Concept:For isomeric hydrocarbons (alkanes), the boiling point heavily depends on the molecular shape, which dictates the total surface area available for London dispersion forces to act upon.
Formula:$$ \text{Boiling Point} \propto \text{Surface Area} $$
Solution:- A straight-chain alkane is long and cylindrical, providing a massive surface area for neighboring molecules to contact, resulting in strong cumulative London dispersion forces.
- When branching increases, the molecule is forced into a much more compact, spherical shape.
- Because a sphere has the lowest possible surface area for a given volume, the molecules have drastically fewer points of physical contact with one another.
- This decreased surface area significantly weakens the overall intermolecular attractive forces, leading to a lower boiling point.
Why other options are incorrect:Branching physically
decreases the available surface area, making Option C completely backwards. Option B states the effect but Option D provides the actual structural
fact (surface area reduction) causing the decrease.
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