Compare and account for differences in the (a) dipole...

Compare and account for differences in the (a) dipole moment, $(b)$ boiling point, $(c)$ density and $(d)$ solubility in water of an alkyl halide RX and its parent alkane RH. (a) $\mathrm{RX}$ has a larger dipole moment because the $\mathrm{C}-\mathrm{X}$ bond is polar. (b) $\mathrm{RX}$ has a higher boiling point since it has a larger molecular weight and also is more polar. (c) $\mathrm{RX}$ is more dense since it has a heavy $\mathrm{X}$ atom; the order of decreasing density is $\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}$. (d) $\mathrm{RX}$, like $\mathrm{RH}$, is insoluble in $\mathrm{H}_2 \mathrm{O}$, but $\mathrm{RX}$ is somewhat more soluble because some $\mathrm{H}$-bonding can occur: $$ \stackrel{\circ}{\mathrm{R}}-\stackrel{\circ-}{\mathrm{X}}:-\cdot \stackrel{\delta}{\mathrm{H}}-\stackrel{\delta}{\mathrm{O}} \mathrm{H} $$ This effect is greatest for RF.

Compare and account for differences in the (a) dipole moment, $(b)$ boiling point, $(c)$ density and $(d)$ solubility in water of an alkyl halide RX and its parent alkane RH.
(a) $\mathrm{RX}$ has a larger dipole moment because the $\mathrm{C}-\mathrm{X}$ bond is polar. (b) $\mathrm{RX}$ has a higher boiling point since it has a larger molecular weight and also is more polar. (c) $\mathrm{RX}$ is more dense since it has a heavy $\mathrm{X}$ atom; the order of decreasing density is $\mathrm{RI}>\mathrm{RBr}>\mathrm{RCl}>\mathrm{RF}$. (d) $\mathrm{RX}$, like $\mathrm{RH}$, is insoluble in $\mathrm{H}_2 \mathrm{O}$, but $\mathrm{RX}$ is somewhat more soluble because some $\mathrm{H}$-bonding can occur:
$$
\stackrel{\circ}{\mathrm{R}}-\stackrel{\circ-}{\mathrm{X}}:-\cdot \stackrel{\delta}{\mathrm{H}}-\stackrel{\delta}{\mathrm{O}} \mathrm{H}
$$

This effect is greatest for RF.

Key Concepts

Molecular Polarity

The concept of molecular polarity involves the uneven distribution of electron density within a molecule due to differences in electronegativity between bonded atoms. In molecules where there is a significant difference, such as between carbon and a halogen, this results in a polar covalent bond and a measurable dipole moment. This polarity is a key factor in determining how molecules interact with each other and with solvents, influencing many physical properties.

Intermolecular Forces and Boiling Point

Intermolecular forces, including dipole-dipole interactions and London dispersion forces, are crucial in determining the boiling point of a substance. Polarity increases the strength of dipole-dipole interactions, while a higher molecular weight generally enhances dispersion forces. Both aspects contribute to higher boiling points as they require more energy to break the intermolecular attractions during phase transitions.

Molecular Weight and Density

The density of a substance is influenced by its molecular weight and the packing of its molecules in the solid or liquid state. Heavier atoms or groups within a molecule, such as halogens, increase the overall mass and, when combined with similar molecular volumes, lead to higher densities. This relationship highlights how the addition of heavier atoms affects fundamental physical properties.

Hydrogen Bonding and Solubility in Water

Hydrogen bonding is an important type of dipole-dipole interaction that can enhance the solubility of compounds in polar solvents like water. Molecules with polar functional groups or atoms capable of forming hydrogen bonds can interact favorably with water, leading to increased solubility. Even relatively nonpolar organic compounds may exhibit enhanced solubility if they can participate in hydrogen bonding, a factor that is significant when considering solute-solvent interactions.

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