Question

How much difference in physical properties would you expect for o-and \(p\) -cyanophenol isomers? Explain.

Short answer

The differences in the physical properties of o-cyanophenol and p-cyanophenol isomers stem from their differing molecular structures and the presence or absence of intramolecular hydrogen bonding. O-cyanophenol, with its intramolecular hydrogen bond, is expected to have higher melting and boiling points but lower solubility in polar solvents than p-cyanophenol. On the other hand, p-cyanophenol forms only intermolecular hydrogen bonds, resulting in higher solubility in polar solvents.

Step-by-step solution

Identify the molecular structures of the isomers.

We'll start by identifying the molecular structures of o-cyanophenol and p-cyanophenol isomers. The "o" (ortho) designation indicates that the cyanide (CN) group is located adjacent to the hydroxyl (OH) group on the phenol ring. The "p" (para) designation indicates that the two functional groups are located opposite each other on the phenol ring. o-cyanophenol: \( \text{C}_{6}\text{H}_{4}\text{(OH)(CN)} \), with the CN group in the 2-position p-cyanophenol: \( \text{C}_{6}\text{H}_{4}\text{(OH)(CN)} \), with the CN group in the 4-position

Analyze the presence and effect of hydrogen bonding.

Both o-cyanophenol and p-cyanophenol have functional groups that can lead to hydrogen bonding. The hydroxyl (OH) group is capable of forming hydrogen bonds because it contains a hydrogen atom bonded to an electronegative oxygen atom. The presence of hydrogen bonding can greatly impact physical properties such as melting point, boiling point, and solubility. For o-cyanophenol, the proximity of the OH and CN functional groups allows the formation of intermolecular hydrogen bonding between the hydrogen from the OH group and the nitrogen from the adjacent CN group within the same molecule. In p-cyanophenol, although the OH group can still participate in hydrogen bonding, the distance between the OH and CN functional groups prevents the formation of intramolecular hydrogen bonds within the same molecule. It can only form intermolecular hydrogen bonding between other p-cyanophenol molecules.

Compare physical properties.

The differences in hydrogen bonding among o-cyanophenol and p-cyanophenol isomers will lead to differences in their physical properties. Here are some key differences to expect: 1. Melting and boiling points: Due to the presence of intramolecular hydrogen bonding in o-cyanophenol, it is expected to have higher melting and boiling points as compared to p-cyanophenol. The intramolecular hydrogen bonding stabilizes the molecule, making it difficult to separate them and therefore requiring more energy. 2. Solubility: Both isomers are polar molecules, but since o-cyanophenol forms intramolecular hydrogen bonds, its solubility in polar solvents is expected to be lower than that of p-cyanophenol. The p-cyanophenol's ability to form intermolecular hydrogen bonds with the polar solvent makes it more soluble. In conclusion, the differences in the physical properties of o-cyanophenol and p-cyanophenol isomers can be attributed to their differing molecular structures and the presence or absence of intramolecular hydrogen bonding. The o-cyanophenol isomer is expected to have higher melting and boiling points but lower solubility in polar solvents as compared to the p-cyanophenol isomer.

Key concepts

Hydrogen Bonding

Understanding hydrogen bonding is crucial when analyzing the physical properties of isomers. It's a specific type of dipole-dipole interaction that occurs between a hydrogen atom, which is covalently bonded to an electronegative atom such as oxygen, nitrogen, or fluorine, and another electronegative atom with a lone pair of electrons.

In the case of isomers like o-cyanophenol and p-cyanophenol, we look at whether these functional groups are positioned to enable hydrogen bonding within the same molecule (intramolecular) or with other molecules (intermolecular). The o-isomer, with groups adjacent to each other, can form strong intramolecular hydrogen bonds that influence its physical state and reactivity. On the other hand, the p-isomer, with groups on opposite sides of the benzene ring, typically forms weaker intermolecular hydrogen bonds.

This difference in hydrogen bonding leads to several physical property changes, largely impacting solubility, as well as melting and boiling points, which are essential for understanding the behavior of these isomers in various conditions and reactions. Intramolecular hydrogen bonding often leads to a more stable and less reactive molecule, while intermolecular hydrogen bonding can facilitate solvation when the molecule is placed in a solvent.

Melting and Boiling Points

The melting and boiling points of a substance are indicative of the strength of the interactions between its molecules. When it comes to isomers, the arrangement of atoms and the type of bonding can lead to different melting and boiling points even though they have the same molecular formula.

Intriguingly, for isomers such as o-cyanophenol, the aforementioned intramolecular hydrogen bonding causes it to have higher melting and boiling points compared to p-cyanophenol. This is because the intramolecular bonds in o-cyanophenol need to be overcome for the substance to change state, from solid to liquid (melting) or from liquid to gas (boiling).

Key Points:

• Intramolecular hydrogen bonding increases melting and boiling points.
• Intermolecular hydrogen bonding affects boiling more than melting points.
• Differences in structure, even with the same chemical formula, can significantly alter these properties.

These variations can be applied in separating and purifying compounds in organic chemistry, making melting and boiling points critical for identification and analysis processes.

Solubility in Organic Chemistry

Solubility is a core concept in organic chemistry that refers to the ability of a substance to dissolve in a solvent. The rule of thumb for predicting solubility is 'like dissolves like.' This means that polar compounds tend to dissolve well in polar solvents, while nonpolar compounds do better in nonpolar solvents.

For our isomers, o-cyanophenol's strong intramolecular hydrogen bonding actually decreases its solubility in polar solvents because it's less available to interact with the solvent. Conversely, p-cyanophenol is more adept at forming intermolecular hydrogen bonds with the solvent, which enhances its solubility. The polarity of isomers also plays a role—the electron-withdrawing cyanide group affects the overall dipole moment of the molecules.

Implications for Solubility:

• Intramolecular hydrogen bonding can reduce solubility.
• Intermolecular interactions with the solvent increase solubility.
• Polarity influences how well a compound will dissolve in a given solvent.

These factors are crucial when performing reactions in organic chemistry, as they determine how substances interact and the conditions necessary for reactions to proceed.