Question

The boiling point of alcohol is high due to: (a) Hydrogen bonding (b) Large size of alcohol (c) Presence of \(-\mathrm{OH}\) group (d) High molecular weight

Short answer

The high boiling point of alcohol is primarily due to hydrogen bonding (a).

Step-by-step solution

Identify Relevant Chemical Properties

To understand why the boiling point of alcohol is high, recognize that boiling points are influenced by molecular interactions and structure. Knowing the characteristics of alcohol is crucial for this.

Review Hydrogen Bonding

Alcohols contain the \(-\mathrm{OH}\) group, which can form hydrogen bonds. Hydrogen bonds are strong intermolecular forces that occur when hydrogen is bonded to electronegative atoms like oxygen, increasing boiling points.

Analyze Influence of Molecular Size

Though larger molecules can lead to higher boiling points due to increased London dispersion forces, the variety in the size of alcohol molecules means this isn't the primary reason for a high boiling point.

Consider the Role of \\(-\\mathrm{OH}\\) Group

The \(-\mathrm{OH}\) group is crucial in forming hydrogen bonds. Its presence promotes hydrogen bonding, suggesting it's significant but not solely the reason for high boiling points by itself.

Evaluate Molecular Weight

High molecular weight can contribute to boiling point by increasing dispersion forces, but significant variation in alcohol weights makes this less relevant as a universal explanation.

Determine the Primary Factor

Hydrogen bonding is the strongest intermolecular force among the provided options and is universally present in alcohols due to the \(-\mathrm{OH}\) group, distinctly affecting all alcohols and increasing their boiling points.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a key concept when understanding the boiling points of alcohols. These special types of molecular interactions involve a hydrogen atom that is bonded to an electronegative atom, such as oxygen. In alcohols, each molecule has an \(-\mathrm{OH}\) group, where the hydrogen is bonded to oxygen.

• These bonds are particularly strong because the shared electrons are attracted very closely to the oxygen.
• Hydrogen bonds form not within the molecule, but between the hydrogen of one molecule and the oxygen of a neighboring molecule.

This formation requires additional energy to break, which means alcohols have higher boiling points than similar sized organic compounds without hydrogen bonding.
A key characteristic of hydrogen bonds is that they are stronger than typical van der Waals forces but weaker than covalent bonds. As a result, they play a major role in determining physical properties like the boiling points of alcohols.

Effect of Molecular Structure on Boiling Points

The molecular structure is a significant factor when examining the boiling points of substances. In alcohols, it specifically refers to variations in the size and shape of alcohol molecules.

• Larger alcohol molecules usually have higher boiling points. This is mainly due to an increase in the number of carbon atoms, which results in stronger London dispersion forces.
• However, the effect of increased size cannot overpower the decisive influence of hydrogen bonds.

Alcohols demonstrate a unique balance; while larger molecules can mean stronger intermolecular interactions, the presence of the \(-\mathrm{OH}\) group leads to significant hydrogen bonding which is a greater determinant of boiling point.
When analyzing boiling points, it is also important to assess the branching within the molecule:

• Branched alcohols typically have lower boiling points compared to their straight-chain isomers, as they have a smaller surface area and can less effectively engage in intermolecular forces.

Intermolecular Forces in Organic Compounds

Understanding different types of intermolecular forces is crucial in grasping why boiling points vary in organic compounds, including alcohols.
There are several key intermolecular forces:

• London Dispersion Forces: Present in all molecules, these arise due to temporary shifts in electron density, leading to weak attractions between molecules.
• Dipole-Dipole Interactions: These occur in polar molecules where partial charges draw molecules together. In alcohols, this occurs but is overshadowed by the stronger hydrogen bonds.
• Hydrogen Bonding: As already discussed, these are the strongest forces affecting alcohols, responsible for their relatively high boiling points.

In summary, while London dispersion forces and dipole-dipole interactions do play roles in determining boiling points, the presence of hydrogen bonds due to the \(-\mathrm{OH}\) group in alcohols significantly elevates their boiling points beyond what molecular size alone would suggest.