Structural Analysis of Molecules
- The chemists' toolkit includes a range of analytical techniques that enable the structure of compounds to be deduced
Summary table of analytical techniques
| Technique | Information provided |
| Mass spectrometry |
Relative atomic mass Isotopes Relative molecular mass Fragments in organic compounds Structure of organic compounds |
| Nuclear magnetic spectroscopy | Structure of organic compounds |
| Infrared spectroscopy |
Types of covalent bonds Functional groups |
- These techniques are rarely used in isolation, but together provide corroborating evidence for elucidating chemical information on newly discovered or synthetic compounds
- Problem solving typically involves taking multiple pieces of spectroscopic data about the same unknown compound and coming up with a likely structure
Worked example
An unknown compound, X, of molecular formula,C4H8O, has the following MS, IR and 1H NMR spectra.
Mass spectrum of X
IR spectrum of X
1H NMR spectrum of X
Deduce the structure of X using the information given and any other additional information in the Data booklet. For each spectrum assign as much spectroscopic information as possible.
Answer
Mass Spectrum (Section 22 of the data booklet)
- The molecular ion peak is at m / z = 72, which corresponds to the relative molecular mass of C4H8O
- Mr = (12 × 4) + (8 × 1) +(16) = 72
- The large peak at m / z = 43 could correspond to CH3CH2CH2+ or CH3CO+ indicating the loss of CH4O or C2H5 from X, that is (Mr - 43)
- The peak at m / z = 29 could correspond to CH3CH2+ indicating the loss of C2H3O from X, that is (Mr - 29)
IR Spectrum (Section 20 of the data booklet)
- There is a strong absorption in the range 1700 - 1750 cm-1 which corresponds to C=O
- This suggests an aldehyde or ketone is present
- It cannot be an ester or a carboxylic acid as there is only one oxygen in the formula
1H NMR Spectrum (Section 21 of the data booklet)
- The 1H NMR spectrum shows three protons environments
- The peak around chemical shift 1.0 ppm could correspond to methyl protons on the end of a chain, -CH3
- The peaks around chemical shift 2.2 - 2.7 ppm could correspond to a proton next to a carbonyl group, R-CH2-CO-
- The peak splitting is a quartet, singlet and triplet
- A quartet and triplet in the same spectrum usually corresponds to an ethyl group, CH3CH2, following the n+1 rule
- The singlet indicates an isolated proton environment
Overall structure
