Question

Evaluate How would you expect the boiling points of propane and 1 -chloropropane to compare? Explain your answer.

Short answer

The boiling point of 1-chloropropane is expected to be higher than that of propane due to its additional intermolecular forces (dipole-dipole interactions) arising from the presence of a polar chlorine atom in its molecular structure. Propane, a non-polar molecule, only has van der Waals forces present and therefore has a lower boiling point.

Step-by-step solution

Identify the chemical structures of propane and 1-chloropropane

Propane has the chemical formula C₃H₈, and its structure is three carbon atoms connected in a straight chain, with hydrogen atoms filling the remaining spaces to satisfy each carbon's four bonds. 1-chloropropane has the chemical formula C₃H₇Cl, and its structure is similar to propane, except one of the hydrogen atoms is replaced by a chlorine atom.

Identify the intermolecular forces in propane and 1-chloropropane

Propane is a non-polar molecule because of its symmetrical structure and the relatively equal electronegativities of carbon and hydrogen. The main intermolecular force present in propane is van der Waals forces, also known as London dispersion forces, which are relatively weak. 1-chloropropane, on the other hand, is a polar molecule due to the presence of the more electronegative chlorine atom. As a result, dipole-dipole interactions are present, in addition to van der Waals forces, which are stronger in polar molecules.

Compare boiling points based on intermolecular forces

More substantial intermolecular forces require more energy to overcome during boiling, leading to higher boiling points. Since 1-chloropropane has stronger dipole-dipole interactions in addition to van der Waals forces, we can expect its boiling point to be higher than that of propane, which only has van der Waals forces present. In conclusion, the boiling point of 1-chloropropane is expected to be higher than that of propane due to its additional intermolecular forces (dipole-dipole interactions) arising from the presence of a polar chlorine atom in its molecular structure.

Key concepts

Intermolecular Forces

Understanding intermolecular forces is crucial when comparing the boiling points of different substances. These forces are the attractions that occur between molecules. The stronger these forces, the more energy is required to overcome them and turn a liquid into a gas, which occurs at the boiling point.

In the case of propane and 1-chloropropane, their boiling points are influenced by the different intermolecular forces present. Propane, being non-polar, experiences only van der Waals forces, while 1-chloropropane, which is polar due to the chlorine atom, experiences both van der Waals and dipole-dipole interactions.

Chemical Structures

The chemical structures of propane and 1-chloropropane play a decisive role in determining their physical properties, like boiling points. Propane's structure is a simple chain of three carbon atoms, with hydrogen atoms filling in the rest. This uniformity results in non-polarity, meaning the molecule has an even charge distribution.

However, by replacing one hydrogen atom in propane with a chlorine atom to form 1-chloropropane, the symmetry is disrupted, creating a polar molecule where the charge distribution is uneven. This small change in chemical structure significantly affects the intermolecular forces present.

Van der Waals Forces

Van der Waals forces are a type of weak intermolecular force that includes attractions and repulsions between atoms, molecules, and surfaces, as well as other intermolecular forces. They result from the interchange of electron positions within molecules, leading to momentary polarity and, thus, a temporary attraction between molecules.

These forces are the only type present in non-polar molecules like propane, resulting in its relatively low boiling point compared to compounds with polar molecules, which exhibit stronger intermolecular forces.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the partial positive charge of one molecule is attracted to the partial negative charge of another. This type of intermolecular force is significantly stronger than van der Waals forces.

1-chloropropane has both van der Waals forces and dipole-dipole interactions due to its polar nature, which is the result of the chlorine atom's high electronegativity compared to hydrogen and carbon. This polarity leads to an asymmetric distribution of electrons, causing these stronger intermolecular forces that contribute to a higher boiling point.