Structural Analysis of Molecules (HL) (DP IB Chemistry): Revision Note
Structural Analysis of Molecules
The chemists' toolkit includes a range of analytical techniques that enable the structure of compounds to be deduced
Summary of analytical techniques
Mass spectrometry provides:
Relative atomic mass
Isotopic composition
Relative molecular mass (Mr)
Fragmentation patterns in organic compounds
Support for determining structure
Nuclear magnetic resonance (NMR) spectroscopy provides:
Information on the structure of organic compounds
Infrared (IR) spectroscopy provides:
Identification of specific covalent bonds
Evidence for functional groups present in the molecule
These techniques are rarely used in isolation
They are typically combined to provide evidence when analysing new or synthetic compounds
Using multiple techniques helps confirm the identity and structure of the compound with greater confidence
Structural problems often require interpreting multiple spectra for the same unknown compound
Each technique offers a different piece of information
The goal is to combine the evidence and propose the most likely structure
Worked Example
Compound X has the molecular formula C4H8O and produces the following mass spectrum, IR spectrum and ¹H NMR spectrum.
Deduce the structure of the compound using the data provided.
Support your answer with evidence from each spectrum and appropriate references to the IB Chemistry data booklet.
Answer
Mass Spectrum - IB Chemistry data booklet (Section 22)
Molecular ion peak, m / z = 72
This corresponds to the relative molecular mass of C4H8O
Mr = (12.01 × 4) + (8 × 101) + (16.00) = 72.12
Large peak at m / z = 43
This could correspond to CH3CH2CH2+ or CH3CO+
Mr of CH3CH2CH2+ = (12.01 x 3) + (7 × 1.01) = 43.10
Mr of CH3CO+ = (12.01 × 2) + (3 × 1.01) + (16.00) = 43.05
This indicates the loss of CH4O or C2H5 from X
Peak at m / z = 29
This could correspond to CH3CH2+
Mr of CH3CH2+ = (12.01 x 2) + (5 × 1.01) = 29.07
This indicates the loss of C2H3O from X
IR Spectrum - IB Chemistry data booklet (Section 20)
There is a strong absorption in the range 1700 - 1750 cm-1
This corresponds to C=O and suggests an aldehyde or ketone
It cannot be an ester or a carboxylic acid because there is only one oxygen in the molecular formula
1H NMR Spectrum - IB Chemistry data booklet (Section 21)
The spectrum shows three distinct proton environments:
A triplet at around 1.0 ppm
This suggests a –CH3 group next to a –CH2– group
A quartet at around 2.4 ppm
This suggests a –CH2 group between a –CH3 group and a carbonyl group
A singlet at around 2.1 ppm
This suggests a –CH3 group attached directly to a carbonyl group, with no adjacent protons
The peak splitting pattern of a quartet and triplet together suggests an ethyl group (CH3CH2–)
The singlet indicates an isolated proton environment
Integration data (3H : 2H : 3H) further confirms the assignment of two methyl groups and one methylene
Overall structure:
The molecular formula is C4H8O
The compound contains a carbonyl group but is not an aldehyde or acid, suggesting it is a ketone
The NMR data support two CH3 groups and one CH2 group in different environments
Mass spectrum fragments match those expected for CH3COCH2CH3
Therefore, the structure of X is butan-2-one (CH3COCH2CH3 ):
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