Comparison of Boiling Points

Oct 13, 2025 by miodeya.com 44 views

Understanding the Boiling Points of Alkanes and Alcohols: A Comparative Analysis

The boiling points of organic compounds are a crucial aspect of their physical properties, which in turn affect their chemical behavior and applications. In this article, we will delve into the boiling points of the first six alkanes and the first six alcohols, exploring the trends and factors that influence these values.

The Table: Boiling Points of Alkanes and Alcohols

Alkane	Boiling Point (°C)	Alcohol	Boiling Point (°C)
Methane	-161.5	Methanol	64.7
Ethane	-88.6	Ethanol	78.3
Propane	-42.2	Propan-1-ol	97.2
Butane	-0.5	Propan-2-ol	82.8
Pentane	36.1	Butan-1-ol	117.2
Hexane	68.7	Butan-2-ol	99.5

As we examine the table, we notice that the boiling points of alkanes generally decrease as the number of carbon atoms increases. This trend is expected, as the intermolecular forces between molecules decrease with increasing molecular weight. In contrast, the boiling points of alcohols increase with the number of carbon atoms, which may seem counterintuitive at first glance.

Intermolecular Forces: A Key Factor

The primary reason for the difference in boiling points between alkanes and alcohols lies in the type and strength of intermolecular forces present in each class of compounds. Alkanes are nonpolar molecules, which means they do not have a permanent electric dipole moment. As a result, they exhibit only weak London dispersion forces between molecules. These forces are relatively weak and decrease rapidly with increasing distance between molecules.

In contrast, alcohols are polar molecules due to the presence of a hydroxyl (-OH) group, which is capable of forming hydrogen bonds with other molecules. Hydrogen bonding is a relatively strong type of intermolecular force that arises from the attraction between a hydrogen atom bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) and another electronegative atom in a different molecule.

Hydrogen Bonding: A Key Factor in Boiling Points

The presence of hydrogen bonding in alcohols leads to a significant increase in their boiling points compared to alkanes. As the number of carbon atoms in an alcohol increases, the number of hydroxyl groups also increases, resulting in a greater number of potential hydrogen bonds between molecules. This, in turn, leads to a stronger intermolecular force and a higher boiling point.

Comparison of Boiling Points

Let us compare the boiling points of the first six alkanes and alcohols:

Methane (alkane): -161.5°C
Methanol (alcohol): 64.7°C
Ethane (alkane): -88.6°C
Ethanol (alcohol): 78.3°C
Propane (alkane): -42.2°C
Propan-1-ol (alcohol): 97.2°C
Butane (alkane): -0.5°C
Butan-1-ol (alcohol): 117.2°C
Pentane (alkane): 36.1°C
Butan-2-ol (alcohol): 99.5°C
Hexane (alkane): 68.7°C
Propan-2-ol (alcohol): 82.8°C

As we can see, the boiling points of alcohols are generally higher than those of alkanes, with some exceptions. The presence of hydrogen bonding in alcohols leads to a significant increase in their boiling points, making them more difficult to vaporize than alkanes.

In conclusion, the boiling points of alkanes and alcohols are influenced by the type and strength of intermolecular forces present in each class of compounds. Alkanes exhibit only weak London dispersion forces, while alcohols exhibit hydrogen bonding, which leads to a significant increase in their boiling points. By understanding the factors that influence boiling points, we can gain a deeper appreciation for the physical properties of organic compounds and their applications in various fields.

Atkins, P. W., & De Paula, J. (2010). Physical chemistry (9th ed.). Oxford University Press.
Brown, T. L., LeMay, J. T., Bursten, B. E., & Murphy, C. (2012). Chemistry: The Central Science (12th ed.). Pearson Education.
McMurry, J. E., & Fay, R. C. (2015). Organic chemistry (9th ed.). Cengage Learning.
Frequently Asked Questions: Boiling Points of Alkanes and Alcohols

Q: What are the main factors that influence the boiling points of alkanes and alcohols?

A: The main factors that influence the boiling points of alkanes and alcohols are the type and strength of intermolecular forces present in each class of compounds. Alkanes exhibit only weak London dispersion forces, while alcohols exhibit hydrogen bonding, which leads to a significant increase in their boiling points.

Q: Why do the boiling points of alcohols increase with the number of carbon atoms?

A: The boiling points of alcohols increase with the number of carbon atoms because the number of hydroxyl groups also increases, resulting in a greater number of potential hydrogen bonds between molecules. This, in turn, leads to a stronger intermolecular force and a higher boiling point.

Q: What is the difference between London dispersion forces and hydrogen bonding?

A: London dispersion forces are weak intermolecular forces that arise from the temporary dipoles that form in nonpolar molecules. Hydrogen bonding, on the other hand, is a relatively strong type of intermolecular force that arises from the attraction between a hydrogen atom bonded to a highly electronegative atom (such as oxygen, nitrogen, or fluorine) and another electronegative atom in a different molecule.

Q: Why are the boiling points of alkanes generally lower than those of alcohols?

A: The boiling points of alkanes are generally lower than those of alcohols because alkanes exhibit only weak London dispersion forces, while alcohols exhibit hydrogen bonding, which leads to a significant increase in their boiling points.

Q: Can you give an example of how the boiling point of an alcohol is affected by the number of carbon atoms?

A: Yes, let's consider the boiling points of the first six alcohols:

Methanol (CH3OH): 64.7°C
Ethanol (C2H5OH): 78.3°C
Propan-1-ol (C3H7OH): 97.2°C
Butan-1-ol (C4H9OH): 117.2°C
Pentan-1-ol (C5H11OH): 138.4°C
Hexan-1-ol (C6H13OH): 159.4°C

As we can see, the boiling point of the alcohol increases with the number of carbon atoms.

Q: What are some practical applications of understanding the boiling points of alkanes and alcohols?

A: Understanding the boiling points of alkanes and alcohols has many practical applications, such as:

Designing efficient distillation processes for separating mixtures of alkanes and alcohols
Developing new fuels and solvents that have optimal boiling points for specific applications
Understanding the behavior of alkanes and alcohols in various industrial processes, such as petroleum refining and chemical synthesis

Q: Can you recommend any resources for further learning about the boiling points of alkanes and alcohols?

A: Yes, here are some resources that you may find helpful:

Atkins, P. W., & De Paula, J. (2010). Physical chemistry (9th ed.). Oxford University Press.
Brown, T. L., LeMay, J. T., Bursten, B. E., & Murphy, C. (2012). Chemistry: The Central Science (12th ed.). Pearson Education.
McMurry, J. E., & Fay, R. C. (2015). Organic chemistry (9th ed.). Cengage Learning.

I hope this Q&A article has been helpful in answering your questions about the boiling points of alkanes and alcohols!