6.9.6 Combined Techniques

• Organic chemists use a variety of techniques to determine an organic structure
  • In real laboratory practice, certain pieces of information might already be known as target molecules are often tested once they are produced to check that the reaction has gone as planned
•  The typical sequence using the common analytical techniques would be:
  1. Elemental analysis - to determine the empirical formula
  2. Mass spectrometry - to determine the molecular mass, and consequently the molecular formula, as well as fragments of the molecule
  3. Infrared spectrometry - to primarily identify functional groups but also identify types of bond, e.g. C-C, C=C
  4. NMR spectrometry - to determine the number of carbon / hydrogen atoms and their environments 
• Each technique provides valuable information that can be used to propose a structure, although some techniques provide more information than others
  • Elemental analysis is sometimes skipped as the combination of mass spectrometry and NMR can suggest the molecular formula without the need for elemental analysis

Answer

Mass Spectrum

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

M_r = (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 (M_r - 43)
    • The peak at  m/z = 29 could correspond to CH₃CH₂⁺  indicating the loss of C₂H₃O from X, that is (M_r - 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