Problem 5 - Chemistry An Atoms First Approach

Question

If you had a group of hydrocarbons, what structural features would you look at to rank the hydrocarbons in order of increasing boiling point?

Step-by-Step Solution

Answer

To rank hydrocarbons in order of increasing boiling point, consider the following factors: 1. Molecular size: Hydrocarbons with more carbon atoms have higher boiling points. 2. Molecular shape: Linear or less branched hydrocarbons have higher boiling points compared to more branched ones. 3. Presence of functional groups: Molecules with more polar functional groups typically have higher boiling points than nonpolar hydrocarbons with the same molecular weight. Compare the hydrocarbons based on these factors, starting with molecular size, followed by shape, and finally, the presence of functional groups.

Step 1: Understand the factors affecting boiling points

Boiling point is the temperature at which a substance changes from liquid to gas. For hydrocarbons, three main factors affect the boiling points: molecular size, molecular shape, and the presence (or absence) of functional groups.

Step 2: Analyze the molecular size

Larger molecules generally have higher boiling points because they have more atoms, which means more electrons, creating stronger London dispersion forces (induced dipole-induced dipole interactions) or van der Waals interactions. As the molecular size of the hydrocarbon increases, the boiling point increases. Therefore, the first structural feature to look for is the number of carbon atoms in the hydrocarbon.

Step 3: Analyze the molecular shape

The shape of the molecule also affects the boiling point. Linear or less branched hydrocarbons have higher boiling points compared to more branched ones with the same molecular weights. This is due to the increased surface area in contact with neighboring molecules, leading to stronger London dispersion forces. Thus, hydrocarbons with more linear structures will have higher boiling points than the ones with more branches.

Step 4: Analyze the presence of functional groups

Although hydrocarbons consist primarily of carbon and hydrogen atoms, they can also contain functional groups. A functional group is an atom or group of atoms responsible for the specific properties of a molecule. Some functional groups, like halogens, oxygen, sulfur, and nitrogen-containing groups, can form dipoles or even hydrogen bonds, which would affect the boiling point. Molecules with more polar functional groups (such as alcohol or amine functional groups) will typically have higher boiling points than nonpolar hydrocarbons with the same molecular weight.

Step 5: Rank the hydrocarbons in order of increasing boiling point

To rank the hydrocarbons in order of increasing boiling point, start by considering the three factors discussed above: 1. Compare the molecular size: look for the number of carbon atoms in the hydrocarbon. 2. Compare the molecular shape: prefer less branched and linear structures. 3. Compare the presence of functional groups: give priority to molecules with more polar functional groups. By considering these three factors, you can rank a group of hydrocarbons in order of increasing boiling points. Begin by comparing the hydrocarbons based on their molecular size, then shape, and finally, the presence of functional groups.