Question

Identify the decreasing order of boiling point of alkanes (i) n-pentane (ii) Isopentane (iii) Neopentane

• A. Isopentane>n-pentane>Neopentane
• B. Neopentane>Isopentane>n-pentane
• C. n-pentane>Isopentane>Neopentane
• D. Isopentane>Neopentane>n-pentane

Hint:

Straight-chain alkanes have higher boiling points than branched ones due to better molecular packing and stronger intermolecular forces.

Answer 1

The Correct Option is C

Step 1: Boiling Point Trend of Alkanes.
In general, the boiling point of alkanes increases with molecular size due to greater London dispersion forces. However, the branching of molecules affects the boiling point. Straight-chain alkanes (like n-pentane) have higher boiling points than their branched counterparts (like isopentane and neopentane), due to their larger surface area and better packing.

Step 2: Analyzing the options.
(A) Isopentane>n-pentane>Neopentane: Incorrect. Isopentane has a lower boiling point than n-pentane.
(B) Neopentane>Isopentane>n-pentane: Incorrect. Neopentane has the lowest boiling point due to its highly branched structure.
(C) n-pentane>Isopentane>Neopentane: Correct. n-pentane has the highest boiling point due to its linear structure, followed by isopentane, with neopentane having the lowest boiling point.
(D) Isopentane>Neopentane>n-pentane: Incorrect. Neopentane is the least boiling among the three.

Step 3: Conclusion.
The correct order is n-pentane>Isopentane>Neopentane, which aligns with the general boiling point trend based on molecular structure.