Question

Evaluate Ethanoic acid (acetic acid) is very soluble in water. However, naturally occurring longchain carboxylic acids, such as palmitic acid \(\left(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{14} \mathrm{COOH}\right)\) , are insoluble in water. Explain.

Short answer

Ethanoic acid (acetic acid) is soluble in water due to its ability to form hydrogen bonds with water molecules and its small hydrocarbon part that does not contribute much to hydrophobic interactions. In contrast, palmitic acid has a long hydrocarbon chain that promotes strong hydrophobic interactions, making it insoluble in water, despite its carboxyl group being capable of forming hydrogen bonds with water molecules.

Step-by-step solution

Comparing the structures of ethanoic acid and palmitic acid

Ethanoic acid (acetic acid) has a simple structure, with the chemical formula CH3COOH. It consists of a small hydrocarbon part (CH3) and a carboxyl group (COOH). On the other hand, palmitic acid has a long hydrocarbon chain consisting of 14 methylene (CH2) groups connected in series, one methyl group (CH3) at the end, and a carboxyl group (COOH) at the other end. Its chemical formula is CH3(CH2)14COOH.

Understanding the role of hydrogen bonding

The solubility of a compound in water depends on its ability to form hydrogen bonds with water molecules. Hydrogen bonding is a strong intermolecular force that occurs between molecules containing highly electronegative elements, such as oxygen and nitrogen, bonded to hydrogen (H). In both ethanoic acid and palmitic acid, the carboxyl group (COOH) contains an oxygen atom that can form hydrogen bonds with water molecules.

Exploring the role of hydrophobic interactions

Hydrophobic interactions play a significant role in the solubility of compounds, particularly those containing long hydrocarbon chains. Hydrocarbon chains are nonpolar and are not attracted to polar water molecules. The longer the hydrocarbon chain, the stronger the hydrophobic interactions and the less soluble the compound is in water.

Analyzing the solubility of ethanoic acid and palmitic acid

Ethanoic acid (acetic acid) is very soluble in water because it has a small hydrocarbon part that does not contribute much to hydrophobic interactions, allowing the carboxyl group to form hydrogen bonds with water molecules. In contrast, palmitic acid has a long hydrocarbon chain that experiences strong hydrophobic interactions, preventing the carboxyl group from effectively forming hydrogen bonds with water molecules. This results in the insolubility of palmitic acid in water.

Conclusion

Ethanoic acid (acetic acid) is soluble in water because it can form hydrogen bonds with water molecules, and its small hydrocarbon part does not create strong hydrophobic interactions. In contrast, palmitic acid has a long hydrocarbon chain that promotes strong hydrophobic interactions, which make it insoluble in water, despite its carboxyl group being capable of forming hydrogen bonds with water molecules.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a key concept when discussing the solubility of carboxylic acids in water. Even though the term 'bonding' suggests an actual bond, these interactions are not as strong as covalent or ionic bonds. They occur when a hydrogen atom, bonded to a highly electronegative atom such as oxygen, creates an attraction to another electronegative atom. This attraction is quite effective in bringing molecules close together.
For acetic acid, the hydrogen bond occurs between the carboxyl group's oxygen and the hydrogen atoms of water molecules. This interaction significantly increases the solubility of acetic acid in water. The ability to engage in hydrogen bonding is a major reason why acetic acid dissolves easily. On the other hand, oxygen in palmitic acid's carboxyl group can also form hydrogen bonds. However, due to its longer hydrocarbon chain, other factors become much more significant in its solubility.

• Hydrogen bonds increase solubility by allowing molecules to intermingle.
• These bonds occur when hydrogen meets electronegative atoms like oxygen.
• Acetic acid's simple structure aids in forming more effective hydrogen bonds.

Hydrophobic Interactions

Hydrophobic interactions are as crucial as hydrogen bonding when understanding how molecules like palmitic acid behave in water. These interactions occur because hydrocarbon chains are nonpolar, meaning they do not have charged ends and do not mix well with polar substances like water.
As the hydrocarbon chain lengthens—as seen in palmitic acid—the number of hydrophobic interactions increases. This is because longer chains aggregate more strongly with each other than with water molecules. Therefore, these chains prefer to stick together rather than mix with water, leading to the molecule being less soluble.

• Hydrophobic interactions prevent mixing with water.
• Longer chains increase these nonpolar interactions.
• These interactions make molecules like palmitic acid less soluble.

Molecular Structure

The structure of a molecule heavily influences how it interacts with solvents like water. Carboxylic acids, such as ethanoic and palmitic acids, have both polar and nonpolar sections, profoundly affecting their solubility.
Ethanoic acid, with its short hydrocarbon tail, allows the polar carboxyl group to dominate, making it water-soluble. This happens because there is less nonpolar surface area to repel water molecules, allowing full utilization of hydrogen bonding potential. Conversely, palmitic acid has a long hydrocarbon chain, overshadowing the polar carboxyl group. The nonpolar nature of the long chain increases hydrophobic interactions, drastically decreasing its solubility.

• Short hydrocarbon chains promote solubility due to fewer nonpolar interactions.
• The long chains in palmitic acid contribute to its resistance to dissolving.
• Greater surface area of nonpolar segments leads to higher hydrophobic interactions.