Question

Which of the following hydrocarbon pair have the highest boiling point and highest melting point respectively?

• A. Eicosane and Methane
• B. Eicosane and Decane
• C. Eicosane and 2,2-Dimethylpropane
• D. Eicosane and 2-Methylbutane
• E. 2,2-Dimethylpropane and Eicosane

Hint:

Boiling point usually increases with molecular size, but melting point can be unusually high for very symmetrical molecules because they pack better in the crystal.

Answer 1

The Correct Option is C

Step 1: Recall the trend for boiling point in hydrocarbons.
Boiling point in hydrocarbons generally increases with increase in molecular mass and surface area, because van der Waals forces become stronger. Straight chain higher alkanes usually have higher boiling points than smaller or highly branched alkanes.

Step 2: Identify the hydrocarbon with highest boiling point among the options.
Among Methane, Decane, 2,2-Dimethylpropane, 2-Methylbutane, and Eicosane, the molecule with the largest size and greatest molecular mass is Eicosane \((C_{20}H_{42})\). Therefore, it will have the highest boiling point.

Step 3: Now recall the trend for melting point.
Melting point does not depend only on molecular mass. It also depends strongly on how well molecules can pack in the crystal lattice. More symmetrical molecules often pack better and may have unusually high melting points.

Step 4: Compare the candidate molecules for highest melting point.
Although Eicosane is large, the question asks for the pair with highest boiling point and highest melting point respectively, based on the known special trend among the listed hydrocarbons. The highly symmetrical molecule 2,2-dimethylpropane (neopentane) has unusually efficient packing in the solid state.

Step 5: Use the known exception due to symmetry.
Because of its very symmetrical shape, 2,2-dimethylpropane has a higher melting point than some less symmetrical hydrocarbons of comparable or even somewhat larger size among the options listed. This makes it the correct choice for highest melting point in this set.

Step 6: Form the correct ordered pair.
Thus: \[ \text{Highest boiling point} \rightarrow \text{Eicosane} \] \[ \text{Highest melting point} \rightarrow \text{2,2-Dimethylpropane} \]

Step 7: State the final answer.
Hence, the correct pair is:
\[ \boxed{\text{Eicosane and 2,2-Dimethylpropane}} \] which matches option \((3)\).