Question

"Ethyl alcohol" and "dimethyl ether" have the same molecular mass. Explain why the boiling point of "ethyl alcohol" \(\left(78^{\circ} \mathrm{C}\right)\) is much higher than the boiling point of "dimethyl ether" \(\left(-25^{\circ} \mathrm{C}\right)\).

Short answer

Ethyl alcohol forms strong hydrogen bonds, requiring more energy to break, leading to a higher boiling point than dimethyl ether, which lacks these bonds.

Step-by-step solution

Identify Chemical Structures

First, understand the structures of ethyl alcohol and dimethyl ether. Ethyl alcohol (C2H5OH) contains a hydroxyl group (-OH), while dimethyl ether (C2H5OC2H5) has an ether group (R-O-R'). These differing structures influence their boiling points through different types of intermolecular forces.

Compare Intermolecular Forces

Ethyl alcohol is capable of forming hydrogen bonds due to the presence of the hydroxyl group (-OH). Hydrogen bonds are strong intermolecular forces that require more energy to break. In contrast, dimethyl ether cannot form hydrogen bonds because it lacks an -OH group, resulting only in weaker van der Waals forces.

Analyze Effect on Boiling Points

Because hydrogen bonds are much stronger than van der Waals forces, more energy is required to disrupt these bonds in ethyl alcohol. This leads to a higher boiling point of 78°C for ethyl alcohol compared to -25°C for dimethyl ether, which relies only on the weaker van der Waals interactions to hold molecules together.

Key concepts

Hydrogen Bonding

Hydrogen bonding plays a crucial role in determining the physical properties of substances. This strong type of intermolecular force occurs when a hydrogen atom is bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine. In the case of ethyl alcohol, the presence of an -OH (hydroxyl) group enables it to form hydrogen bonds. These specific interactions happen between the hydrogen of the hydroxyl group and the oxygen from another molecule's -OH group.

What makes hydrogen bonds so special? They are significantly stronger than many other types of intermolecular forces. Because of this strength, substances with hydrogen bonding typically have higher boiling points and melting points. In essence, you need more energy, which translates to higher temperatures, to break these bonds and allow the substance to change state.

van der Waals Forces

Van der Waals forces are another category of intermolecular forces. These forces are generally weaker than hydrogen bonds and occur due to temporary dipoles that form as electrons move around a molecule. There are different types of van der Waals forces, but the most common ones include London dispersion forces and dipole-dipole interactions.

Dimethyl ether mainly relies on these van der Waals forces. Since it lacks the -OH group found in ethyl alcohol, it can't form hydrogen bonds. The forces acting in dimethyl ether are much weaker, making it easier for its molecules to separate and move into the gaseous phase, hence the lower boiling point. Generally, the strength of van der Waals forces depends on the size and shape of molecules, but they never quite match the robust strength of hydrogen bonds.

Chemical Structures

Understanding chemical structures is key to comprehending why substances behave differently in terms of boiling points, solubility, and other properties. The structure of a molecule not only defines its basic identity but also the types of intermolecular forces it can participate in. In ethyl alcohol versus dimethyl ether, both have similar molecular formulas but different structures.

Ethyl alcohol has a hydroxyl group (-OH) that enables hydrogen bonding. Dimethyl ether features an ether group (R-O-R') that precludes such interactions. This difference in presence or absence of functional groups capable of hydrogen bonding significantly influences their physical characteristics, despite having identical molar masses. A structural formula helps us visualize these subtleties, which further explains why ethyl alcohol's boiling point far exceeds that of dimethyl ether.

Boiling Point

The boiling point of a substance represents the temperature at which its liquid form transitions into a gas. This point is highly influenced by the type and strength of intermolecular forces present in the liquid. Substances like ethyl alcohol, which engage in strong hydrogen bonds, have higher boiling points.

In contrast, dimethyl ether, which relies only on relatively weak van der Waals forces, boils at a much lower temperature. This stark difference in boiling points between these two chemicals, despite sharing a similar molecular weight, reflects the stronger and more energetically intense hydrogen bonds in ethyl alcohol. Thus, recognizing the boiling point helps to predict the energy needed for a compound to change states, which in turn provides insight into its molecular structure and bonding characteristics.