Explain why ethanol has a higher boiling point than bromoethane and the hydrocarbons?
Ethanol has a higher boiling point compared to bromoethane and most hydrocarbons due to its intermolecular hydrogen bonding and polarity. To understand why ethanol has a higher boiling point, we need to consider the types of intermolecular forces present in each compound.
In ethanol (C2H5OH), there are hydrogen (H) atoms bonded to an oxygen (O) atom. This oxygen atom is highly electronegative, meaning it attracts electrons more strongly. Consequently, the oxygen atom pulls the shared electrons towards itself, creating a partial negative charge (δ-) on the oxygen and partial positive charges (δ+) on the hydrogen atoms.
These partial charges in ethanol result in the formation of intermolecular hydrogen bonds. Hydrogen bonding occurs when the positive hydrogen atom of one ethanol molecule is attracted to the negative oxygen atom of a neighboring molecule. These hydrogen bonds are relatively strong and require a higher amount of energy to break. Therefore, a higher temperature, or boiling point, is necessary to convert ethanol from a liquid to a gas phase.
On the other hand, bromoethane (C2H5Br) is a halogenated hydrocarbon that lacks the presence of hydrogen bonding. Although bromoethane is polar due to the difference in electronegativity between carbon, hydrogen, and bromine atoms, the polarity of this compound is not as pronounced as in ethanol. The strength of dipole-dipole interactions in bromoethane is weaker than that of hydrogen bonds, resulting in a lower boiling point.
When we consider hydrocarbons without any halogen or functional groups, such as methane (CH4), ethane (C2H6), and octane (C8H18), the boiling points are even lower. Hydrocarbons are entirely non-polar, making them unable to form strong dipole-dipole interactions. Instead, they exhibit relatively weaker van der Waals forces (London dispersion forces), which primarily depend on the size and shape of the molecules. These forces are much weaker than hydrogen bonding or dipole-dipole interactions, so the boiling points of hydrocarbons are significantly lower than those of ethanol and bromoethane.
In summary, ethanol has a higher boiling point than bromoethane and hydrocarbons due to the presence of intermolecular hydrogen bonding caused by the polarity of the compound. The stronger hydrogen bonds require more energy to break, leading to a higher boiling point.