Combining IR and Mass Spectra

No single technique proves the structure of an unknown compound. Combining evidence from different techniques narrows the options until only one structure fits.What each technique tells you
• Elemental analysis (percentage composition) gives the empirical formula.
• The mass spectrum gives M_r from the molecular ion peak, so the molecular formula; fragment peaks show parts of the structure.
• The IR spectrum shows which functional groups are present, such as C=O, O-H and C-O.Strategy
1. Find the molecular formula (empirical formula and M⁺).
2. Use IR to identify the functional groups.
3. Draw all structures that fit.
4. Use fragment peaks to decide between them.Worked example: compound Y contains only C, H and O and has one C=O group. Its molecular ion peak is at m/z 58.
• C₃H₆O has M_r = 36 + 6 + 16 = 58, so the molecular formula is C₃H₆O.
• IR: a strong trough at 1715 cm⁻¹ (C=O) and no broad O-H trough, so Y is an aldehyde or a ketone.
• Possible structures: propanal, CH₃CH₂CHO, or propanone, CH₃COCH₃.
• The mass spectrum has a large fragment peak at m/z 43 (CH₃CO⁺) and a peak at 15 (CH₃⁺), with no large peak at 29. Propanal would give a large peak at m/z 29 from C₂H₅⁺.
So Y is propanone.