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H3 Chemistry 9813 · Study focus: H3 Chemistry: Suggest major fragment ions without rearrangement

H3 Chemistry: Suggest major fragment ions without rearrangement

Start from the governing chemical model, test it against evidence, then transfer the reasoning to an unfamiliar case.

Your success criteria

  • Suggest major fragment ions without rearrangement
  • Use named chemical evidence.
  • Transfer the governing reason to an unfamiliar case.
Diagnose this objective

Assign formula before mass

A fragment peak requires a charged formula and nominal mass, not only a named neutral loss.

CH3CO+ has m/z43; C2H5CO+ has m/z57.

Explore this H3 topic and lesson sequence.

Preserve connectivity

Fragments must be obtainable by cleaving bonds in the supplied structure while retaining charge on one product.

Do not assemble atoms from disconnected ends without an allowed rearrangement.

  1. For this outcome, test simple bond cleavage unless rearrangement is explicitly supplied.

  2. Balance the complementary neutral radical for an odd-electron parent.

H3 Chemistry: Suggest major fragment ions without rearrangement: move from the evidence or givens, through the governing Chemistry idea, to a conclusion that stays inside the selected course boundary.
H3 Chemistry: Suggest major fragment ions without rearrangement evidence representation. Fixed parent structures and cleavage arrows force connectivity and charge checks.
Cleavage of CH₃–COCleavage of CH₃–COEI radical cation α-cleavageCH₃CCH₂CH₃O⁺•C₂H₅CO⁺ m/z 57+CH₃•two single-electron fishhooks; charge stays on acylium ionCleavage of CO–CH₂Cleavage of CO–CH₂EI radical cation α-cleavageCH₃CCH₂CH₃O⁺•CH₃CO⁺ m/z 43+C₂H₅•two single-electron fishhooks; charge stays on acylium ion

Text alternative: Text gives structures, broken bond, formulas, charges and mass checks.

Use the no-rearrangement boundary

For this outcome, test simple bond cleavage unless rearrangement is explicitly supplied.

Balance the complementary neutral radical for an odd-electron parent.

Fragment butan-2-one

Cleavage beside C=O gives CH3CO+43 + C2H5• or C2H5CO+57 + CH3•.

Both channels preserve connectivity and atom balance.

  • Write both channels.
Open the feedback checkpoint after attempting
  • Credit charged formula, neutral partner and m/z.

Fragment propan-1-ol

α-cleavage can give CH2OH+ at31 plus C2H5•.

A proposed C2H5O+45 must be checked against the actual broken bond and parent connectivity.

  • Assign m/z31.
Open the feedback checkpoint after attempting
  • Credit CH2OH+ mass31.

Start the diagnostic and follow its feedback

Cross-check neutral loss

If M+•=74 and fragment=59, the neutral loss is15, consistent with CH3•.

Mass difference supports but does not alone prove a formula; connectivity must agree.

  • Evaluate M−15.
Open the feedback checkpoint after attempting
  • Credit mass and structural checks.

Repair mass-only guessing

Different ions can share nominal m/z, so formula and precursor structure matter.

A peak43 is not automatically CH3CO+ in every molecule.

  • Repair: ‘m/z43 always means acylium.’
Open the feedback checkpoint after attempting
  • Require formula candidates and connectivity evidence.

Combine the spectrum

For butan-2-one, M+•72 with43 and57 supports two α-cleavages.

Next: stereochemical projections at /learning/h3-stereochemistry-projections-use-lesson.html.

  • Audit the fixed spectrum.
Open the feedback checkpoint after attempting
  • Credit parent formula and both balanced fragments.