The correct order of increasing boiling points of the following compounds C3H7OH, C7H16 and C4H10 is?

Analysis of the Question

To determine the order of increasing boiling points, we must look at the intermolecular forces present in each compound. The compounds are:

1. $C_4H_{10}$ (Butane): An alkane.
2. $C_3H_7OH$ (Propanol): An alcohol.
3. $C_7H_{16}$ (Heptane): An alkane.

Concept Explanation

Boiling point is determined by the strength of the intermolecular forces holding the molecules together:

• Van der Waals forces (London Dispersion Forces): These are the primary forces in alkanes. The strength of these forces increases as the molecular mass (or chain length) increases. Therefore, $C_7H_{16}$ (a larger molecule) has stronger Van der Waals forces than $C_4H_{10}$ (a smaller molecule).
• Hydrogen Bonding: Alcohols ($R-OH$) possess a hydroxyl group, which allows for hydrogen bonding. Hydrogen bonds are significantly stronger than the Van der Waals forces found in simple alkanes of similar size.

Comparing the compounds:

• $C_4H_{10}$: Smallest alkane, weakest forces, lowest boiling point.
• $C_3H_7OH$: Contains hydrogen bonding. Even though it is a smaller molecule than $C_7H_{16}$, the hydrogen bonding gives it a higher boiling point than the small alkane ($C_4H_{10}$), but it is generally lower than a long-chain alkane like $C_7H_{16}$ due to the significant increase in Van der Waals forces in the much larger $C_7$ chain.
• $C_7H_{16}$: Largest molecule with the strongest Van der Waals forces, resulting in the highest boiling point.

Step-by-Step Solution

1. Identify the lowest: $C_4H_{10}$ is a small alkane with weak dispersion forces. It has the lowest boiling point.
2. Identify the middle: $C_3H_7OH$ has hydrogen bonding, which makes its boiling point higher than the small alkane $C_4H_{10}$.
3. Identify the highest: $C_7H_{16}$ is a much larger hydrocarbon. The cumulative effect of the Van der Waals forces in a 7-carbon chain is strong enough to exceed the boiling point of the 3-carbon alcohol.
4. Order: $C_4H_{10} < C_3H_7OH < C_7H_{16}$.

Conclusion

Exam Tip

When comparing boiling points of organic compounds, always check for Hydrogen Bonding first (Alcohols, Carboxylic acids). If no hydrogen bonding is present, simply look at the Molecular Mass—the heavier the molecule, the higher the boiling point!