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Chemistry Question 62 – NEET-UG 2024

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Given below are two statements: Statement I : The boiling point of three isomeric pentanes follows the order n-pentane > isopentane > neopentane Statement II : When branching increases, the molecule attains a shape of sphere. This results in smaller surface area for contact, due to which the intermolecular forces between the spherical molecules are weak, thereby lowering the boiling point. In the light of the above statements, choose the most appropriate answer from the options given below:

Recall how structural isomerism, particularly branching, affects the physical properties of alkanes like boiling point.

Step 1: Analyze Statement I: Boiling Point Order of Pentane Isomers✦ Active

Statement I claims the boiling point order for isomeric pentanes is n-pentane > isopentane > neopentane. n-pentane is a straight-chain alkane, isopentane (2-methylbutane) has one branch, and neopentane (2,2-dimethylpropane) is highly branched and nearly spherical. Increased branching reduces the surface area available for intermolecular contact, leading to weaker London dispersion forces. Therefore, the boiling point decreases with increasing branching. The given order is consistent with this principle. Thus, Statement I is correct.

Step 2: Analyze Statement II: Effect of Branching on Boiling Point○ Expand

Statement II explains that increased branching leads to a more spherical molecular shape, which results in a smaller surface area for contact. This reduction in surface area weakens the intermolecular forces (specifically London dispersion forces) between molecules, thereby lowering the boiling point. This statement accurately describes the underlying reason for the trend observed in Statement I. Thus, Statement II is correct.

Step 3: Conclusion○ Expand

Since both Statement I and Statement II are correct, the most appropriate answer is the option that states both are correct.

💡 Teacher's Secret Hint

Remember that London dispersion forces are the only intermolecular forces present in nonpolar alkanes, and their strength is highly dependent on molecular surface area.

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