Question

Which molecule would you expect to be more soluble in water: \(\mathrm{CCl}_{4}\) or \(\mathrm{CH}_{2} \mathrm{Cl}_{2} ?\)

Short answer

\(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) is more soluble in water than \(\mathrm{CCl}_{4}\) due to its polarity.

Step-by-step solution

Understanding Solubility Principles

The solubility of a molecule in water is greatly influenced by the molecule's polarity. Polar molecules, which have a distribution of charge that leads to positive and negative regions, are more likely to be soluble in water due to the ability to form hydrogen bonds or engage in dipole-dipole interactions with water. Nonpolar molecules, on the other hand, do not have these interactions and are less soluble in water.

Comparing the Polarity of the Molecules

Analyze the polarity of both \(\mathrm{CCl}_{4}\) and \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\). \(\mathrm{CCl}_{4}\) has a tetrahedral geometry with symmetrically distributed chlorine atoms, which makes it nonpolar. \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\), however, also has a tetrahedral geometry but with two hydrogen atoms and two chlorine atoms that create a polar molecule due to the difference in electronegativity between hydrogen and chlorine, resulting in a dipole moment.

Determining Relative Solubility in Water

Since \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) is a polar molecule and \(\mathrm{CCl}_{4}\) is nonpolar, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) is more likely to interact with water and thus be more soluble.

Key concepts

Polarity of Molecules

The concept of polarity is pivotal when discussing the solubility of molecules in water. A molecule is considered polar if there is a significant difference in electronegativity between the atoms, leading to an uneven distribution of electron density.

Polar molecules have positive and negative poles, resembling the poles on a magnet. This uneven distribution allows them to form attractions with other polar molecules, such as water, which is also polar. In the context of our textbook exercise, CCl₄ and CH₂Cl₂ are compared. CH₂Cl₂, with its collective polar bonds, becomes a polar molecule capable of interacting favorably with water, enhancing its solubility.

Hydrogen Bonding

Hydrogen bonding is a specific and strong type of dipole-dipole interaction that significantly influences the solubility of a molecule in water. It occurs when a hydrogen atom bonded to a highly electronegative atom, like oxygen or nitrogen, is attracted to another electronegative atom with a lone pair of electrons.

In our exercise, while CCl₄ cannot form hydrogen bonds due to the lack of hydrogen, CH₂Cl₂ could potentially engage in this type of interaction depending on its structural context. However, it's the overall polarity of CH₂Cl₂ due to the differing electronegativities of hydrogen and chlorine that increases its solubility in water, not direct hydrogen bonding.

Dipole-dipole Interactions

Dipole-dipole interactions are attractive forces between the positive end of one polar molecule and the negative end of another. These interactions are less strong than hydrogen bonds but still play a critical role in the solubility of polar substances in water.

In the exercise, CH₂Cl₂ has a dipole moment due to its polar covalent bonds. This creates partial positive and negative charges that can align with the opposite charges on water molecules. On the other hand, the symmetry in CCl₄'s structure means that its dipole moments cancel out, making dipole-dipole interactions with water molecules nonviable and thus reducing its solubility.

Molecular Geometry

The molecular geometry or shape of a molecule can determine its polarity and, consequently, its solubility in water. When atoms in a molecule are arranged symmetrically, the molecule may be nonpolar because the individual dipole moments can cancel each other out.

For the exercise provided, the symmetrical tetrahedral geometry of CCl₄ results in a nonpolar molecule. In contrast, CH₂Cl₂, while also tetrahedral, has an asymmetrical distribution of different atoms (chlorines and hydrogens) that leads to a net dipole moment. This makes CH₂Cl₂ polar and affects its interaction with water, enhancing its solubility.