Rationalize the difference in boiling points for each of the...

Rationalize the difference in boiling points for each of the following pairs of substances: a. $n$ -pentane $\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \quad 36.2^{\circ} \mathrm{C}$ $$\begin{array}{c} \mathrm{CH}_{3} \\ \text { neopentane } \mathrm{H}_{3} \mathrm{C}-\mathrm{C}-\mathrm{CH}_{3} \quad 9.5^{\circ} \mathrm{C} \\\mathrm{CH}_{3} \end{array}$$ $$\begin{aligned} &\begin{array}{lr} \text { b. } \mathrm{HF} & 20^{\circ} \mathrm{C} \\ \text { HCl } & -85^{\circ} \mathrm{C} \\ \text { c. } \mathrm{HCl} & -85^{\circ} \mathrm{C} \\ \text { LiCl } & 1360^{\circ} \mathrm{C} \\ \text { d. } n \text { -pentane } & \mathrm{CH}_{3} \\ n \text { -hexane } & \mathrm{CH}_{3} \end{array}\\ &\begin{array}{ll} \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 36.2^{\circ} \mathrm{C} \\ \mathrm{SH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 69^{\circ} \mathrm{C} \end{array} \end{aligned}$$

Rationalize the difference in boiling points for each of the following pairs of substances:
a. $n$ -pentane $\quad \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} \quad 36.2^{\circ} \mathrm{C}$
$$\begin{array}{c}
\mathrm{CH}_{3} \\
\text { neopentane } \mathrm{H}_{3} \mathrm{C}-\mathrm{C}-\mathrm{CH}_{3} \quad 9.5^{\circ} \mathrm{C} \\\mathrm{CH}_{3}
\end{array}$$
$$\begin{aligned}
&\begin{array}{lr}
\text { b. } \mathrm{HF} & 20^{\circ} \mathrm{C} \\
\text { HCl } & -85^{\circ} \mathrm{C} \\
\text { c. } \mathrm{HCl} & -85^{\circ} \mathrm{C} \\
\text { LiCl } & 1360^{\circ} \mathrm{C} \\
\text { d. } n \text { -pentane } & \mathrm{CH}_{3} \\
n \text { -hexane } & \mathrm{CH}_{3}
\end{array}\\
&\begin{array}{ll}
\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 36.2^{\circ} \mathrm{C} \\
\mathrm{SH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} & 69^{\circ} \mathrm{C}
\end{array}
\end{aligned}$$

Key Concepts

Intermolecular Forces

Intermolecular forces are the weak attractions between molecules that govern many physical properties, including boiling points. These forces include dispersion interactions, dipole–dipole interactions, and hydrogen bonding. The type, strength, and number of these forces present in a substance determine the energy needed to separate the molecules during phase changes. For instance, substances with strong hydrogen bonds or ionic interactions require more energy to boil than those that only experience weak dispersion forces.

Molecular Geometry and Surface Area

Molecular geometry and branching play a critical role in determining how closely molecules can pack together. In general, linear or less-branched molecules have a larger surface area that can come into contact with neighboring molecules, allowing stronger dispersion forces to act. Branching reduces the accessible surface area, diminishing the attractive van der Waals forces between molecules and often leading to lower boiling points, as seen when comparing isomeric compounds.

Hydrogen Bonding

Hydrogen bonding is a specific type of dipole–dipole interaction that occurs when hydrogen is covalently bonded to highly electronegative atoms such as fluorine, oxygen, or nitrogen. These interactions are considerably stronger than typical dipole–dipole forces and significantly raise boiling points. Molecules capable of hydrogen bonding, even if small, may exhibit much higher boiling points compared to larger molecules that lack this ability.

Ionic Bonding and Ion–Dipole Interactions

Ionic bonding involves the electrostatic attraction between positive and negative ions and is generally much stronger than intermolecular forces found in covalent compounds. Substances composed of ions, or those that can form ion–dipole interactions, have very high boiling points because breaking these strong attractions requires a substantial amount of energy.

Dispersion Forces and Molecular Size

Dispersion forces, also known as London forces, are present in all molecules and become more significant as the size and polarizability of a molecule increases. Larger molecules with more electrons have stronger instantaneous dipoles, resulting in stronger dispersion forces. Consequently, even subtle increases in molecular mass or chain length can lead to appreciably higher boiling points due to the cumulative effect of these weak but numerous attractions.

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