Question

Compounds that contain an \(\mathrm{N}-\mathrm{H}\) group associate by hydrogen bonding. (a) Do you expect this association to be stronger or weaker than that of compounds containing an \(\mathrm{O}-\mathrm{H}\) group? (b) Based on your answer to part (a), which would you predict to have the higher boiling point, 1 -butanol or 1 -butanamine?

Short answer

Answer: 1-butanol is likely to have a higher boiling point because it contains an O-H group, which exhibits stronger hydrogen bonding due to oxygen's higher electronegativity compared to nitrogen in the N-H group found in 1-butanamine. Stronger hydrogen bonding results in stronger intermolecular forces and higher boiling points.

Step-by-step solution

Understand the molecules involved

First, let's recall that hydrogen bonding occurs most strongly between very electronegative atoms like nitrogen (N), oxygen (O), and fluorine (F) bonded to hydrogen (H). Now, let's consider the two compounds: 1. Compounds with an N-H group (e.g., 1-butanamine): These have a nitrogen atom (N) bonded to a hydrogen atom (H). 2. Compounds with an O-H group (e.g., 1-butanol): These have an oxygen atom (O) bonded to a hydrogen atom (H).

Compare hydrogen bonding strength

To determine which type of hydrogen bonding is stronger, we need to compare the electronegativity of the nitrogen and oxygen atoms. Electronegativity is a measure of an atom's ability to attract electrons in a bond between it and another atom. The higher an atom's electronegativity, the stronger the hydrogen bond it can form. Oxygen has an electronegativity of 3.44, while nitrogen has an electronegativity of 3.04. So, oxygen is more electronegative than nitrogen.

Determine the stronger hydrogen bonding for association

As oxygen is more electronegative than nitrogen, compounds containing an O-H group will exhibit stronger hydrogen bonding than compounds with an N-H group.

Compare boiling points based on hydrogen bonding strength

Boiling points of compounds are often related to how strongly the molecules interact with each other (intermolecular forces). Stronger hydrogen bonding leads to stronger intermolecular forces, resulting in a higher boiling point. Since compounds with an O-H group (like 1-butanol) exhibit stronger hydrogen bonding than those with an N-H group (like 1-butanamine), we can predict that 1-butanol likely has a higher boiling point than 1-butanamine.

Key concepts

Electronegativity

Electronegativity is a fundamental concept in chemistry, determining how atoms attract bonding electrons. In hydrogen bonding, this property is pivotal as it describes why certain atoms, like oxygen, nitrogen, and fluorine, can form stronger bonds when attached to hydrogen.

The more electronegative an atom is, the greater its ability to draw electrons towards itself, creating a stronger partial negative charge. This leads to a more potent hydrogen bond because the hydrogen atom, which is already partially positive, is more strongly attracted to the electronegative atom. For instance, oxygen's electronegativity rating of 3.44 means it has a stronger pull on electrons than nitrogen, which has an electronegativity of 3.04. This difference accounts for the varying strength of hydrogen bonds in compounds with O-H or N-H groups.

Intermolecular Forces

Intermolecular forces are the forces that mediate interactions between molecules. These include London dispersion forces, dipole-dipole interactions, and hydrogen bonds. Hydrogen bonding, a type of dipole-dipole interaction, is significant due to its strong influence on physical properties like melting and boiling points.

When analysing compounds such as 1-butanamine (N-H group) and 1-butanol (O-H group), the strength of their intermolecular forces is crucial. Stronger hydrogen bonds will result in a more profound connection between molecules, making substances with such connections harder to vaporize. This underscores how understanding the nature of these forces helps predict the behavior of substances under various conditions.

Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding it, causing the substance to change from liquid to gas. This physical property is an excellent indicator of the intermolecular forces at play.

A higher boiling point signifies stronger intermolecular attractions, requiring more heat energy to separate the molecules. In the case of organic compounds with varying hydrogen bonding capabilities, the disparity in boiling points is evident. Oxygen's higher electronegativity leads to stronger hydrogen bonds in 1-butanol, compared to the weaker bonds in 1-butanamine. Thus, we expect 1-butanol to have a higher boiling point, requiring more energy to reach the point of vaporization.

Organic Compounds

Organic compounds are largely comprised of carbon atoms combined with other elements like hydrogen, oxygen, and nitrogen. The structure and functional groups within these compounds significantly impact their physical and chemical properties.

Take, for instance, 1-butanamine and 1-butanol, both of which are organic compounds with different functional groups (N-H and O-H, respectively). Not only do these functional groups define the compound's classification (amine and alcohol), but they also play a critical role in how the compounds interact through intermolecular forces. This connection between structure and property is a central theme in organic chemistry, informing the understanding of how such compounds behave in various environments.