Structural Analysis of Molecules (HL) (DP IB Chemistry) : Revision Note

An unknown compound, X, of molecular formula,C₄H₈O, has the following MS, IR and ¹H NMR spectra.

Mass spectrum of X

IR spectrum of X

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.

Answer

Mass Spectrum (Section 22 of the data booklet)

• 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 (Section 20 of the data booklet)

• There is a strong absorption in the range 1700 - 1750 cm^-1 which corresponds to C=O

• This suggests an aldehyde or ketone is present

  • It cannot be an ester or a carboxylic acid as there is only one oxygen in the formula

¹H NMR Spectrum (Section 21 of the data booklet)

• The ¹H NMR spectrum shows three protons environments

  • The peak around chemical shift 1.0 ppm could correspond to methyl protons 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, R-CH₂-CO-

• 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

Overall structure