Place these hydrocarbons in order of decreasing boiling point (highest to lowest): 1. Paraffin wax 2. Octadecane 3. Hexane 4. Neohexane 5. Ethane.

Introduction / Context:
This organic chemistry question tests understanding of how molecular size and branching affect boiling points of hydrocarbons. Boiling point trends are important for predicting physical properties, separating mixtures using distillation, and understanding how structure influences intermolecular forces. The compounds listed include paraffin wax (a mixture of long chain alkanes), octadecane (C18H38), hexane (C6H14), neohexane (2,2 dimethylbutane), and ethane (C2H6). We must order them from the highest boiling point to the lowest.

Given Data / Assumptions:
- Hydrocarbons considered: paraffin wax, octadecane, hexane, neohexane, and ethane.
- All are non polar molecules where London dispersion forces dominate intermolecular attractions.
- Paraffin wax is a general term for a mixture of long chain alkanes with higher molar mass than octadecane.
- We assume normal atmospheric pressure for comparing boiling points.

Concept / Approach:
In non polar hydrocarbons, boiling point mainly depends on the strength of London dispersion forces. These forces increase with molar mass and with surface area. Longer, unbranched chains have higher boiling points than shorter or more highly branched chains. Therefore, the longest chains boil at the highest temperatures, and the smallest, most branched molecules boil at the lowest. Paraffin wax contains very long chains and so has the highest boiling point. Octadecane, a long unbranched C18 alkane, comes next. Among C6 isomers, linear hexane has a higher boiling point than its highly branched isomer neohexane. Ethane, the smallest molecule listed, has the lowest boiling point.

Step-by-Step Solution:
Step 1: Recognise that paraffin wax is a mixture of high molar mass long chain alkanes, typically longer than C18, so it has very strong London dispersion forces.
Step 2: Octadecane is a straight chain C18 alkane, which has a high molar mass but is generally somewhat smaller than paraffin wax components, so its boiling point is lower than that of paraffin wax but still quite high.
Step 3: Hexane is a straight chain C6 alkane with a moderate boiling point. It is much smaller than octadecane and paraffin but larger than ethane.
Step 4: Neohexane (2,2 dimethylbutane) is a highly branched isomer of C6H14. Branching reduces surface area and lowers boiling point compared with the straight chain isomer hexane.
Step 5: Ethane is the smallest molecule listed, with only two carbon atoms. It has the weakest London dispersion forces and therefore the lowest boiling point.
Step 6: Combining these comparisons, the decreasing order of boiling point is Paraffin > Octadecane > Hexane > Neohexane > Ethane.

Verification / Alternative check:
Data tables show that highly branched alkanes have lower boiling points than their straight chain isomers, and that boiling points increase with carbon number. For example, hexane boils at a higher temperature than its isomer neohexane, and both boil well above ethane. Similarly, heavy paraffin fractions in petroleum distillation have the highest boiling ranges. This experimental information confirms that the correct order places paraffin first, followed by octadecane, then hexane, neohexane, and finally ethane at the lowest boiling point.

Why Other Options Are Wrong:
- Paraffin > Octadecane > Neohexane > Hexane > Ethane: This incorrectly places neohexane above hexane, ignoring the effect of branching in lowering boiling point.

Why Other Options Are Wrong (continued):
- Hexane > Paraffin > Octadecane > Neohexane > Ethane: This incorrectly suggests that hexane boils higher than paraffin and octadecane, which contradicts both theory and data.
- None of the above: This is incorrect because one provided order exactly matches the correct reasoning based on molar mass and branching.

Common Pitfalls:
Students sometimes think that any C6 compound must have a similar boiling point, forgetting the strong effect of branching. Others may under estimate the impact of very long chains found in paraffin wax. Another common error is to focus only on molecular weight and ignore branching altogether, leading to incorrect ordering among isomers. To avoid these mistakes, always remember that, for alkanes, boiling point increases with chain length and decreases with increased branching.

Final Answer:
Paraffin > Octadecane > Hexane > Neohexane > Ethane is the correct order of decreasing boiling point.