + acid chloride overshoots
- Carbon count and disconnection shown at the start
Extra mark
- Alternative synthetic routes
- Purification methods
- (b)(i) Write the steps involved in the given conversion. 10 marks
explain— definition/context → points in order → small example → short close
Must cover
- tBuOK deprotonates tert-butyl chloroacetate at the α-carbon to give the α-chloro ester enolate
- Michael addition of the enolate to ethyl acrylate gives the ester enolate three carbons away
- That enolate displaces chloride intramolecularly (3-exo-tet ring closure): a Michael-initiated ring closure, giving the cyclopropane-1,2-dicarboxylate (tert-butyl and ethyl esters), mainly trans
- Arrows for each step and the reason a cyclopropane rather than open-chain product forms
Loses marks
- Carbene or Simmons–Smith mechanisms
- Ester exchange as the main event
- Missing the Michael step
Earns more
- Name the sequence (MIRC / cyclopropanation via 1,3-elimination)
- Comment on trans selectivity
Extra mark
- Named reaction identification
- Alternative pathways
- (b)(ii) Elucidate the structure of C and D in the given conversion. 5 marks
explain— definition/context → points in order → small example → short close
Must cover
- C = cyclooctyne (C₈H₁₂; 3 degrees of unsaturation: ring + C≡C)
- Ozonolysis cleaves the ring C≡C to give suberic acid HO₂C(CH₂)₆CO₂H as the single product
- D = cyclooctane (C₈H₁₆) from H₂/Pt
- 1,5-cyclooctadiene ruled out (would give two succinic acids)
Loses marks
- Incorrect structures
- Missing reasoning
- Inconsistent with molecular formulas
Earns more
- Degrees of unsaturation calculated
- Ozonolysis of an alkyne gives two –COOH groups
- Ring strain of cyclooctyne noted
- Molecular formula of D stated
Extra mark
- Cyclooctyne as the smallest isolable cycloalkyne
- Use of cyclooctyne in strain-promoted click chemistry
- (c)(i) Provide the structure of missing reagents and intermediates in the given synthesis. 10 marks
describe— define → structure or process in order → labelled diagram → significance
Must cover
- M = PCC or Dess–Martin periodinane (3-methylbutan-1-ol → 3-methylbutanal)
- N = NaCN/H⁺ (or TMSCN)
- O = cyanohydrin (CH₃)₂CHCH₂CH(OH)CN (2-hydroxy-4-methylpentanenitrile)
- P = LiAlH₄ or H₂/Ni → 1-amino-4-methylpentan-2-ol
Loses marks
- Incorrect intermediate structure
- Missing reagents
- Wrong reaction sequence
Earns more
- Mechanism of cyanohydrin formation
- Why a mild oxidant avoids over-oxidation to the acid
- Chain extended by one carbon
- Stereocentre formed as a racemate
Extra mark
- Named reaction identification
- Alternative pathways
- (c)(ii) Write the structure of the product formed and the steps involved in the given reaction. 10 marks
explain— definition/context → points in order → small example → short close
Must cover
- Wolff rearrangement: hν expels N₂ from the α-diazoketone to give a ketocarbene
- 1,2-shift of the adjacent ring carbon gives a ketene with ring contraction
- Methanol adds to the ketene C=O/C=C to give the ester
- Ring-contracted methyl ester drawn
Loses marks
- Incorrect product structure
- Missing steps
- Wrong reagents
Earns more
- Arrow-pushing for each step
- Migrating group retains its configuration
- Concerted vs carbene pathway mentioned
- Use in Arndt–Eistert homologation mentioned
Extra mark
- Named reaction identification
- Alternative pathways
What "Elucidate" is asking you to do
Make a stated proposition plain and then prove it with instances. Elucidate stems almost always carry a claim or a named concept, and very often the words “with examples” or “with suitable diagrams” — the illustration is part of the directive, not decoration.
Structure that answers it
Plain-language statement of what the proposition means → the part that is obscure, resolved → first illustration → second illustration → why the proposition holds
Where marks are lost
Adding terminology; elucidate rewards removing it. The commoner loss is a clean explanation with no example, when the stem asked for examples.
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