Question

Use molecular structures and noncovalent interactions to explain why dimethyl ether, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{O},\) is completely miscible in water, but dimethylsulfide, \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{~S},\) is only slightly water soluble.

Short answer

Dimethyl ether is miscible in water due to hydrogen bonding; dimethyl sulfide is not because it lacks effective hydrogen bonding.

Step-by-step solution

Understanding Molecular Structures

Analyze the structural components of dimethyl ether (\((\mathrm{CH}_{3})_{2} \mathrm{O}\)) and dimethyl sulfide (\((\mathrm{CH}_{3})_{2} \mathrm{~S}\)). Dimethyl ether consists of an oxygen atom bonded to two methyl groups, while dimethyl sulfide has a sulfur atom bonded to two methyl groups. Both molecules are similar, but the oxygen atom in dimethyl ether can form hydrogen bonds, unlike sulfur in dimethyl sulfide.

Examining Noncovalent Interactions in Dimethyl Ether

Dimethyl ether is polar due to the electronegative oxygen atom, which allows it to engage in hydrogen bonding with water molecules. The partially negative oxygen in the ether can attract the partially positive hydrogen atoms of the water molecules, leading to strong intermolecular interactions.

Examining Noncovalent Interactions in Dimethyl Sulfide

Dimethyl sulfide has a sulfur atom that does not engage effectively in hydrogen bonding due to its lower electronegativity compared to oxygen. This results in weaker Van der Waals forces and dipole-dipole interactions with water, reducing solubility.

Comparing Solubility Based on Interactions

Since dimethyl ether can form hydrogen bonds with water, it leads to a higher degree of interaction and hence is completely miscible. On the other hand, dimethyl sulfide can only slightly interact with water through weaker forces, resulting in limited solubility.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom is directly bonded to a highly electronegative atom, like oxygen or nitrogen. This creates a significant difference in electronegativity, causing the hydrogen to carry a partial positive charge, while the electronegative atom, like oxygen, holds a partial negative charge.

• In dimethyl ether (((\(\mathrm{CH}_{3}\))_{2}\(\mathrm{O}\))), the oxygen atom is electronegative enough to form hydrogen bonds with water.
• The oxygen in dimethyl ether can attract hydrogen atoms from water, making the molecules interact strongly and mix well.
• This hydrogen bonding is responsible for the complete miscibility of dimethyl ether in water.

Dimethyl sulfide (((\(\mathrm{CH}_{3}\))_{2}\(\mathrm{S}\))), on the other hand, lacks this capability since sulfur is not as electronegative as oxygen. As a result, hydrogen bonds are not formed effectively, limiting its interaction and subsequent solubility with water.

Polarity

Polarity plays a crucial role in determining how molecules interact with each other. It arises from a difference in electronegativity between atoms in a molecule, leading to a dipole moment.

• Dimethyl ether (((\(\mathrm{CH}_{3}\))_{2}\(\mathrm{O}\))) is a polar molecule due to the presence of oxygen, which attracts electrons towards itself, creating a dipole where the oxygen is partially negative.
• Polarity allows dimethyl ether to dissolve well in polar solvents, like water, due to similar polar characteristics.
• In contrast, dimethyl sulfide has less polarity than dimethyl ether because sulfur does not create such a strong dipole, reducing its affinity for polar solvents like water.

This difference in polarity explains why dimethyl ether mixes completely with water, while dimethyl sulfide does not.

Solubility

Solubility refers to the extent to which a solute dissolves in a solvent. Solubility hinges on the balance of various molecular interactions between the solute and solvent.

• The ability of dimethyl ether to dissolve completely in water is due to its ability to form strong interactions through hydrogen bonding and dipole interactions.
• Water can surround dimethyl ether molecules due to these effective interactions, making it highly soluble.
• Dimethyl sulfide, however, is only slightly soluble in water since it cannot effectively form strong interactions like hydrogen bonds.
• The limited interaction strength leads to only slight solubility, as water molecules cannot adequately attract and dissolve dimethyl sulfide molecules.

Overall, solubility is heavily dependent on the nature and strength of interactions a molecule can form with water.

Van der Waals Forces

Van der Waals forces are weak intermolecular forces arising from temporary dipoles that occur when electron clouds fluctuate within molecules.

• These forces are generally weak, especially in comparison to hydrogen bonds and significant dipole interactions.
• In both dimethyl ether and dimethyl sulfide, van der Waals forces are present but play a smaller role in solubility compared to stronger forces like hydrogen bonds.
• Dimethyl sulfide relies more on van der Waals forces due to its inability to engage in hydrogen bonding. This reliance results in limited solubility in water.

Because van der Waals forces are not strong enough to make dimethyl sulfide soluble in water, this force alone cannot satisfy the needs for greater solubility without the presence of other interactions.

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 stronger than van der Waals forces but weaker than hydrogen bonds.

• Dimethyl ether, being a polar molecule, engages in dipole-dipole interactions which allow it to align well with the polarity of water.
• This interaction supports its solubility by facilitating the mixing of its molecules with water.
• Dimethyl sulfide has weaker dipole-dipole interactions because its polarity is less than that of dimethyl ether, reducing its ability to align and dissolve in water.

In summary, dipole-dipole interactions enhance miscibility when polar substances like dimethyl ether are mixed with water. Dimethyl sulfide, having less polarity, does not benefit as much from these interactions, explaining its limited water solubility.