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?

Short answer

To rank hydrocarbons in order of increasing boiling point, consider their molecular size, molecular weight, and strength of dispersion forces. Larger molecules and those with greater molecular weight generally have higher boiling points due to increased dispersion forces. Hydrocarbons with a higher number of carbons, and stronger dispersion forces will have higher boiling points. Arrange the hydrocarbons based on the number of carbons and structural features, considering the strength of their dispersion forces.

Step-by-step solution

Understand the factors affecting boiling points of hydrocarbons

Boiling point is influenced by molecular size, molecular weight, and intermolecular forces. Larger molecules and molecules with more molecular weight generally have higher boiling points due to increased dispersion forces. The boiling point also depends on the strength of the intermolecular forces, including dispersion forces, dipole-dipole interactions, and hydrogen bonding.

Examine the molecular size of the hydrocarbons

Larger molecules have more atoms, leading to a larger surface area that can interact with other surrounding molecules. This increased interaction leads to stronger dispersion forces, which cause a higher boiling point. So, it is important to evaluate the number of carbons in the hydrocarbon molecules in order to determine their relative sizes.

Consider the molecular weight of the hydrocarbons

Similarly, hydrocarbons with more molecular weight tend to have stronger dispersion forces due to an increase in the number of electrons. To compare the molecular weights of the hydrocarbons, count the total number of atoms (carbon and hydrogen) present in each hydrocarbon molecule.

Evaluate the intermolecular forces of the hydrocarbons

Hydrocarbons are nonpolar molecules due to the similar electronegativity of carbon and hydrogen atoms. Thus, they do not have dipole-dipole interactions or hydrogen bonding. However, all hydrocarbons experience dispersion forces (also known as London dispersion forces or van der Waals forces), which are weaker intermolecular forces that arise from the temporary fluctuations of electron density around the atoms. The strength of dispersion forces increases with molecular size and molecular weight of the hydrocarbon molecules.

Rank the hydrocarbons in order of increasing boiling point

Considering the factors discussed in previous steps, rank the hydrocarbons based on molecular size, molecular weight, and the strength of dispersion forces. Hydrocarbons with larger molecular size, greater molecular weight, and stronger dispersion forces will have a higher boiling point. To do this, first arrange the hydrocarbons in increasing order of the number of carbons, ensuring that you consider the presence of any branching or other structural features. Then, if there are any isomers with the same number of carbons but different structural features, arrange them based on the strength of their dispersion forces. Finally, arrange the hydrocarbons in the order of increasing boiling point, considering all of the factors above.