Problem 14 - Chemistry and Chemical Reactivity

Question

Acetone, \(\mathrm{CH}_{3} \mathrm{COCH}_{3}\), is quite soluble in water. Explain why this should be so. (EQUATION CAN'T COPY)

Step-by-Step Solution

Answer

Acetone is soluble in water due to hydrogen bonding between its polar carbonyl group and water molecules.

Step 1: Identify Acetone's Structure

Acetone (\( \mathrm{CH}_3 \mathrm{COCH}_3 \)) is a simple ketone, with the structural formula of \( (\mathrm{CH}_3)_2\mathrm{CO} \). It consists of a central carbonyl group (\( \mathrm{C} = \mathrm{O} \)) bonded to two methyl groups (\( \mathrm{CH}_3 \)).

Step 2: Consider Intermolecular Forces

Water is a polar molecule, known for its hydrogen bonding capabilities. Acetone's carbonyl group is very polar due to the oxygen's high electronegativity, which can form dipole-dipole interactions with water's hydrogen atoms.

Step 3: Describe Solubility Mechanism

The polar interaction between water and acetone is significant. The carbonyl oxygen in acetone can attract the hydrogen in water molecules, leading to hydrogen bonding, a strong intermolecular force contributing to acetone's solubility in water.

Step 4: Assess Solubility in Polar Solvents

Since acetone can hydrogen bond with water, a highly polar solvent, it is soluble due to effective interaction between acetone's polar groups and water molecules, aligning with the principle that 'like dissolves like'.

Key Concepts

Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion between molecules. They are crucial in determining how substances interact and stick together. There are several types of intermolecular forces, but three commonly discussed ones are London dispersion forces, dipole-dipole interactions, and hydrogen bonding. Each type holds molecules together with varying strength.
Water and acetone interact primarily through dipole-dipole interactions and hydrogen bonding. Dipole-dipole interactions occur when the positive end of a polar molecule is attracted to the negative end of another. Acetone, with its polar carbonyl group, can align its negative dipole (oxygen) towards the positive dipole (hydrogen) in water.
These interactions are stronger than London dispersion forces, which occur even between nonpolar molecules, and they are particularly significant in helping acetone to dissolve in water.

Hydrogen Bonding

Hydrogen bonding is a specific and particularly strong type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like oxygen, nitrogen, or fluorine. This makes hydrogen bonding especially important in substances like water, which is a prime example of extensive hydrogen bonding;
it is responsible for many of water's unique properties, such as high surface tension and boiling point.

• In the case of acetone interacting with water, the carbonyl oxygen in acetone can act as a hydrogen bond acceptor.
• Water's hydrogen atoms, being very partially positive, are attracted and can form bonds with the electron-rich oxygen in acetone.
• This hydrogen bonding is stronger than typical dipole-dipole interactions, significantly enhancing acetone's solubility in water.

Hydrogen bonds involve partial electrostatic attraction which might be likened to the 'Velcro' of molecular interactions, grabbing onto any compatible, proximate hydrogen atom on a neighboring molecule.

Polar Molecules

Polar molecules are molecules in which there is an uneven distribution of electron density, resulting in regions of partial positive and negative charge. This polarity is often due to differences in electronegativity between atoms involved in chemical bonds.
In acetone, the carbonyl group (C=O) is quite polar; oxygen is much more electronegative than carbon, drawing electron density and creating a partial negative charge on the oxygen, and a partial positive charge on the carbon atom.
Water is also a polar molecule, with its oxygen atom holding a partial negative charge and the hydrogen atoms holding partial positive charges. This polarity means that water can engage in strong dipole-dipole interactions with other polar molecules, like acetone.
Because both acetone and water are polar, they can interact favorably, allowing acetone to dissolve readily in water—a classic case of "like dissolves like." This principle explains why polar molecules are generally soluble in polar solvents.