![Q. 45 The standard free energy (ΔG′) values of reactions catalyzed by citrate lyase and citrate synthetase are −670 cal/mol and −8192 cal/mol, respectively. Citrate lyase reaction: Citrate ⇌ Acetate + Oxaloacetate ΔG1′ = −670 cal/mol Citrate synthetase reaction: Acetyl-CoA + Oxaloacetate + H2O ⇌ Citrate + CoA ΔG2′ = −8192 cal/mol The standard free energy (in cal/mol) of acetyl-CoA hydrolysis is _____ (correct to integer number). Benzoic acid's mass spectrum shows a prominent base peak at m/z 105 due to the stable benzoyl cation. 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. Molecular ion peak appears at m/z 122 (weak, ~10-20% abundance). Base peak at m/z 105: [M - OH]⁺, highly stable due to resonance between phenyl and carbonyl. m/z 77: C₆H₅⁺ (phenyl cation) from further loss of CO (mass 28) from m/z 105. 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: 105 (Correct): Most abundant due to benzoyl stability; 100% base peak. 122: Molecular ion [M]⁺- ; present but weak as aromatics fragment easily. 77: Significant (phenyl ion) but secondary (~50-70% abundance). 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.](data:image/svg+xml;base64,PHN2ZyB2aWV3Qm94PSIwIDAgMTkyMCAxMDgwIiB3aWR0aD0iMTkyMCIgaGVpZ2h0PSIxMDgwIiB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciPjwvc3ZnPg==)
Q. 45
The standard free energy (ΔG′) values of reactions catalyzed by
citrate lyase and citrate synthetase
are −670 cal/mol and −8192 cal/mol, respectively.
Citrate lyase reaction:
Citrate ⇌ Acetate + Oxaloacetate
ΔG1′ = −670 cal/mol
Citrate synthetase reaction:
Acetyl-CoA + Oxaloacetate + H2O ⇌ Citrate + CoA
ΔG2′ = −8192 cal/mol
The standard free energy (in cal/mol) of acetyl-CoA hydrolysis is
_____ (correct to integer number).
Benzoic acid’s mass spectrum shows a prominent base peak at m/z 105 due to the stable benzoyl cation.
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.
-
Molecular ion peak appears at m/z 122 (weak, ~10-20% abundance).
-
Base peak at m/z 105: [M – OH]⁺, highly stable due to resonance between phenyl and carbonyl.
-
m/z 77: C₆H₅⁺ (phenyl cation) from further loss of CO (mass 28) from m/z 105.
-
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:
-
105 (Correct): Most abundant due to benzoyl stability; 100% base peak.
-
122: Molecular ion [M]⁺- ; present but weak as aromatics fragment easily.
-
77: Significant (phenyl ion) but secondary (~50-70% abundance).
-
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.
