Question

In which of the" following compounds would you expect intramolecular hy drogen bonding to occur; o-nitroaniline, o-cresol, o-hydroxybenzoic acid (salicylic acid), o-hydro-benzaldehyde (salicylaldehyde), o-fluorophenol, o-hydroxy- benzonitrile.

Short answer

Intramolecular hydrogen bonding is expected to occur in the following compounds: o-nitroaniline, o-hydroxybenzoic acid (salicylic acid), o-hydro-benzaldehyde (salicylaldehyde), o-fluorophenol, and o-hydroxybenzonitrile. Only o-cresol does not exhibit intramolecular hydrogen bonding.

Step-by-step solution

Identify functional groups

First, let's identify the functional groups of each compound to identify potential hydrogen bond donors and acceptors. 1. o-nitroaniline: Amino (-NH2) and nitro (-NO2) groups. 2. o-cresol: Hydroxyl (-OH) and methyl (-CH3) groups. 3. o-hydroxybenzoic acid (salicylic acid): Carboxyl (-COOH) and hydroxyl (-OH) groups. 4. o-hydro-benzaldehyde (salicylaldehyde): Hydroxyl (-OH) and aldehyde (-CHO) groups. 5. o-fluorophenol: Hydroxyl (-OH) and fluorine (-F) groups. 6. o-hydroxybenzonitrile: Hydroxyl (-OH) and nitrile (-CN) groups.

Determine potential hydrogen bonding

Now we will determine which compounds have adjacent hydrogen bond donor and acceptor: 1. o-nitroaniline: Amino (-NH2) acts as a hydrogen bond donor, and nitro (-NO2) acts as a hydrogen bond acceptor. Adjacent, intramolecular hydrogen bonding is possible. 2. o-cresol: Hydroxyl (-OH) acts as a hydrogen bond donor, but there is no adjacent hydrogen bond acceptor. No intramolecular hydrogen bonding. 3. o-hydroxybenzoic acid (salicylic acid): Carboxyl (-COOH) can act as both a hydrogen bond donor and acceptor. The hydroxyl (-OH) group acts as a hydrogen bond donor. Intramolecular hydrogen bonding is possible. 4. o-hydro-benzaldehyde (salicylaldehyde): Hydroxyl (-OH) acts as a hydrogen bond donor, and the aldehyde's carbonyl oxygen can act as a hydrogen bond acceptor. Intramolecular hydrogen bonding is possible. 5. o-fluorophenol: Hydroxyl (-OH) acts as a hydrogen bond donor and fluorine (-F) acts as a hydrogen bond acceptor. Intramolecular hydrogen bonding is possible. 6. o-hydroxybenzonitrile: Hydroxyl (-OH) acts as a hydrogen bond donor, and nitrile (-CN) (specifically, the nitrogen atom) can act as a hydrogen bond acceptor. Intramolecular hydrogen bonding is possible.

Conclusion

Compounds which are expected to have intramolecular hydrogen bonding include: o-nitroaniline, o-hydroxybenzoic acid (salicylic acid), o-hydro-benzaldehyde (salicylaldehyde), o-fluorophenol, o-hydroxybenzonitrile. Out of the given compounds, only o-cresol does not exhibit intramolecular hydrogen bonding.

Key concepts

Functional Groups Identification

Functional groups are specific groups of atoms within molecules that determine the characteristic chemical reactions of those molecules. Identifying these groups is crucial because they are key to understanding the behavior of organic compounds.

• Amino group (-NH2): Often found in amino acids and can act as a hydrogen bond donor.
• Nitro group (-NO2): Common in explosives; it can be a hydrogen bond acceptor due to oxygen's electronegativity.
• Hydroxyl group (-OH): Present in alcohols; known for its ability to both donate and accept hydrogen bonds.
• Carboxyl group (-COOH): Found in acids; can engage in hydrogen bonding as both donor and acceptor.
• Aldehyde group (-CHO): Part of sugars and organic solvents; primarily acts as a hydrogen bond acceptor.
• Fluorine (-F): Highly electronegative, making it a potential hydrogen bond acceptor.
• Nitrile group (-CN): Contains a triple bond between carbon and nitrogen, with the nitrogen able to accept hydrogen bonds.

By identifying these groups, you're taking the first step toward predicting potential interactions in a molecule.

Hydrogen Bond Donors and Acceptors

Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom interacts with another electronegative atom bearing a lone pair of electrons. To predict intramolecular hydrogen bonding, it's crucial to identify potential hydrogen bond donors and acceptors.

• Hydrogen bond donors are typically hydrogen atoms bound to electronegative elements like oxygen or nitrogen.
• Hydrogen bond acceptors are typically non-bonded electron pairs found on electronegative atoms such as oxygen, nitrogen, or fluorine.

In the context of the compounds mentioned, the hydroxyl (-OH) groups are consistently acting as hydrogen bond donors. Meanwhile, groups like nitro (-NO2), carbonyl (from aldehydes), and nitrile (-CN) can serve as acceptors. Recognizing these roles helps in establishing whether intra or intermolecular interactions are feasible.

Organic Compound Structure Analysis

Analyzing the structure of organic compounds helps us understand their reactivity and interactions. Organic chemistry is largely based on this structure-function relationship.
The compounds mentioned have ortho-positions (designated "o-") in their names, indicating that functional groups are next to each other on the benzene ring. This proximity is key for potential intramolecular interactions. When functional groups like amino and nitro in o-nitroaniline are adjacent:

• The structure allows for the molecules to form hydrogen bonds within themselves.
• Proximity facilitates internal bonding, leading to potential stability in the structure without needing external interaction.

Examining the structural arrangement, particularly the ortho-positions, is essential for predicting which compounds will engage in intramolecular hydrogen bonding.

Intramolecular Interactions in Chemistry

Intramolecular interactions, such as intramolecular hydrogen bonding, occur within a single molecule. These interactions give insight into the molecule's stability and reactivity.
Several factors influence intramolecular hydrogen bonding:

• Proximity of hydrogen bond donors and acceptors due to the molecule's structure increases the likelihood of bonding.
• Strength of bonds can affect the physical properties of the molecule, like melting and boiling points.

For the compounds listed, those that exhibit these intramolecular interactions become more rigid and may have reduced solubility in water due to reduced availability of hydrogen bonding sites for intermolecular interactions. Recognizing the conditions that favor these intramolecular interactions allows chemists to predict the behavior and stability of organic compounds.