Q. 44 The mass spectrum of benzoic acid will generate the fragment as a base peak ( 𝟏𝟎𝟎% relative
abundance) of 𝒎/𝒛 (mass to charge ratio) at ____ (correct to integer number).
Benzoic acid’s mass spectrum shows a prominent base peak at m/z 105 due to the stable benzoyl cation. This article explains the fragmentation,
Correct Answer
The base peak (100% relative abundance) in benzoic acid’s electron ionization mass spectrum occurs at m/z 105. This fragment forms from the molecular ion (m/z 122) losing a hydroxyl radical (- OH, mass 17), yielding the resonance-stabilized benzoyl cation [C₆H₅CO]⁺. For Q.44, enter the integer 105.
Fragmentation Pathways
Benzoic acid (C₆H₅COOH, MW 122) undergoes characteristic cleavages in EI-MS.
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Molecular ion peak appears at m/z 122 (weak, ~10-20% abundance).
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Base peak at m/z 105: [M – OH]⁺, highly stable due to resonance between phenyl and carbonyl.
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m/z 77: C₆H₅⁺ (phenyl cation) from further loss of CO (mass 28) from m/z 105.
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Minor peaks: m/z 51 (C₄H₃⁺, tropylium-like), m/z 123 (M+1 from ¹³C isotope).
Common Exam Options Explained
Typical multiple-choice distractors test fragmentation knowledge:
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105 (Correct): Most abundant due to benzoyl stability; 100% base peak.
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122: Molecular ion [M]⁺- ; present but weak as aromatics fragment easily.
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77: Significant (phenyl ion) but secondary (~50-70% abundance).
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51 or 45: Smaller fragments (e.g., [C₆H₅COOH – COOH]⁺ or COOH⁺); low intensity.
Why m/z 105 Dominates
Resonance in [C₆H₅CO]⁺ delocalizes charge across oxygen and ring, enhancing survival to detector. This pattern repeats in aromatic carboxylic acids. Spectra from NIST or textbooks confirm this for benzoic acid identification.
