Q. 97 The molecules, hexanoic acid, lysine, histidine and glucose, each contain 6 carbon atoms, but have
completely different properties due to the presence of different functional groups. Which ONE of these
molecules has a high calorific value?
(A) Lysine
(B) Hexanoic acid
(C) Glucose
(D) Histidine
Hexanoic acid stands out with the highest calorific value among these six-carbon molecules due to its fatty acid structure, which yields more energy per gram upon oxidation compared to carbohydrates or amino acids. This makes it the correct answer in exams testing metabolic energy yields.
Correct Answer
Hexanoic acid (B) has the highest calorific value.
Calorific Value Basics
Calorific value measures energy released from complete oxidation of a molecule, typically in kJ/mol or kJ/g, relevant in nutrition and bomb calorimetry. Fats like hexanoic acid provide about 37-39 kJ/g, exceeding carbohydrates (15-17 kJ/g) and proteins (17-20 kJ/g). All options have C6 formulas but differ in H:C ratios and functional groups affecting oxidation energy.
Option Breakdown
| Molecule | Formula | Type | Approx. Calorific Value | Reason for Value |
|---|---|---|---|---|
| Lysine (A) | C6H14N2O2 | Amino acid | ~20 kJ/g | Nitrogen content lowers energy; used in protein synthesis, not primary fuel. |
| Hexanoic acid (B) | C6H12O2 | Fatty acid | ~37 kJ/g (~3500 kJ/mol) | High C-H bonds; lipid-like, maximal oxidation energy from low oxygen. |
| Glucose (C) | C6H12O6 | Carbohydrate | ~15.6 kJ/g (2800 kJ/mol) | Oxygen-rich; efficient cell fuel but lower yield than fats. |
| Histidine (D) | C6H9N3O2 | Amino acid | ~18-20 kJ/g | Imidazole ring and nitrogens reduce energy density. |
Why Hexanoic Acid Wins
Hexanoic acid’s structure maximizes hydrocarbon chains with minimal oxygen, yielding higher ΔH combustion (e.g., -3490 kJ/mol liquid). Glucose’s polyol groups pre-oxidize parts of the molecule, cutting energy output. Amino acids like lysine and histidine incorporate nitrogen, diverting metabolism to urea synthesis and lowering net calories. In biology, this mirrors why fats store more energy than carbs or proteins gram-for-gram.
Exam Relevance
This question from GATE Life Sciences tests functional group impacts on bioenergetics. For students in genetics or biochemistry, note how C6 isomers highlight metabolism: glucose for glycolysis, hexanoic acid for beta-oxidation yielding more ATP.
