Structure Elucidation from Spectroscopic and Analytical Data
Structure Elucidation from Spectroscopic and Analytical Data
This domain tests whether you can assemble a structure from independent evidence rather than pattern-matching one spectrum.
Core rules
- Start with molecular formula and degrees of unsaturation: (2C + 2 + N − H − X)/2. A value of 5 with eight carbons suggests a benzene ring plus one additional ring or double bond.
- ¹H NMR gives chemical shift, integration, and multiplicity. Use integrals to count protons, and coupling constants to distinguish cis/trans and axial/equatorial relationships. Exchangeable OH/NH protons may be broad or absent. ¹³C and DEPT-135 separate CH/CH3 (up) from CH2 (down); quaternary carbons are absent. HSQC connects C–H; HMBC gives two- and three-bond correlations.
- IR identifies functional groups: broad O–H near 3300 cm⁻¹, sharp C=O near 1700 cm⁻¹, C≡N near 2250 cm⁻¹. The fingerprint region supports, but rarely proves, a structure.
- MS gives the molecular ion, isotope pattern (Cl 3:1, Br 1:1), nitrogen rule, and fragmentation. Exact mass distinguishes formulas that differ by a few millidaltons.
- Cross-check every assignment; a structure must explain all data, not most of it.
Worked example
A compound analysing as C8H8O has degrees of unsaturation = (2×8 + 2 − 8)/2 = 5. IR shows a strong band at 1685 cm⁻¹, consistent with an aryl ketone and not an aldehyde (no C–H stretch near 2720 cm⁻¹). ¹H NMR shows a three-proton singlet at δ 2.55 and a five-proton multiplet at δ 7.2–7.9; there is no aldehydic proton near δ 10. The only structure fitting a methyl ketone with a monosubstituted benzene ring is acetophenone, C6H5COCH3.
Traps
Ignoring exchangeable protons; treating integration as exact when baseline is poor; misreading DEPT-135; assigning a molecular ion without checking isotopes; accepting a structure that leaves one signal unexplained.