Ethylene glycol (HOCH2CH2OH), the major substance in antifreeze, has a normal boiling point of 198 °C. By comparison, ethyl alcohol (CH3CH2OH) boils at 78 °C at atmospheric pressure. Ethylene glycol dimethyl ether (CH3OCH2CH2OCH3) has a normal boiling point of 83 °C, and ethyl methyl ether (CH3CH2OCH3) has a nomral boiling point of 11 °C. (a) Explain why replacement of a hydrogen on the oxygen by a CH3 group generally results in a lower boiling point.

Verified step by step guidance

1. Identify the functional groups in each compound and understand their influence on intermolecular forces. Ethylene glycol has hydroxyl groups (OH), ethyl alcohol also has a hydroxyl group, while the ethers (ethylene glycol dimethyl ether and ethyl methyl ether) contain ether groups (C-O-C).
2. Recognize that hydroxyl groups can form hydrogen bonds, which are strong intermolecular forces that significantly increase the boiling points of compounds.
3. Understand that when a hydrogen in a hydroxyl group is replaced by a methyl group (CH3), the ability to form hydrogen bonds is lost, leading to weaker intermolecular forces in ethers compared to alcohols.
4. Compare the boiling points of the compounds to see the effect of the functional groups. Note that compounds with hydroxyl groups (capable of hydrogen bonding) have higher boiling points than those with ether groups, which mainly exhibit dipole-dipole interactions and London dispersion forces.
5. Conclude that the replacement of a hydrogen atom on the oxygen by a methyl group in ethers reduces the boiling point because it prevents the formation of hydrogen bonds, leading to weaker intermolecular forces compared to alcohols.

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a strong type of dipole-dipole attraction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen or nitrogen. In molecules like ethylene glycol and ethyl alcohol, hydrogen bonds significantly increase boiling points due to the energy required to break these interactions. When a hydrogen atom is replaced by a methyl group (CH3), the ability to form hydrogen bonds is reduced, leading to lower boiling points.

Molecular Structure and Polarity

The molecular structure and polarity of a compound influence its boiling point. Polar molecules, which have an uneven distribution of charge, tend to have higher boiling points due to stronger intermolecular forces. In the case of ethers like ethyl methyl ether, the presence of nonpolar methyl groups decreases overall polarity, resulting in weaker intermolecular forces and lower boiling points compared to more polar alcohols.

Intermolecular Forces

Intermolecular forces are the forces of attraction between molecules, which include hydrogen bonds, dipole-dipole interactions, and London dispersion forces. The strength and type of these forces directly affect a substance's boiling point. When a hydrogen atom is replaced by a methyl group, the overall intermolecular forces are weakened, leading to a decrease in boiling point as less energy is needed to separate the molecules during the phase change from liquid to gas.