Ethanoic acid reacts with ethanol in the presence of catalytic H₂SO₄ to yield ethyl…
CHEMISTRY · Multi-part · A Level · Chemistry · Introduction to organic chemistry · Introduction to organic chemistry
Problem
Ethanoic acid reacts with ethanol in the presence of catalytic H₂SO₄ to yield ethyl ethanoate and water. Time-resolved vibrational spectroscopy on the acid–alcohol mixture at 55 °C reveals a short-lived species whose C=O stretching frequency is shifted by 45 cm⁻¹ relative to free ethanoic acid. The species appears within 50 µs of mixing and decays within 2 ms.
Two general mechanistic pathways are under debate:
Pathway I: Direct nucleophilic attack of ethanol on the protonated carbonyl of ethanoic acid, forming a tetrahedral cationic intermediate that subsequently eliminates water.
Pathway II: Concerted "cyclic" transition in which proton transfer, C–O bond formation, and water loss occur in a single elementary step with no discrete tetrahedral intermediate.
Assume all other reaction conditions are identical and that the detected vibrational band is accurately assigned.
(a) Explain, with reference to molecular structure, why a 45 cm⁻¹ shift in the C=O stretching frequency is consistent with formation of a tetrahedral intermediate.
(b) Discuss how the lifetime of the detected species helps distinguish between Pathway I and Pathway II.
(c) Predict how increasing the concentration of ethanol relative to ethanoic acid would affect the maximum transient concentration of the detected species if Pathway I is operative. Justify your prediction.
(d) Suggest one kinetic or spectroscopic experiment (other than IR spectroscopy) that could test whether the reaction proceeds through a discrete tetrahedral intermediate. State the expected observation for Pathway I.