IB Chemistry HL

Revision Notes

21.1.3 Structure Identification Problems

Structure Identification Problems

  • The chemists’ toolkit includes a range of analytical techniques that enable the structure of compounds to be deduced

Summary table of analytical techniques





  • 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 unknow 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.






Figure 1

MS of X






Figure 2

IR spectrum of X






Figure 3

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.


Mass Spectrum

  • The molecular ion peak is at m/z = 72, which corresponds to the relative molecular mass of C4H8O

M = (12 x 4) + (8 x 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 (M – 43)
  • The peak at  m/z = 29 could correspond to CH3CH2+  indicating the loss of C2H3O from X, that is (M – 29)

IR Spectrum

  • There is a strong absorption in the range 1700-1750 cm-1 which corresponds to C=O, based on Section 26 of the Data book
  • This suggests an aldehyde or ketone is present ( it cannot be an ester or carboxylic acid as only one oxygen is in the formula)

1H NMR Spectrum

  • The 1H NMR spectrum shows three protons environments
  • The peak around chemical shift 1.0 ppm could correspond to a proton 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 RCH2CO-
  • 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

Putting the information together the structure of X is




The structure of X is butanone


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