The boiling point of \(\mathrm{CH} 3 \mathrm{OH}\) is \(65^{\circ} \mathrm{C}\), while the boiling point of \(\mathrm{CH} 3 \mathrm{SH}\) is only \(6^{\circ} \mathrm{C}\). Explain this difference in boiling point.

Understanding intermolecular forces is essential when determining the physical properties of substances, such as boiling points. These forces are the attractions that occur between neighboring molecules. They vary in strength based on the types of particles involved and their proximity. In our exercise, we notice a significant difference in the boiling points of two organic compounds, methanol (CH3OH) and methanethiol (CH3SH), which can be attributed to the kinds of intermolecular forces present.

There are three main types of intermolecular forces: London dispersion forces, dipole-dipole interactions, and hydrogen bonding. London dispersion forces are the weakest and come into play between all molecules, whether polar or nonpolar, due to the momentary distribution of electrons. Dipole-dipole interactions occur when the positive end of a polar molecule is attracted to the negative end of another. These are stronger than London forces but still weaker than the third type, which greatly affects boiling points—hydrogen bonding.

Role in Boiling Points

The energy required to boil a liquid is essentially the energy needed to overcome its intermolecular forces. Substances with stronger intermolecular forces have higher boiling points because it takes more energy to separate the molecules. This is why the study of these forces is critical when analyzing why molecules behave differently under heat.

Hydrogen bonding, a unique and strong type of dipole-dipole interaction, plays a pivotal role in explaining the higher boiling point of methanol compared to methanethiol. For hydrogen bonds to occur, a molecule must feature a hydrogen atom bonded to an electronegative element such as oxygen, nitrogen, or fluorine. This hydrogen develops a strong attraction to a lone pair of electrons on an electronegative atom in another molecule, creating a bridge-like bond.

Methanol exhibits hydrogen bonding because it has an O-H bond with oxygen's electronegativity pulling electron density away from hydrogen, making it partially positive. This enables it to strongly attract nearby electronegative atoms with lone pairs, just like magnets. Methanethiol, however, contains an S-H bond, and since sulfur is less electronegative than oxygen, the attraction is weaker, and hydrogen bonds do not form significantly.

Impact on Physical Properties

Hydrogen bonds are intrinsically strong due to the significant difference in electronegativity between hydrogen and atoms like oxygen. This strength needs more thermal energy to be broken, leading to high boiling points. Methanol's capacity to form these bonds results in a boiling point of 65°C. Methanethiol cannot establish these strong interactions and has a much lower boiling point of only 6°C.

Electronegativity is a concept that helps us understand why different atoms have different abilities to attract electrons when forming chemical bonds. Atoms closer to the top right of the periodic table – except for the noble gases – are generally more electronegative. Oxygen, for instance, is highly electronegative and strongly attracts electrons in covalent bonds.

The disparity in electronegativity between elements is a crucial factor when considering the strength of hydrogen bonds. For hydrogen bonding to be robust, as seen in methanol, the bonded electronegative atom must be able to attract electrons towards itself effectively, creating a significant partial positive charge on the hydrogen atom. In contrast, sulfur is less electronegative than oxygen, so the S-H bond in methanethiol generates a less pronounced charge separation, leading to weaker attractions between molecules and thus a lower boiling point.

Effects on Boiling Points

The stronger the electronegative pull in a molecule, the greater the charge difference, and subsequently, the stronger the intermolecular attractions. Hence, electronegativity directly influences the boiling point of substances, with higher electronegativity correlating to higher boiling points through stronger intermolecular forces like hydrogen bonding.