Question

Which of the following classes of compounds can form hydrogen bonds? (a) carboxylic acids (b) esters (c) amides

Short answer

Carboxylic acids and amides can form hydrogen bonds, but esters typically cannot.

Step-by-step solution

Understand Hydrogen Bonding

A hydrogen bond occurs when a hydrogen atom is covalently bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) and is attracted to another electronegative atom nearby, which must have a lone pair of electrons.

Examine Carboxylic Acids

Carboxylic acids contain a hydroxyl (-OH) group, where the hydrogen is directly attached to the oxygen. Additionally, the carbonyl (C=O) group provides an electronegative oxygen that can participate in hydrogen bonding, both as a donor and an acceptor.

Consider Esters

Esters contain a carbonyl (C=O) and an alkoxy (O-R) group. Here, the hydrogen is not bonded to the electronegative oxygen, which limits esters' ability to participate in hydrogen bonding typically only as an acceptor, not effectively as a donor.

Evaluate Amides

Amides contain a carbonyl (C=O) group and an amine (NH2) group. The hydrogen atoms in the amine group are directly bonded to nitrogen, an electronegative atom, making them capable of participating in hydrogen bonding both as donors and acceptors.

Key concepts

Carboxylic Acids

Carboxylic acids are organic compounds that consist of a carboxyl group, which is a combination of a carbonyl group (C=O) and a hydroxyl group (-OH). This unique structure makes carboxylic acids particularly adept at forming hydrogen bonds.

• The hydroxyl part of the carboxyl group contains hydrogen directly bonded to oxygen, which is a highly electronegative atom.
• This positioning allows the hydrogen to donate a hydrogen bond to electronegative atoms in other molecules.
• Simultaneously, the carbonyl oxygen can act as a hydrogen bond acceptor due to its lone pairs of electrons.

As a result, carboxylic acids can both donate and accept hydrogen bonds, a dual functionality not found in all compounds. This capability often makes carboxylic acids appear in higher boiling points compared to similar molecular size compounds. The significant hydrogen bonding they possess can lead to the formation of dimers in the vapor phase, further illustrating their hydrogen bonding potential.

Esters

Esters are a class of organic compounds derived from carboxylic acids and alcohols. They contain a characteristic carbonyl group (C=O) adjacent to an alkoxy group (O-R). While esters have a carbonyl oxygen that can participate in hydrogen bonding, they are limited to acting only as hydrogen bond acceptors.

• In esters, the hydrogen is not directly bonded to the electronegative oxygen.
• This setup restricts them from efficiently donating hydrogen bonds, a role possible only if hydrogen is directly connected to an electronegative atom.
• Esters are generally less polar than carboxylic acids and amides and typically don't form hydrogen bonds with each other.

These attributes of esters affect their physical properties. For instance, they generally have lower boiling points compared to carboxylic acids and amides of similar molecular size, owing to the limited hydrogen bonding interactions.

Amides

Amides are organic compounds featuring a carbonyl group (C=O) adjacent to an amine group (NH2). This placement of structures in amides permits robust hydrogen bonding, with opportunities to both donate and accept hydrogen bonds.

• The amine hydrogen atoms, bonded to nitrogen, contribute to hydrogen bonding as donors.
• The carbonyl oxygen, bearing lone pairs, allows for hydrogen bond acceptance.
• This dual capacity enhances the overall cohesion and physical stability of amides.

Due to this strong hydrogen bonding potential, amides generally display higher boiling and melting points compared to other less polar compounds of similar size. This feature is particularly evident in biological systems, where amides form the backbone of proteins, thanks to peptide bonds. The extensive hydrogen bonding is crucial for maintaining protein structure, influencing properties such as solubility and reactivity.