Question

(a) What atoms must a molecule contain to participate in hydrogen bonding with other molecules of the same kind? (b) Which of the following molecules can form hydrogen bonds with other molecules of the same kind: \(\mathrm{CH}_{3} \mathrm{~F}, \mathrm{CH}_{3} \mathrm{NH}_{2}, \mathrm{CH}_{3} \mathrm{OH}, \mathrm{CH}_{3} \mathrm{Br} ?\)

Short answer

To form hydrogen bonds, molecules must have hydrogen bonded to F, O, or N. Thus, \( \mathrm{CH}_{3}\mathrm{NH}_{2} \) and \( \mathrm{CH}_{3}\mathrm{OH} \) can form hydrogen bonds with similar molecules; \( \mathrm{CH}_{3}\mathrm{F} \) and \( \mathrm{CH}_{3}\mathrm{Br} \) cannot.

Step-by-step solution

Understanding Hydrogen Bonds

A hydrogen bond occurs when a hydrogen atom is covalently bonded to a strongly electronegative atom such as fluorine (F), oxygen (O), or nitrogen (N), and this hydrogen forms a weak bond with a lone pair of electrons on another electronegative atom.

Identifying the Required Atoms

To participate in hydrogen bonding with molecules of the same kind, a molecule must contain a hydrogen atom bonded directly to fluorine (F), oxygen (O), or nitrogen (N). This creates the necessary condition for hydrogen bonds due to the electronegativity of these atoms.

Evaluating CH₃F

In  \( \mathrm{CH}_{3}\mathrm{~F} \) , hydrogen is not bonded to the fluorine atom; instead, it is bonded to carbon. Therefore, it cannot participate in hydrogen bonding with its own kind as it lacks the necessary bond between hydrogen and an electronegative atom.

Evaluating CH₃NH₂

In  \( \mathrm{CH}_{3}\mathrm{NH}_{2} \) , there is a hydrogen atom bonded directly to nitrogen. Nitrogen is a highly electronegative atom, enabling \( \mathrm{CH}_{3}\mathrm{NH}_{2} \) to form hydrogen bonds with other \( \mathrm{CH}_{3}\mathrm{NH}_{2} \) molecules.

Evaluating CH₃OH

In \( \mathrm{CH}_{3}\mathrm{OH} \) , there is a hydrogen atom bonded directly to oxygen, which is another highly electronegative atom. This allows \( \mathrm{CH}_{3}\mathrm{OH} \) to form hydrogen bonds with other \( \mathrm{CH}_{3}\mathrm{OH} \) molecules.

Evaluating CH₃Br

In  \( \mathrm{CH}_{3}\mathrm{Br} \) , hydrogen is bonded to carbon and not to an electronegative atom like fluorine, oxygen, or nitrogen. Therefore, it cannot form hydrogen bonds with molecules of the same kind.

Key concepts

Electronegativity

Electronegativity is a crucial concept in chemistry, particularly when understanding the behaviors of molecules and the types of bonds they can form.
It refers to the tendency of an atom to attract electrons toward itself when it is part of a compound. Generally, atoms with high electronegativity values pull electrons more strongly. The most electronegative elements include fluorine (F), oxygen (O), and nitrogen (N). This characteristic is vital for forming hydrogen bonds. Why? Because a hydrogen bond forms best when a hydrogen atom is attached to a highly electronegative atom like F, O, or N.
This bond creates a partial positive charge on the hydrogen, which attracts the lone pair of electrons belonging to another nearby electronegative atom, making them more likely to form such bonds. Some key points to remember about electronegativity:

• The greater the difference in electronegativity between two bonded atoms, the more polar their bond is.
• In molecules, the polarity of their bonds due to differing electronegativities contributes significantly to how they interact with other molecules.
• Fluorine is the most electronegative element, followed by oxygen and nitrogen.

Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. It's essential for determining the molecule's physical and chemical properties.
The structure affects how molecules interact with each other, and whether they can engage in hydrogen bonding. To partake in hydrogen bonding, a molecule must have at least one hydrogen atom directly bonded to an electronegative atom like F, O, or N. The arrangement will mean that the hydrogen becomes slightly positive. This slight positive charge allows it to attract lone pairs from electronegative atoms in nearby molecules.
For example, the molecular structure of water ( H₂O) includes two hydrogen atoms bonded to oxygen, which makes water a classic case of a hydrogen-bonding molecule. Important things about molecular structures related to hydrogen bonding:

• Molecules with branched or complicated structures may still participate in hydrogen bonding if they contain the necessary hydrogen-electronegative atom bond.
• The ability of a molecule to engage in hydrogen bonding significantly affects its boiling and melting points. Molecules with more opportunities for hydrogen bonding will generally have higher boiling and melting points.
• Hydrogen bonding influences the molecular geometry, often bending or angling the molecule due to these interactions.

Intermolecular Forces

Intermolecular forces, including hydrogen bonds, are the forces of attraction or repulsion between molecules, differing from the forces holding molecules (intra-molecular forces) together.
When atoms form a molecule, they share electrons and form chemical bonds. But in a collection of molecules, it's the intermolecular forces that hold them closely together. There are several kinds of intermolecular forces:

• Hydrogen Bonds: The strongest type of dipole-dipole interaction. It occurs when a hydrogen atom bonded to F, O, or N in one molecule is attracted to the electronegative atom of another molecule.
• Dipole-Dipole Forces: Occur between molecules that have permanent dipole moments (i.e., molecules with a distinct positive and negative pole).
• London Dispersion Forces: These are the weakest and are present in all molecules, caused by temporary shifts in electron density within a molecule.

Hydrogen bonds play a significant role in biological systems too. They hold together strands of DNA and are crucial for the properties of water. Understanding these forces helps make sense of why substances have the properties they do, like why water has such a high surface tension or why ice floats.
Appreciating the nuances of intermolecular forces will deepen your overall understanding of chemistry and molecular interactions.