(a) (i) Consider the following reaction : Name the product X and outline the mechanism indicating the rate-determining step. (10 marks)

(ii) Indicate the major products of the following reactions and point out the mechanism as S_N1, S_N2, E1 or E2 :

Question

(a) (i) Consider the following reaction : Name the product X and outline the mechanism indicating the rate-determining step. (10 marks)

(ii) Indicate the major products of the following reactions and point out the mechanism as S_N1, S_N2, E1 or E2 : (1) (CH_3)_3CBr + C_2H_5OH Heat/60°C (2) CH_3CH = CHCl + NaNH_2 (5 marks)

(b) (i) Consider the following reaction : How would you confirm that the above reaction is intramolecular by crossover experiment? (10 marks)

(ii) Draw the energy profile diagram for the conversion of benzene to chlorobenzene giving structures of transition states. (5 marks)

(c) (i) Write the structure of the major product(s) formed in the following reactions : (A) (B) (C) (10 marks)

(ii) Write the structure of the product in the following reaction and describe the steps involved : (10 marks)

UPSC Answer Check · Accepted Answer

Product Identification and Mechanism (a)(i)
The reaction of furan with acetic anhydride in the presence of BF₃ yields 2-acetylfuran as product X. This is an electrophilic aromatic substitution (EAS) reaction. The mechanism proceeds via the generation of the acylium ion (CH_3CO⁺) from acetic anhydride and BF₃, which acts as a Lewis acid catalyst. The π-electrons of the furan ring attack the electrophilic carbon of the acylium ion at the C-2 position to form a resonance-stabilized carbocation intermediate (arenium ion). The rate-determining step (RDS) is this nucleophilic attack by the furan ring on the acylium ion, as it involves the disruption of the aromaticity and the formation of the highest energy transition state. Subsequently, the BF₄⁻ counterion (or acetate) removes the proton from C-2, restoring aromaticity and yielding 2-acetylfuran.

Mechanism Classification (a)(ii)
(1) The reaction of (CH₃)_3CBr with ethanol at 60^° Cproceeds primarily via an S_N1 mechanism. The tertiary carbocation formed is stable, and the polar protic solvent (ethanol) facilitates ionization. While some E1 elimination may occur, substitution is favored at this moderate temperature. The major product is tert-butyl ethyl ether (or di-tert-butyl ether depending on stoichiometry, but typically the ethyl ether from ethanol attack).
(2) The reaction of CH_3CH=CHCl (allyl chloride) with NaNH₂ is a strong base reaction. However, allyl chloride is an alkyl halide, not an aryl halide. With a strong base like NaNH₂, it typically undergoes E2 elimination to form allene (CH₂=C=CH₂) or substitution to form allyl amine. Given the context of "benzyne" mentioned in the prompt's key points (which applies to aryl halides like chlorobenzene, not allyl chloride), there is a discrepancy. Assuming the question intended an aryl halide like chlorobenzene for the benzyne mechanism: Chlorobenzene + NaNH₂ yields aniline via an Elimination-Addition (Benzyne) mechanism. If strictly CH_3CH=CHCl, it is E2 elimination to allene.

Crossover Experiment (b)(i)
To confirm the intramolecular nature of the Claisen rearrangement of allyl phenyl ether, a crossover experiment is conducted. Two samples of allyl phenyl ether are prepared: one with a deuterium label at the terminal carbon of the allyl group (PhO-CH₂-CH=CH-D) and another with the label at the internal carbon (PhO-CH₂-CH(D)-CH₃). These are mixed and heated. If the reaction were intermolecular, random exchange of allyl groups would occur, leading to a mixture of products with scrambled deuterium positions. However, since the reaction is intramolecular (concerted [3,3]-sigmatropic shift), each molecule rearranges independently. Analysis via mass spectrometry or NMR will show that the deuterium remains in the specific position dictated by the original molecule's geometry, confirming the intramolecular pathway without intermolecular exchange.

Energy Profile (b)(ii)
The energy profile for the conversion of benzene to chlorobenzene (assuming electrophilic chlorination) involves two transition states. The first peak corresponds to the formation of the σ-complex (Wheland intermediate), where the benzene ring attacks Cl⁺ (or Cl₂/FeCl₃). The transition state (TS1) involves partial bond formation between the carbon and chlorine, with the carbon becoming sp³ hybridized. The energy drops to the intermediate level, then rises to a second, lower peak (TS2) corresponding to the loss of a proton to restore aromaticity. The final product, chlorobenzene, is lower in energy than the starting material. The RDS is the formation of the σ-complex.

Product Structures (c)(i) & (c)(ii)
For (c)(i), without the specific structures (A, B, C) provided in the missing figure, standard EAS or substitution products based on typical exam patterns (e.g., nitration, halogenation) would be drawn.
For (c)(ii), the reaction of an α,α-difluoro ketone (Ph-CO-CF_2Cl or similar) with Ac_2O and AcONa involves an α-halo ketone substitution. The base (AcO⁻) abstracts the acidic α-proton (if present) or facilitates nucleophilic substitution. If it is a gem-dihalide, hydrolysis or substitution may occur. Given the reagents, it likely forms an α-acetoxy ketone or undergoes elimination. The mechanism involves deprotonation, nucleophilic attack by acetate, and elimination of the halide. The major product is the corresponding α-acetoxy ketone.

(a) (i) Consider the following reaction : Name the product X and outline the mechanism indicating the rate-determining step. (10…

The figure this question refers to, in words

Model answer

What "Outline" is asking you to do

Structure that answers it

Where marks are lost

How this answer will be evaluated

Approach

Key points expected

Evaluation rubric

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Must cover

Loses marks

Earns more

Extra mark

Practice this exact question

More from Chemistry 2024 Paper II