[elli]

Hi! Can anyone explain the relationship behind the heat of combustion and the structure of monosaccharide and disaccharide? For our report, we are supposed to explain the calculated heat of combustion by analyzing the chemical structure of α-D-galactose, α-D-glucose, β-D-fructose, and sucrose. I understand that sucrose is the only disaccharide but I don't understand how come the remaining three has different heat of combustions when they have the same chemical formula and α-D-glucose and β-D-fructose have the same combustion process. Please help

[Corribus]

The differences are quite small, less than 10 kJ/mol (that's less than 0.4%), which may be less than the error of the measurement. Supposing that the numbers are meaningful, the molecules have different structures and nuclear connectivity, which can easily lead to small differences in the combustion energy. As an example, fructose forms a 5 member ring in its closed chain form, whereas glucose forms a 6 member ring. These would have some differences in strain energy associated with them. Also differences in condensed phase intermolecular interactions (crystallinity, packing density, hydrogen bonding, and so forth) can influence the measured combustion energy.