Question

Ethane-1,2-diol, \(\mathrm{C}_{2} \mathrm{H}_{6} \mathrm{O}_{2},\) has one OH bonded to each carbon. (a) Draw the Lewis dot structure of ethane-1,2-diol. (b) Draw the Lewis dot structure of chloroethane, \(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{Cl}\). (c) Chloroethane has a slightly higher molar mass than ethane- 1,2 -diol but a much lower boiling point \(3{ }^{\circ} \mathrm{C}\) versus \(198^{\circ} \mathrm{C}\) ). Explain.

Short answer

Ethane-1,2-diol has hydrogen bonding, raising its boiling point despite chloroethane's higher molar mass.

Step-by-step solution

Understanding the structure of Ethane-1,2-diol

Ethane-1,2-diol, also known as ethylene glycol, consists of two carbon atoms, each bonded to an oxygen atom and remaining hydrogen atoms. It has the formula \( \mathrm{C}_2 \mathrm{H}_6 \mathrm{O}_2 \). Start by drawing two carbon atoms connected by a single bond. Each carbon will have one OH group attached and remaining valence fulfilled by hydrogen atoms.

Drawing the Lewis Structure for Ethane-1,2-diol

Place the two carbon (C) atoms side by side. Draw a single bond connecting them. Attach an OH group to each carbon atom, signifying the presence of one oxygen (O) bonded to each carbon along with a hydrogen (H). Complete the structure by adding hydrogen atoms to ensure that each carbon atom has four bonds.

Understanding the structure of Chloroethane

Chloroethane has a similar structure to ethane but with one chlorine (Cl) atom replacing a hydrogen. The chemical formula is \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \). We need to have two carbon atoms connected by a single bond, one attached to a chlorine atom, and the rest of the bonds filled by hydrogen atoms.

Drawing the Lewis Structure for Chloroethane

Draw two carbon atoms connected by a single bond. Attach a chlorine atom to one of the carbon atoms. Add hydrogen atoms so that each carbon has four bonds filled. This completes the structure of chloroethane.

Explaining Molar Mass and Boiling Points

Chloroethane has a slightly higher molar mass due to the higher atomic mass of chlorine compared to oxygen. However, ethane-1,2-diol can form hydrogen bonds because of the -OH groups, leading to strong intermolecular interactions. These interactions significantly increase its boiling point compared to chloroethane, which forms weaker van der Waals forces.

Key concepts

Molecular Structure

The molecular structure of a chemical compound provides a detailed map of how the atoms within the molecule are bonded and arranged. Understanding the molecular structure is like solving a puzzle to see the full picture of a molecule's behavior.

When we look at ethane-1,2-diol, commonly known as ethylene glycol, its molecular structure consists of two carbon atoms. These carbons are joined by a single carbon-carbon bond, and each carbon atom is also bonded to an OH group. This arrangement leads to the chemical formula \( \mathrm{C}_2 \mathrm{H}_6 \mathrm{O}_2 \).

• The carbons are at the core, establishing a backbone for the molecule.
• The OH groups bonded to each carbon are what lends ethylene glycol its ability to form hydrogen bonds.

In contrast, chloroethane has a molecular structure where one hydrogen in ethane is replaced by a chlorine atom, thus having the formula \( \mathrm{C}_2 \mathrm{H}_5 \mathrm{Cl} \). Both compounds have carbons as their central bonding sites, but their molecular structures influence their respective physical properties.

Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine many physical properties such as melting and boiling points. These forces vary depending on the types of atoms and groups within molecules.

Ethane-1,2-diol features strong hydrogen bonding due to the presence of its OH groups. Hydrogen bonds are special attractive forces that occur when hydrogen is bonded to a more electronegative atom, such as oxygen. This results in a partial positive charge on the hydrogen atom. In ethylene glycol, these hydrogen bonds create a strong network of attractions between the molecules, significantly influencing its boiling point.

• Hydrogen bonds are stronger than van der Waals forces or dipole-dipole interactions found in molecules like chloroethane.
• These forces make ethane-1,2-diol more cohesive, resulting in higher energy needed to break the intermolecular bonds.

Chloroethane, lacking the OH groups, primarily exhibits van der Waals forces. These are weaker compared to hydrogen bonds, leading to less energy needed to separate the molecules during a phase change like boiling.

Molar Mass

Molar mass is a measure of the mass of a given substance (chemical element or chemical compound) divided by the amount of substance in moles. Though chloroethane has a slightly higher molar mass than ethane-1,2-diol, the effects of mass on physical properties like boiling points are outweighed by intermolecular forces.

Chloroethane's molar mass is higher because chlorine, with an atomic mass of around 35.5 amu, is heavier than hydrogen or oxygen found in ethane-1,2-diol. However, in ethane-1,2-diol, the oxygen atoms play a crucial role in forming hydrogen bonds, intensifying intermolecular interactions beyond what molar mass typically contributes.

• In general, heavier molecules with similar intermolecular forces tend to have higher boiling points. But hydrogen bonding can cause exceptions to this trend.
• This explains ethane-1,2-diol’s unusually high boiling point despite having a lower molar mass.

Boiling Point

Boiling point refers to the temperature at which a substance changes from a liquid to a gas. This property is largely affected by the types and strengths of intermolecular forces present between molecules.

Ethane-1,2-diol has a significantly higher boiling point than chloroethane. Ethane-1,2-diol boils at 198°C, mainly due to its capacity to form hydrogen bonds. These bonds necessitate more energy (in the form of heat) to sever, thus raising the boiling point.

• Hydrogen bonding in ethane-1,2-diol serves as a robust "glue" holding molecules together in the liquid phase.
• The high boiling point indicates a more cohesive molecular structure.

On the flip side, chloroethane has a boiling point of just 3°C because it can't form hydrogen bonds. It only relies on van der Waals forces, which are much weaker, making it quicker to evaporate when heated. Thus, understanding and comparing the boiling points of these substances reveals much about the strength of the forces at work within them.