Indian Forest Service (Mains), Chemistry optional · 2023 · Paper II
Q1(b). Evidence for a tetrahedral intermediate
Question
How can isotopic labelling show that BAc2 ester hydrolysis involves a true intermediate rather than only a transition state?
Answer in brief
Demonstrate carbonyl-oxygen exchange in recovered, unhydrolysed ester: reversible addition can return to ester after the two oxygen atoms interchange roles.
Key points
- Use a carbonyl-¹⁸O-labelled ester and unlabelled aqueous hydroxide, or labelled water. Stop the reaction at partial conversion and isolate the unchanged ester.
- Hydroxide addition creates a tetrahedral species containing the old carbonyl oxygen and the incoming oxygen; proton exchange lets either become the carbonyl oxygen.
- Loss or incorporation of carbonyl label in recovered ester therefore records reversible addition before productive alkoxide departure.
- A single irreversible displacement transition state cannot explain this exchange in unchanged ester; the evidence supports the stepwise tetrahedral-intermediate mechanism.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversisotopic labelling¹⁸O exchangetetrahedral intermediateBAc2 ester hydrolysisBender experiment
All parts of Question 1
- 1(a) Aromaticity of heterocycles5 marks
- 1(b) Evidence for a tetrahedral intermediate5 marks
- 1(c) Benzyne capture by tetracyclone5 marks
- 1(d) Para-alkyl effects on benzyl solvolysis5 marks
- 1(e) Dehydrobromination through an ion pair5 marks
- 1(f) Why nucleophilic shifts are common5 marks
- 1(g) A double oxy-Cope rearrangement5 marks
- 1(h) Substituent effects in Cannizzaro reaction5 marks