When an atom or group of atoms is substituted for an H atom in benzene (C6 H6), the boiling poin

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When an atom or group of atoms is substituted for an H atom...

When an atom or group of atoms is substituted for an $\mathrm{H}$ atom in benzene $\left(\mathrm{C}_{6} \mathrm{H}_{6}\right),$ the boiling point changes. Explain the order of the following boiling points: $\mathrm{C}_{6} \mathrm{H}_{6}\left(80{ }^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}$ $\left(132^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\left(156^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\left(182^{\circ} \mathrm{C}\right)$

When an atom or group of atoms is substituted for an $\mathrm{H}$ atom in benzene $\left(\mathrm{C}_{6} \mathrm{H}_{6}\right),$ the boiling point changes. Explain the order of the following boiling points: $\mathrm{C}_{6} \mathrm{H}_{6}\left(80{ }^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Cl}$
$\left(132^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{Br}\left(156^{\circ} \mathrm{C}\right), \mathrm{C}_{6} \mathrm{H}_{5} \mathrm{OH}\left(182^{\circ} \mathrm{C}\right)$

Key Concepts

Hydrogen Bonding

Hydrogen bonding is a particularly strong type of intermolecular force that occurs when hydrogen is directly bonded to highly electronegative atoms such as oxygen. In compounds like phenol, the presence of an -OH group allows for hydrogen bonding, which greatly elevates the boiling point relative to similar molecules without such bonding capabilities.

Molecular Mass and Polarizability

The molecular mass and polarizability of substituents also affect boiling points. Larger, more polarizable atoms (like bromine compared to chlorine) can enhance dispersion forces even if they are less electronegative, which contributes to incremental increases in boiling point as the mass and size of the substituent increase.

Intermolecular Forces

Intermolecular forces are the attractions between molecules that determine many physical properties, including boiling points. Their strengths vary depending on molecular structure and functional groups, and stronger intermolecular forces require more energy to break, leading to higher boiling points.

Substituent Effects on Molecular Polarity

Changing an atom in benzene to a different substituent alters the molecule's polarity and electron distribution. When polar groups are introduced, such as in chlorobenzene or bromobenzene, dipole-dipole interactions become more significant compared to nonpolar benzene, increasing the boiling point.

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