The following figure shows the molecular structures of four constitutional isomers of heptane (C7H16- black atoms are Cs and grey atoms are Hs). Constitutional isomers have the same molecular formula, but a different structural shape. A) Which has the highest boiling point, a, b, c, or d? Briefly, explain why? B) Which represents the least viscous isomer, a, b, c, or d? Why do you think your choice is the correct answer? (a) (b) (c) (d)
The Correct Answer and Explanation is:
ChatGPT said:
Correct Answers:
A) Structure (a) has the highest boiling point.
B) Structure (d) represents the least viscous isomer.
Explanation:
Boiling Point – Structure (a):
Boiling point is influenced primarily by the strength of intermolecular forces and surface area. Among constitutional isomers, straight-chain alkanes typically exhibit higher boiling points than their branched counterparts due to their greater surface area. This extensive surface area allows for stronger London dispersion forces (van der Waals forces) between molecules, requiring more energy to separate them during phase change.
Structure (a) is a straight-chain heptane. This linear arrangement maximizes surface contact between molecules, enhancing intermolecular attractions. In contrast, structures (b), (c), and (d) show increasing degrees of branching. As branching increases, the molecule becomes more compact, reducing the effective surface area and consequently weakening intermolecular forces. Thus, structure (a) exhibits the highest boiling point among the four isomers.
Viscosity – Structure (d):
Viscosity refers to a fluid’s resistance to flow, which is influenced by molecular shape and intermolecular interactions. Branched molecules tend to be less viscous because their compact structures reduce entanglement and sliding resistance between molecules.
Structure (d) is the most branched among the options. Its compact geometry leads to minimal molecular interaction and reduced surface contact. As a result, molecules move past each other more easily, leading to lower internal friction and thus lower viscosity.
On the other hand, straight-chain molecules like in structure (a) exhibit greater resistance to flow due to chain entanglement and stronger intermolecular attractions. This increases viscosity.
In conclusion, structure (a) has the highest boiling point due to its linear shape, and structure (d) is the least viscous due to its highly branched, compact form.
