A chemist is studying two structural isomers with molecular formula
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A chemist is studying two structural isomers with molecular formula C2H4O2.
• Molecule X is acetic acid, CH3COOH.
• Molecule Y is methyl formate, HCOOCH3.
Using X-ray diffraction, the chemist obtains the following experimental data:
• In molecule X the C–O bond in the hydroxyl group is 133pm.
• In molecule Y the corresponding C–O bond is 143pm.
A compilation of typical single-bond radii predicts a C–O single bond length of 143pm and a C=O double bond length of 121pm.
(1) Part 1: (a) Draw the most important Lewis structure(s) for each molecule, including any significant resonance contributors, and assign formal charges.
(2) Part 2: (b) For each molecule, identify every C–O bond and state whether resonance would be expected to alter its idealized single- or double-bond character.
(3) Part 3: (c) Use resonance and formal-charge arguments to explain why the C–O bond length in the hydroxyl group of acetic acid is shorter than the corresponding bond in methyl formate.
(4) Part 4: (d) Predict which of the two molecules will have the higher average C–O bond dissociation energy and provide a quantitative estimate for that bond energy by interpolating between the tabulated single- and double-bond values. Show your reasoning.