Question

Infer how the boiling and freezing points of alkynes compare with those of alkanes with the same number of carbon atoms. Explain your reasoning, then research data to see it if supports your idea.

Short answer

The boiling and freezing points of alkynes are generally lower than those of alkanes with the same number of carbon atoms. This is due to their molecular structure and the strength of London dispersion forces. Alkynes have higher bond energy, less surface area available for interaction, and weaker dispersion forces compared to alkanes, resulting in lower boiling and freezing points. Data comparing the boiling points of alkanes and alkynes supports this hypothesis, although additional factors like branching and steric effects may influence this trend for larger molecules.

Step-by-step solution

Alkanes are hydrocarbons with single bonds between carbon atoms (C-C), while alkynes have at least one carbon-carbon triple bond (C≡C). Due to this difference in bonding, the shapes, molecular sizes, and characteristic properties of alkynes and alkanes are different, which can potentially influence their boiling and freezing points. #Step 2#: Consider intermolecular forces among alkanes and alkynes.

Alkanes and alkynes are nonpolar molecules with no permanent dipole moment. As a result, the primary intermolecular forces between these molecules are London dispersion forces. These are temporary attractive forces that arise from the induced electron cloud fluctuation. The strength of these dispersion forces typically increases with the size of the molecule and the amount of surface area available for interaction between neighboring molecules. #Step 3#: Infer boiling and freezing points based on intermolecular forces.

Since London dispersion forces are the dominant intermolecular forces in both alkanes and alkynes, the relative strength of these forces in each class of compounds will mainly determine the differences in their boiling and freezing points. In general terms, larger molecules or those with more surface area for interaction will exhibit stronger dispersion forces and thus have higher boiling and freezing points. Considering the difference in bonding, alkynes have higher bond energy than alkanes and are likely to have less surface area available for interaction due to more carbon atoms being involved in triple bonds. As a result, we can infer that the boiling and freezing points of alkynes should be lower than those of alkanes with the same number of carbon atoms. #Step 4#: Research data to support the hypothesis.

We will now research data on the boiling and freezing points of alkanes and alkynes to see if it supports our inference. 1. Ethane (2 carbons, alkane): Boiling point = -88.6°C Ethyne (2 carbons, alkyne): Boiling point = -84°C 2. Propane (3 carbons, alkane): Boiling point = -42.1°C Propyne (3 carbons, alkyne): Boiling point = -23°C 3. Butane (4 carbons, alkane): Boiling point = -0.5°C 1-Butyne (4 carbons, alkyne): Boiling point = 8.1°C Comparing the boiling points of alkanes and alkynes with the same number of carbon atoms, we can observe that the boiling points of alkynes are generally lower than those of corresponding alkanes, which supports our hypothesis. It is essential to note that this hypothesis may not hold true universally, especially for larger molecules where additional factors, such as branching and steric effects, may come into play. Also, the specific boiling and freezing points of each compound may vary slightly depending on the data source and experimental conditions. However, the general trend that alkynes have lower boiling and freezing points than alkanes with the same number of carbon atoms appears to be valid and supported by the data found.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules that influence the physical properties of substances, such as boiling and freezing points. These forces vary in strength and are crucial in determining how molecules interact with each other.
For hydrocarbons like alkanes and alkynes, they mostly exhibit nonpolar characteristics due to their symmetric structure and lack of significant differences in electronegativity between atoms.
As a result, the primary intermolecular forces at play between alkanes and alkynes are van der Waals forces, primarily known as London dispersion forces. These are the weakest of all intermolecular forces, yet they are substantial enough to influence the state and phase changes of these molecules.
Understanding how intermolecular forces work helps us predict the boiling and freezing points of different hydrocarbons, providing key insights into their properties and how they might behave under various conditions.

London Dispersion Forces

London dispersion forces are a type of van der Waals force and the only intermolecular force that nonpolar molecules, like alkanes and alkynes, experience. These forces result from temporary fluctuations in the electron distribution within molecules.
When two nonpolar molecules come close, a temporary dipole can induce a similar dipole in a neighboring molecule, leading to a weak and transient attraction. This interaction is stronger in larger molecules because they have more electrons and a greater surface area, producing a larger temporary dipole moment.
Therefore, the boiling and freezing points are influenced by these interactions:

• Larger hydrocarbons have stronger London dispersion forces.
• Stronger forces increase both boiling and freezing points.
• Shape affects surface area and thus influences force strength.

This concept helps explain why some hydrocarbons have higher or lower boiling points than others. In alkanes versus alkynes, even though alkynes have triple bonds, disrupting a smooth surface area interaction, this affects how effectively these forces can act.

Alkanes vs Alkynes

When comparing alkanes and alkynes, it's essential to understand the structural differences. Alkanes are saturated hydrocarbons, meaning they only have single bonds between carbon atoms. In contrast, alkynes have at least one carbon-carbon triple bond, which makes them less saturated.
Due to these structural differences, alkanes typically have more flexibility in their molecular structure than alkynes. Alkynes, with their triple bonds, have a more linear and rigid structure. This structural rigidity in alkynes means less surface area is available for neighboring molecules to interact through dispersion forces, resulting in generally lower boiling and freezing points compared to alkanes.
This trend is evident when reviewing the data on boiling points:

• Alkynes tend to have lower boiling points relative to alkanes with the same carbon count, as less effective London dispersion forces act upon them.
• The triple bond in alkynes accounts for a higher bond energy but also limits interactions from dispersion forces due to less available surface area.

Understanding these differences allows us to predict how new hydrocarbons might behave regarding phase changes and temperature when designing or analyzing chemical compounds.