Mass Spectrum Fragmentation Pattern

Steps:

• Identify molecular weight as 60 from M+ peak at m/z=60 (base peak).
• Eliminate B (CH3OCH3, MW=46) as it cannot produce M+ at 60.
• For A (HCOOCH3), expect strong M+ (observed) and fragments at m/z=45 ([M-CH3]+) and m/z=31 (CH3O+ from C-O cleavage).
• Compare to C and D: both have MW=60 but show weak M+ and different major fragments (C: m/z=43; D: m/z=42,45), mismatching the strong M+ and minor m/z=45.

Why A is correct:

• Esters like methyl formate exhibit a stable, intense M+ peak (base at m/z=60) due to resonance stabilization, with minor [M-CH3]+ at 45 and CH3O+ at 31 from alpha-cleavage.

Why the others are wrong:

• B: Molecular formula CH3OCH3 gives MW=46, incompatible with m/z=60 M+.
• C: Ethanoic acid shows weak M+ (<20% intensity) and strong m/z=43 (CH3CO+), not matching the base M+ or given peaks.
• D: Propan-2-ol has weak M+ and prominent m/z=45 (from alpha-cleavage) plus m/z=42 (M-H2O), contradicting the minor m/z=45 and absent m/z=42.

Final answer: A