Question

Predict and explain the relative boiling points of \(\mathrm{CH}_{3} \mathrm{CO}_{2} \mathrm{H}\) and \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CHO}\).

Short answer

The boiling point of ethanoic acid, CH3CO2H, is predicted to be higher than that of propanal, CH3CH2CHO, due to its stronger intermolecular hydrogen bonding.

Step-by-step solution

Identify Intermolecular Forces

First, identify the types of intermolecular forces present in each molecule. CH3CO2H (Ethanoic acid) has strong hydrogen bonds because of the -CO2H group, while CH3CH2CHO (Propanal) has weaker dipole-dipole interactions due to the polar C=O and C-H bonds, but no hydrogen bonding.

Analyze Hydrogen Bonding

Next, recognize that Ethanoic acid, CH3CO2H, with its ability to form two hydrogen bonds per molecule due to the two electronegative oxygen atoms on the carboxylic acid group, exhibits stronger intermolecular forces compared to Propanal, CH3CH2CHO, which cannot participate in hydrogen bonding.

Conclude Boiling Point Prediction

Since a stronger intermolecular force results in more energy required to break that force, it's concluded that the boiling point of ethanoic acid CH3CO2H would be higher than propanal CH3CH2CHO.

Key concepts

Boiling Point Prediction

Understanding the boiling point of a substance is a fundamental aspect of chemistry that provides insight into its physical properties and behavior. The boiling point of a substance is the temperature at which it changes from a liquid to a gas state. To predict the boiling points of different substances, it's crucial to look at the types of intermolecular forces present because these forces determine how tightly molecules hold onto each other.

One can generally predict that a molecule with stronger intermolecular forces will have a higher boiling point. This is because more energy, in the form of heat, is required to overcome these forces during the transition from liquid to vapor. The exercise posed involves two organic compounds, ethanoic acid, and propanal.

In the step-by-step solution, we identify that ethanoic acid has stronger hydrogen bonds due to the -CO2H group, while propanal exhibits weaker dipole-dipole interactions. Based on this data, it can be predicted that ethanoic acid will have a higher boiling point than propanal as it has stronger intermolecular forces that require more energy to overcome.

Hydrogen Bonding

Hydrogen bonding is a unique and particularly strong type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine. This bonding is a significant factor in determining the boiling point of compounds.

In the context of our exercise, ethanoic acid (\(\mathrm{CH}_{3}\mathrm{CO}_{2}\mathrm{H}\)) can form two hydrogen bonds per molecule, which makes its network of intermolecular forces particularly robust. These hydrogen bonds form when the hydrogen attached to the electronegative oxygen atom of one molecule is attracted to the oxygen atom of another molecule. This interaction leads to the formation of a strong link between the molecules, thus requiring more heat energy to break these links and causing a higher boiling point compared to molecules with weaker or no hydrogen bonding.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another molecule. They arise due to the uneven distribution of electrons within the molecule, creating a permanent dipole moment. While these interactions are significant, they are generally weaker than hydrogen bonds.

As seen in propanal (\(\mathrm{CH}_{3}\mathrm{CH}_{2}\mathrm{CHO}\)), these interactions are due to the polar C=O and C-H bonds. However, unlike hydrogen bonds that require a hydrogen bonded to a highly electronegative atom, dipole-dipole interactions can occur with any polar molecules. This explains why propanal, which is unable to form hydrogen bonds, has weaker intermolecular forces and, therefore, a lower boiling point than ethanoic acid.

The strength of dipole-dipole interactions depends on the polarity of the molecule - the more polar the molecule, the stronger these interactions will be. Because boiling points are directly related to the strength of intermolecular forces present, understanding dipole-dipole interactions is critical for predicting the boiling points of polar substances.