Question

Explain why methyl alcohol is soluble in water in all proportions, while stearyl alcohol \(\left[\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{OH}\right]\) is a waxy solid that is not soluble in water.

Short answer

Methyl alcohol is soluble in water in all proportions due to its polar molecular structure, which allows it to form hydrogen bonds with polar water molecules. Stearyl alcohol, however, is insoluble in water due to its long nonpolar hydrocarbon chain, which limits its interaction with water molecules despite having a polar OH group. This difference in solubility stems from the "like dissolves like" concept, where polar substances dissolve in polar solvents and nonpolar substances dissolve in nonpolar solvents.

Step-by-step solution

Learn "like dissolves like" concept

Solubility of a substance in a solvent mainly depends on the nature of both the solute and the solvent. A rule of thumb determining solubility is "like dissolves like," which means that polar substances (with charges) tend to dissolve in polar solvents, while nonpolar substances (without charges) dissolve in nonpolar solvents.

Understand water's polar nature

Water is a polar solvent due to its molecular structure. It has a bent molecular geometry with oxygen being more electronegative than hydrogen, causing a partial negative charge on the oxygen atom and partial positive charges on the hydrogen atoms. This leads to the formation of hydrogen bonds between water molecules, which allow polar solutes such as ionic compounds and polar covalent molecules to dissolve in water.

Analyze methyl alcohol's molecular structure

Methyl alcohol, also known as methanol or CH3OH, is a simple alcohol that consists of a hydroxyl group (-OH) bound to a methyl group (-CH3). The oxygen atom in the hydroxyl group is more electronegative than the hydrogen and carbon atoms, resulting in a polar molecule.

Explain methyl alcohol's solubility in water

Methyl alcohol's polarity allows it to form hydrogen bonds, making it miscible (fully mixable) with the polar solvent water. This is because the polar molecules in methyl alcohol can interact with polar water molecules through hydrogen bonding, leading to easy solubility.

Analyze stearyl alcohol's molecular structure

Stearyl alcohol, or CH3(CH2)16OH, is a long-chain alcohol composed of 18 carbon atoms and a hydroxyl group (-OH) at one end. The hydroxyl group is polar due to oxygen's electronegativity, but the long hydrocarbon chain is nonpolar as it mostly consists of nonpolar carbon-hydrogen bonds that have similar electronegativities.

Explain stearyl alcohol's insolubility in water

The long nonpolar hydrocarbon chain in stearyl alcohol renders it less soluble in the polar solvent water. Although it has a polar OH group that could potentially form hydrogen bonds with water molecules, the majority of the molecule is composed of nonpolar C-H bonds. This makes the overall molecule's interaction with water molecules limited, causing it to be insoluble in water and therefore form a waxy solid. Thus, based on the molecular structures and interactions with water, we can conclude that methyl alcohol is soluble in water in all proportions, while stearyl alcohol is not soluble in water due to its long nonpolar hydrocarbon chain.

Key concepts

Polar and Nonpolar Substances

Exploring the nature of substances, we classify them as either polar or nonpolar, based on the distribution of electric charge across the molecule. Polar substances have uneven charge distribution, creating partial positive and negative ends. Imagine a magnet with distinct north and south poles; polar molecules have similar characteristics with 'poles' of charges.

On the flip side, nonpolar substances boast a uniform distribution of electrical charge, meaning there are no distinct poles or charge imbalances within the molecule. Think of them like a perfectly balanced seesaw that stays level regardless of where you place weight on it. This simplicity in balance is why nonpolar substances do not mix well with polar substances, as their charge distribution doesn't allow for the necessary interaction needed to dissolve.

Hydrogen Bonding

Diving into hydrogen bonding, it's a special type of dipole-dipole interaction that occurs specifically between a hydrogen atom bonded to a highly electronegative atom (like oxygen, nitrogen, or fluorine) and another electronegative atom. This bond is like a strong handshake between two individuals; it's much stronger than other van der Waals forces but weaker than covalent or ionic bonds.

Hydrogen bonds are the reason why water has such a high boiling point relative to its molecular weight and why it's such an excellent solvent for many substances. These bonds aren't just about attraction; they are protocol for how water behaves and interacts with other molecules, permitting vital biochemical reactions and processes!

Molecular Structure

The term molecular structure is the 3D arrangement of atoms within a molecule. The geometry can determine the molecule's polarity—linear, trigonal, tetrahedral, to name a few structures. Each geometric arrangement carries its own electrical personality, much like how different architectural designs give buildings their unique characters.

Methyl Alcohol vs. Stearyl Alcohol

Consider methyl alcohol and stearyl alcohol as two buildings with different architectures. Methyl alcohol has a simple, compact 'building' with a polar hydroxyl group making it friendly with water. In contrast, stearyl alcohol is like a long, sprawling complex with a tiny polar 'entrance' and a vast nonpolar structure, which makes it less approachable for water molecules. The structural differences are fundamental to understanding why they do or do not mix with water.

Like Dissolves Like

The idiom 'like dissolves like' is more than just a catchy phrase—it's an essential principle in chemistry that governs dissolution. Polarity is the key; polar solvents (like water) are social butterflies at parties for polar substances, mixing and mingling effortlessly. The same goes for nonpolar solvents that are the perfect hosts for nonpolar substances.

But why don't polar and nonpolar substances get along? It boils down to their molecular characteristics—their 'personalities' are too different to blend. Just as people with similar interests tend to flock together, molecules do the same. This principle is fundamental when understanding why certain substances dissolve in water while others do not.