The ion shown below is commonly called the amide anion of acetamide
CHEMISTRY · Multi-part · A Level · Chemistry · Introduction to organic chemistry · Introduction to organic chemistry
Problem
The ion shown below is commonly called the amide anion of acetamide. Hydrogen atoms attached to carbon are omitted for clarity.
O−
∥
CH3–C–NH−
Experimental studies reveal the following bond lengths in acetamide anion:
• C–N: 128pm
• C–O: 124pm
A chemist proposes adding an electron-withdrawing fluorine atom to the carbonyl carbon to give the ion CFH2C(O)NH− and wonders how the C–N and C–O bond lengths will be affected.
(a) Draw all major resonance contributors for the original acetamide anion, labeling formal charges.
(b) Use your resonance contributors to explain the observed C–N and C–O bond lengths relative to standard single and double bonds.
(c) Draw the dominant resonance contributors for the fluorinated anion CFH2C(O)NH− and discuss how the presence of the electronegative fluorine atom changes the relative importance of each contributor.
(d) Predict qualitatively (shorter, longer, unchanged) how each of the two bond lengths (C–N and C–O) will shift upon fluorination, and justify your predictions by invoking resonance, inductive effects, and formal charge distribution.
(e) Propose one experimental technique, other than X-ray diffraction, that could be used to verify your predicted changes in bond lengths, and briefly describe the type of evidence it would provide.