Question

Although carboxylic acids and alcohols both contain an \- OH group, one is acidic in water and the other is not. Explain the difference.

Short answer

In summary, the difference in acidity between carboxylic acids and alcohols in water is due to their chemical structures. Carboxylic acids (R-COOH) can donate a proton, forming a stable carboxylate ion (R-COO^-), because the negative charge can be stabilized through resonance. In contrast, alcohols (R-OH) have a hydroxyl group with low acidic properties, as the resulting alkoxide ion (R-O^−) cannot effectively stabilize the negative charge. So, despite both containing an -OH group, they exhibit different behaviors in water.

Step-by-step solution

Identify the chemical structures of carboxylic acids and alcohols

Carboxylic acids have the general structure R-COOH, where R is an alkyl or aryl group, and COOH is the carboxyl group. Alcohols, on the other hand, have the general structure R-OH, where R is an alkyl or aryl group, and OH is the hydroxyl group.

Look at the acidic properties of carboxylic acids

Carboxylic acids are acidic because of their carboxyl group (R-COOH) which can donate a proton (H+) to a base or to water. Upon this donation, the carboxyl group (COOH) gets ionized into its conjugate base, the carboxylate ion (R-COO-), and a hydrogen ion (H+). The equilibrium reaction can be represented as: \[ R-COOH \rightleftharpoons R-COO^- + H^+ \] Carboxylic acids have a relatively high tendency to lose a proton and form the carboxylate ion because the COO- group can stabilize the negative charge on the oxygen atom through resonance. Consequently, carboxylic acids are strong acids in comparison to alcohols.

Analyze the lack of acidic properties in alcohols

Alcohols, with a general structure of R-OH, do not exhibit acidic behavior in water because the hydroxyl group (OH) has a very low tendency to donate its hydrogen ion (H+) to water or other available bases. This is because the resulting alkoxide ion (R-O^−) cannot stabilize the negative charge on the oxygen atom effectively. Therefore, alcohols are considered to be very weak acids.

Explain the difference between carboxylic acids and alcohols in water

In summary, the main difference between carboxylic acids and alcohols in water is their acidity. Carboxylic acids are acidic due to the ability of their carboxyl group (R-COOH) to donate a proton, resulting in a stable conjugate base (carboxylate ion). On the other hand, alcohols possess a hydroxyl group (R-OH) with low acidic properties, since the alkoxide ion formed after proton donation cannot effectively stabilize the negative charge. Thus, although both groups contain an -OH moiety, their chemical behaviors in water are significantly different.

Key concepts

Carboxyl Group

The carboxyl group is an essential functional group found in carboxylic acids and is represented as COOH. It consists of a carbon atom double-bonded to an oxygen atom (carbonyl group) and also bonded to a hydroxyl group (OH).

The presence of both the carbonyl and hydroxyl components within the same group creates a highly polar environment. This polarity has a significant effect on the group's properties, notably its ability to engage in hydrogen bonding and its acidic behavior. The carbonyl group specifically plays a vital role in the acidity of carboxylic acids. This is because when the hydroxyl hydrogen is donated as a proton (H+), the remaining oxygen-carbon bond takes on a negative charge, which can be delocalized across the two oxygens via resonance, providing stability to the resulting carboxylate ion.

Hydroxyl Group

A hydroxyl group is a functional group that consists of an oxygen atom bonded to a hydrogen atom, denoted as OH. This group is the defining feature of alcohols, characterized by the presence of one or more of these groups attached to a saturated carbon atom.

The hydroxyl group is polar due to the difference in electronegativity between oxygen and hydrogen, making alcohols generally hydrophilic or water-compatible. Despite its similarity to the hydroxyl part of the carboxyl group, the hydroxyl group's ability to stabilize the negative charge after proton donation is much weaker. As a result, alcohols do not exhibit the same level of acidic behavior as carboxylic acids.

Resonance Stabilization

Resonance stabilization is a concept in chemistry where the real structure of a molecule is represented as an average of several contributing structures, known as resonance forms or canonical structures. These forms differ only in the position of electrons, not atoms.

In the context of carboxylic acids, resonance occurs when the carboxyl group loses a proton, creating a carboxylate ion. The negative charge on the oxygen atom can be delocalized across the adjacent oxygen, thus stabilizing the molecule. It is this delocalization that greatly increases the acidity of the carboxylic acids. Alcohols lack this extended system of pi bonds and, therefore, cannot benefit from resonance stabilization to the same extent.

Proton Donation

Proton donation, or deprotonation, is the act of an acid releasing a hydrogen ion (H+) thus behaving as a proton donor. The strength of an acid can be gauged by its propensity to donate a proton. In acidic solutions, the equilibrium between the protonated and deprotonated forms is crucial.

Carboxylic acids readily donate a proton due to the stability afforded by the resonance of the negative charge on the carboxylate ion. However, when alcohols donate a proton, forming an alkoxide ion, there's no resonance stabilization to distribute the resulting negative charge. Hence, alcohols are considerably less acidic and are more likely to exist in their protonated form in aqueous solutions.