Question

Which of the following in each pair is likely to be more soluble in hexane, ${\mathrm{C}}_6{\mathrm{H}}_{14}$ (a) ${\mathrm{CCl}}_4$ or ${\mathrm{CaCl}}_2;$ (b) benzene $\left({\mathrm{C}}_6{\mathrm{H}}_6\right)$ or glycerol, ${\mathrm{CH}}_2(\mathrm{OH})\mathrm{CH}(\mathrm{OH}){\mathrm{CH}}_2\mathrm{OH};$ (c) octanoic acid, ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COOH}$, or acetic acid, ${\mathrm{CH}}_3\mathrm{COOH}$. Explain your answer in each case

Short answer

The more soluble compounds in hexane among the given pairs are ${\mathrm{CCl}}_4$, benzene, and octanoic acid.

Step-by-step solution

Compare Polarity of Hexane and ${\mathrm{CCl}}_4$ and ${\mathrm{CaCl}}_2$

Hexane (${\mathrm{C}}_6{\mathrm{H}}_{14}$) is a non-polar hydrocarbon. Carbon tetrachloride (${\mathrm{CCl}}_4$) is also non-polar, while calcium chloride (${\mathrm{CaCl}}_2$) is ionic in nature and polar. Since hexane and ${\mathrm{CCl}}_4$ are both non-polar, they are more likely to dissolve each other.

Compare Polarity of Hexane and Benzene and Glycerol

Benzene (${\mathrm{C}}_6{\mathrm{H}}_6$) is a non-polar hydrocarbon just like hexane, while glycerol (${\mathrm{CH}}_2(\mathrm{OH})\mathrm{CH}(\mathrm{OH}){\mathrm{CH}}_2\mathrm{OH}$) is an alcohol with three polar hydroxyl (-OH) groups, making it significantly more polar than hexane. As a result, benzene is likely to be more soluble in hexane compared to glycerol.

Compare Polarity of Hexane and Octanoic Acid and Acetic Acid

Octanoic acid (${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{COOH}$) and acetic acid (${\mathrm{CH}}_3\mathrm{COOH}$) are both carboxylic acids containing polar -COOH groups. However, octanoic acid has a longer non-polar hydrocarbon chain than acetic acid. Since hexane is non-polar, the solubility of these acids will depend on the balance between the polar -COOH group and the non-polar hydrocarbon chain. Octanoic acid, having a larger non-polar part, is likely to be more soluble in hexane compared to acetic acid. In conclusion, the more soluble compounds in hexane among the given pairs are ${\mathrm{CCl}}_4$, benzene, and octanoic acid.

Key concepts

Polarity

Polarity is a key concept when discussing solubility. It refers to the distribution of electrical charge over the atoms in a molecule. Molecules can be polar or non-polar.

• Polar molecules have areas of positive and negative charge due to an unequal sharing of electrons.
• Non-polar molecules have a more equal distribution of charge, resulting in no distinct poles.

The rule of thumb in solubility is that "like dissolves like." This means polar substances dissolve well in polar solvents. Similarly, non-polar substances dissolve better in non-polar solvents. That is why understanding polarity is crucial when examining the solubility of different substances.

Non-polar Compounds

Non-polar compounds have molecules with a nearly equal sharing of electrons. This equality means that there are no distinct charged regions or poles. Instead, the electrons are spread out evenly.

• An example of a non-polar compound is hexane (${\mathrm{C}}_6{\mathrm{H}}_{14}$
• Non-polar compounds are generally soluble in other non-polar substances.

The solubility of a compound in hexane, for instance, largely depends on its polarity. Since hexane is non-polar, it dissolves other non-polar compounds more effectively.

Carbon Tetrachloride

Carbon tetrachloride (${\mathrm{CCl}}_4$ is composed of one carbon atom bonded to four chlorine atoms. This compound is non-polar, as the symmetrical arrangement causes the charges to cancel out.

• Due to its non-polar nature, it easily dissolves in non-polar solvents like hexane.
• This characteristic makes it a good example of the "like dissolves like" principle at work.

In contrast to polar compounds, ${\mathrm{CCl}}_4$ does not mix well with water, a polar solvent.

Benzene

Benzene (${\mathrm{C}}_6{\mathrm{H}}_6$ is a non-polar hydrocarbon. It has a ring structure where each carbon atom is bonded similarly, resulting in an even distribution of charge.

• Because benzene is non-polar, it mixes well with other non-polar solvents like hexane.
• Benzene does not dissolve in polar solvents such as water.

Understanding benzene's non-polar nature is crucial in comprehending why it is more soluble in hexane compared to polar compounds like glycerol. Its solubility behavior aligns with the fundamental chemical principle that similar polarities result in better solubility.