Question

Rationalize the fact that hydrogen bonding has not been observed between \(\mathrm{CH}_{4}\) molecules.

Short answer

Hydrogen bonding is not observed in \(\text{CH}_4\) because it lacks highly electronegative atoms and lone pairs required for such bonding.

Step-by-step solution

Understand Hydrogen Bonding

Hydrogen bonding occurs when a hydrogen atom, which is covalently bonded to a highly electronegative atom such as oxygen (O), nitrogen (N), or fluorine (F), is electrostatically attracted to a lone pair of electrons on another electronegative atom.

Identify Electronegativity in \(\textbf{CH}_4\)

Methane (\(\text{CH}_4\)) consists of one carbon (C) atom bonded to four hydrogen (H) atoms. The carbon-hydrogen bond involves a carbon atom with an electronegativity of 2.55 and hydrogen with an electronegativity of 2.20, leading to a relatively nonpolar bond.

Check for Electronegativity Difference

The electronegativity difference between carbon (2.55) and hydrogen (2.20) is 0.35. This difference is too small to make the carbon-hydrogen bond significantly polar, which is a prerequisite for hydrogen bonding.

Assess Lone Pair Availability

A lone pair is necessary for hydrogen bonding. In the case of \( \text{CH}_4 \), neither the carbon nor the hydrogen has lone pairs of electrons available to participate in hydrogen bonding.

Conclude Based on Requirements

Given that \(\text{CH}_4\) lacks highly electronegative atoms (such as O, N, or F) and lone pairs, it cannot engage in hydrogen bonding. Therefore, no hydrogen bonding is observed between methane molecules.

Key concepts

hydrogen bonding

Hydrogen bonding is a special type of dipole-dipole interaction and an important concept in chemistry. It occurs when a hydrogen atom, that is covalently bonded to a highly electronegative atom like oxygen (O), nitrogen (N), or fluorine (F), forms a temporary bond with a lone pair of electrons on another electronegative atom.
This bond is a result of the attraction between the partial positive charge on the hydrogen atom and the partial negative charge on the lone pair of the second electronegative atom.
Some key features of hydrogen bonding include:

• It is stronger than regular dipole-dipole interactions but weaker than covalent and ionic bonds.
• Hydrogen bonds significantly affect the physical properties of compounds, like boiling and melting points.
• Hydrogen bonding is crucial in biological systems; for example, it helps stabilize the structure of proteins and DNA.
• Water's unique properties, such as its high boiling point and surface tension, are due to hydrogen bonding.

electronegativity

Electronegativity is a measure of an atom's ability to attract and hold onto shared electrons in a chemical bond. The higher an atom's electronegativity, the more it pulls electrons towards itself.
This property is key to understanding why certain atoms form hydrogen bonds.
Some characteristics and impacts of electronegativity include:

• Fluorine (F) is the most electronegative element, followed by oxygen (O) and nitrogen (N).
• Electronegativity values can be found using the Pauling scale where fluorine has a value of 3.98, oxygen 3.44, and nitrogen 3.04.
• In water, for example, the oxygen atom's high electronegativity leads it to attract electrons more strongly than hydrogen, creating a polar molecule that can form hydrogen bonds.
• Small differences in electronegativity between bonded atoms (like in C-H bonds) lead to nonpolar bonds, while larger differences result in polar bonds, which are crucial for hydrogen bonding.

In the case of methane (CH₄), the difference in electronegativity between carbon (2.55) and hydrogen (2.20) is only 0.35, making the molecule relatively nonpolar and unable to participate in hydrogen bonding.

lone pairs

Lone pairs are pairs of valence electrons that are not involved in chemical bonding. They are found on the outermost shells of an atom.
Lone pairs are important for the formation of hydrogen bonds because they provide the site where the hydrogen atom from another molecule can be attracted.
Key points about lone pairs include:

• Atoms like oxygen, nitrogen, and fluorine typically have lone pairs available for hydrogen bonding.
• In water (H₂O), the oxygen atom has two lone pairs, which makes it highly capable of forming hydrogen bonds with other water molecules.
• Molecules lacking lone pairs, such as methane (CH₄), cannot participate in hydrogen bonding as they have no lone pairs of electrons to attract other hydrogen atoms.
• This is why methane does not form hydrogen bonds; both carbon and hydrogen in methane do not have lone pairs available.