Organic analysis — Question 77

The molecular ion is the ionised intact molecule; the base peak is the most intense fragment/ion and is set to 100%. Propanone is more strongly supported because loss of CH₃ gives CH₃CO⁺.

$\mathrm{C}{\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{C}{\mathrm{H}}_3^{+\mathrm{•}}\to \mathrm{C}{\mathrm{H}}_3\mathrm{C}{\mathrm{O}}^{+}+\mathrm{C}{\mathrm{H}}_3^{\mathrm{•}}$. The molecular-ion mass is consistent with both candidates, while the assigned acylium ion directly follows from cleavage in propanone. This fragment alone is supportive, not absolute proof.

• Defines molecular ion. [1 marks]
• Defines base peak as maximum relative intensity. [1 marks]
• Selects propanone. [1 marks]
• Explains loss of methyl/formation of CH₃CO⁺. [1 marks]