Question

The following substances have boiling points as indicated: ethyl ethanoate \(\left(77^{\circ}\right)\) ethanoic anhydride \(\left(140^{\circ}\right)\) ethanoic acid \(\left(118^{\circ}\right)\) ethanamide \(\left(221^{\circ}\right)\) Account for these differences on the basis of molecular weight and hydrogen bonding.

Short answer

Boiling points vary due to molecular weights and hydrogen bonding, with strong hydrogen bonding leading to higher boiling points.

Step-by-step solution

Identify Molecular Weights

First, calculate the molecular weights of each compound. Ethyl ethanoate (C₄H₈O₂) has a molecular weight of 88 g/mol, ethanoic anhydride (C₄H₆O₃) has a molecular weight of 102 g/mol, ethanoic acid (C₂H₄O₂) has a molecular weight of 60 g/mol, and ethanamide (C₂H₅NO) has a molecular weight of 59 g/mol.

Analyze Hydrogen Bonding Capacity

Next, identify hydrogen bonding capabilities. Ethanoic acid and ethanamide both have strong hydrogen bonding due to the presence of -OH and -NH₂ groups, respectively. Ethyl ethanoate and ethanoic anhydride have little or no hydrogen bonding, lacking these groups.

Link Boiling Points to Molecular Interactions

Now, connect boiling points to molecular interactions. Higher boiling points result from stronger intermolecular forces. Ethanamide has the highest boiling point due to strong hydrogen bonding. Ethanoic acid also has significant hydrogen bonding, giving it a high boiling point, but not as high as ethanamide. Ethanoic anhydride has a relatively high boiling point due to its higher molecular weight and some dipole interactions. Ethyl ethanoate has the lowest boiling point, attributed to its lower molecular weight and minimal hydrogen bonding.

Key concepts

Molecular Weight

Molecular weight, often referred to as molar mass, is the sum of the atomic weights of all atoms in a molecule. It plays a crucial role in determining various physical properties of a substance. In the context of boiling points, molecular weight can influence the energy required to transition a substance from liquid to gas.
Generally, compounds with higher molecular weights will have higher boiling points. This is because more energy is required to overcome the greater number of interactions between the larger number of atoms. For example, ethanoic anhydride \(C_4H_6O_3\), with a molecular weight of 102 g/mol, has a relatively high boiling point of 140°C. This is due in part to its larger molecular structure compared to ethanoic acid \(C_2H_4O_2\) with 60 g/mol. However, molecular weight isn't the sole factor; intermolecular forces also play a significant role.

Boiling Points

Boiling points are a measure of the temperature at which a substance transitions from liquid to gaseous state. This phase change is strongly influenced by the nature of the forces acting between the molecules, known as intermolecular forces.
For instance, ethanamide \(C_2H_5NO\) exhibits a very high boiling point of 221°C because it can form strong hydrogen bonds due to the presence of \(-NH_2\) groups. Hydrogen bonds are much stronger than other types of dipole-dipole interactions. The boiling point of a compound is thus not only a factor of molecular weight but also the strength and nature of its intermolecular forces.
Therefore, assessing both molecular weight and intermolecular interactions can give a comprehensive view of why certain substances boil at higher temperatures.

Intermolecular Forces

Intermolecular forces are the forces that mediate interaction between molecules, including forces of attraction or repulsion which act between molecules and other types of neighboring particles. These forces determine a wide range of physical properties, including boiling points.
One of the strongest types of intermolecular forces is hydrogen bonding. This specific interaction occurs when a hydrogen atom, covalently bonded to a highly electronegative atom like oxygen or nitrogen, attracts another electronegative atom. For example, ethanoic acid \(C_2H_4O_2\) can form hydrogen bonds because it contains hydroxyl \(-OH\) groups, which increases its boiling point to 118°C.
In contrast, ethyl ethanoate \(C_4H_8O_2\) lacks these groups and thus experiences weaker Van der Waals forces only, resulting in a lower boiling point of 77°C. Therefore, the strength of intermolecular forces like hydrogen bonding is key to understanding the differences in boiling points among substances.