Question

Which of the following hydrocarbons has stronger London dispersion forces ?

• A. Methane
• B. Ethane
• C. n-pentane
• D. neo-pentane

Hint:

London dispersion forces grow with the surface area and number of electrons, so a long unbranched chain beats a compact one.

Answer 1

The Correct Option is C

Step 1: Understand the concept
London dispersion forces arise from temporary dipoles in non-polar molecules. Their strength increases with the number of electrons and with the surface area available for contact.

Step 2: Compare the sizes
Methane (\(\text{CH}_4\)) has 10 electrons, ethane (\(\text{C}_2\text{H}_6\)) has 18, and both pentane isomers (\(\text{C}_5\text{H}_{12}\)) have 42. So the pentanes are far ahead of methane and ethane.

Step 3: Compare the pentane isomers
n-Pentane is a long, straight chain that lies along its neighbours and has a large surface of contact. Neopentane is a compact, nearly spherical molecule with little surface in contact.

Step 4: Conclusion
So n-pentane has stronger London forces, which is why it boils at 309 K while neopentane boils at 283 K.

Final Answer:
The long straight chain of n-pentane makes more contact than neopentane. This is option (C). \[ \boxed{\text{(C) }\text{n-Pentane}} \]