Question

The following groups are found in some organic molecules. Which are hydrophilic and which are hydrophobic: (a) -OH; (b) \(-\mathrm{CH}_{2} \mathrm{CH}_{3} ;\) (c) \(-\mathrm{CONH}_{2} ;\) (d) \(-\mathrm{Cl}\) ?

Short answer

Hydrophilic groups: (a) -OH, (c) -CONH2, (d) -Cl; Hydrophobic group: (b) -CH2CH3.

Step-by-step solution

Determine Hydrophilic Groups

A group is considered hydrophilic if it can form hydrogen bonds with water or dissolve in water due to its polarity.

Determine Hydrophobic Groups

A group is hydrophobic if it is nonpolar and does not interact well with water, often consisting mostly of carbon and hydrogen atoms.

Analyze Group (a) -OH

The hydroxyl group (-OH) is polar and can form hydrogen bonds with water, making it hydrophilic.

Analyze Group (b) -CH2CH3

The ethyl group (-CH2CH3) is nonpolar and does not form hydrogen bonds with water, making it hydrophobic.

Analyze Group (c) -CONH2

The amide group (-CONH2) is polar due to the presence of both a carbonyl and an amino group. It can form hydrogen bonds with water, making it hydrophilic.

Analyze Group (d) -Cl

The chloride group (-Cl) is polar and can interact with water, making it hydrophilic.

Key concepts

Understanding Hydrogen Bonds

Hydrogen bonds are vital interactions that occur between a hydrogen atom covalently bonded to a more electronegative atom, such as oxygen or nitrogen, and another electronegative atom with a lone pair of electrons. The result is a special type of dipole-dipole attraction, which, while weaker than a covalent bond, is stronger than other types of van der Waals interactions.

When it comes to hydrophilic groups in organic molecules, their ability to engage in hydrogen bonding plays a crucial role in their affinity for water. For instance, a hydroxyl group \textbf{(-OH)} found in alcohols can form hydrogen bonds with water due to the electronegative oxygen atom. Similarly, the amide group \textbf{(-CONH\(_2\))} is drawn to water through hydrogen bonds formed by the nitrogen's hydrogen atoms and the oxygen in water. It's the presence of these polar bonds that increases the solubility of the compound in water. As a student, recognizing whether a functional group can form hydrogen bonds can greatly aid in predicting the solubility and behavior of organic molecules in aqueous environments.

Polarity of Organic Compounds

Polarity in organic compounds is a measure of how unequally electrons are distributed within the molecule. Molecules with polar covalent bonds have a partial positive charge on one end and a partial negative charge on the other, which occurs due to a difference in electronegativity between bonded atoms.

Polar molecules like \textbf{(-Cl)} interact well with water, which is also a polar molecule, leading to better solubility. The concept of 'like dissolves like' is helpful to remember: polar solutes dissolve well in polar solvents. A molecule's polarity can be attributed to functional groups, which range from polar hydrophilic to nonpolar hydrophobic. For example, the ethyl group \textbf{(-CH\(_2\)CH\(_3\))} is nonpolar and consists of carbon and hydrogen atoms that share electrons more or less equally, resulting in a lack of significant charge separation. This is why it is categorized as hydrophobic and does not mix well with water.

Solubility in Water

The solubility of organic compounds in water is directly related to their ability to form intermolecular associations with water molecules. This is predominantly determined by the presence of hydrophilic groups within the molecule that can participate in hydrogen bonds or other polar interactions.

Hydrophilic groups often contain atoms such as oxygen, nitrogen, or halogens that are more electronegative compared to carbon and hydrogen. These atoms' partial negative charges attract the partial positive charges of the hydrogen atoms in water molecules, resulting in solubilization. Compounds such as \textbf{(-OH)}, \textbf{(-CONH\(_2\))}, and \textbf{(-Cl)} have such features and therefore typically show good solubility in water.

On the contrary, hydrophobic groups, which are predominantly nonpolar, do not mix well with water as they lack the polar characteristics needed for interaction. Understanding this concept can significantly facilitate the prediction of a molecule's behavior in biological systems and industrial processes, enhancing a student's ability to approach complex problems in chemistry.