Combining Spectral Data to Determine Structure HL Only

Full Notes

When analysing organic compounds, a range of techniques are often used to determine molecular structure.

Each technique gives partial information:

• MS tells you the molecular ion (M⁺) and fragments.
• IR tells you about the types of bonds (e.g. O–H, C=O).
• ¹H NMR tells you:
  • How many different H environments there are
  • The types of environments
  • The relative number of Hs in each
  • Neighbouring Hs through splitting patterns

Used together, these tools can help identify a compound and deduce its structure.

Matt’s exam tip

Focus on your working when analysing data for structure questions. Show clearly what each piece of data tells you before drawing the final structure. You can still earn good marks just by interpreting the data well.

Deducing Organic Structures

You should be able to use (and combine) data from the following techniques:

Elemental Analysis

Gives empirical formula.

Example Find the empirical formula for the compound with a composition by mass of C 52.2%, H 13.0% and O 34.8%

Mass Spectrometry

Determines molecular mass and possible fragments.

Example C₄H₁₀ isomers

Hydrocarbons A and B both have a molecular formula of C₄H₁₀ (same molecular ion peak at 58), however they have different fragment patterns in their spectra, showing different structures.

Fragment peaks at 15 and 43 show a CH₃ fragment and C₃H₇ fragment. However, no fragment at 29 means no C₂H₅ group. This means the likely possible structure is CH₃CH(CH₃)CH₃.

The extra peak at m/z 29 for Hydrocarbon B means it has a C₂H₅ group in its structure (as well as a CH₃ and C₃H₇ group). This would indicate CH₃CH₂CH₂CH₃ as its structure.

IR Spectroscopy

Shows presence of functional groups:

• C=O: ~1700 cm⁻¹
• O–H (broad): ~3200–3600 cm⁻¹

Example Ethanoic acid (CH₃COOH)

Ethanoic acid has two absorbances in its IR spectra that help identify it: one for the O–H bond (2500 to 3000) and one for the C=O bond (1680 to 1750).

NMR Spectroscopy

Provides detailed framework of molecule.

Example Ethanol (CH₃CH₂OH)

• 3 peaks in the ¹H NMR show 3 unique H environments
• CH₃ group shows a triplet (next to CH₂).
• CH₂ group shows a quartet (next to CH₃).
• OH appears as a singlet (no splitting).

Summary

• MS provides molecular mass and fragments.
• IR identifies functional groups via bond vibrations.
• ¹H NMR reveals hydrogen environments, shifts, integration and splitting.
• Combining these gives an accurate molecular structure.