21.1.3 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 unknown compound and coming up with a likely structure

Worked Example

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