The isomers cyclopropane and propylene both have the formula C3H6

The isomers cyclopropane and propylene both have the formula C3H6 , and they are shown below.

Complete the following statements that compare these two constitutional isomers.

Word Box (can use more than once)

gas

liquid

solid

are important

are not important

cyclopropane

prophylene

1. At 25∘C cyclopropane is a ( ) and propylene is a ( ) , which means that lattice effects ( ) .
2. Comparing the structure of the two compounds indicates that the isomer with greater free rotation is ( ).
3. Given the analysis of lattice effects and structure, the isomer with a higher standard molar entropy at 25∘C is ( ).

The Correct Answer and Explanation is :

1. At 25°C, cyclopropane is a (gas) and propylene is a (liquid), which means that lattice effects (are important).

Cyclopropane and propylene are both structural isomers with the same molecular formula (C3H6), but they differ significantly in their physical properties. Cyclopropane is a gas at room temperature, while propylene is a liquid. The difference in their states can be explained by the molecular structure of each compound. Cyclopropane has a three-membered ring with significant angle strain, which makes the molecule more volatile and less stable, hence it exists as a gas under normal conditions. Propylene, on the other hand, is a more stable molecule with a double bond between two carbon atoms and a single bond to a third carbon atom. This structure allows it to interact more strongly with other molecules through Van der Waals forces, enabling it to exist as a liquid.

Since cyclopropane is a gas and propylene is a liquid, lattice effects (which refer to the interactions between molecules that affect the solid-state structure) play a significant role. In cyclopropane, these effects are minimal in the gaseous state, while in propylene, the stronger intermolecular forces in the liquid state cause more pronounced lattice effects, influencing its properties such as boiling point, density, and entropy.

2. Comparing the structure of the two compounds indicates that the isomer with greater free rotation is (propylene).

Propylene (C3H6) contains a double bond between two carbon atoms and a single bond to a third carbon atom. The single bonds between carbon atoms in propylene allow for free rotation around the bonds, though it is somewhat restricted by the double bond’s planarity. However, in cyclopropane, the three carbon atoms are part of a strained three-membered ring. This ring structure introduces significant angle strain, and the carbon-carbon bonds in the ring are unable to rotate freely. This restricted rotation in cyclopropane limits its flexibility compared to propylene.

3. Given the analysis of lattice effects and structure, the isomer with a higher standard molar entropy at 25°C is (cyclopropane).

Entropy is a measure of disorder or randomness in a system, and it tends to be higher in gases compared to liquids, as gases have more freedom of movement. Cyclopropane, being a gas at 25°C, has more freedom of motion than propylene, which is a liquid. The higher mobility of molecules in the gaseous state generally results in greater entropy. Therefore, cyclopropane has a higher standard molar entropy at 25°C than propylene.