Question

Carbexylic acids generally have \(\mathrm{p} K_{2}\) 's of \(-5\), but alcohols have \(\mathrm{pK}_{\mathrm{y}}\) 's of \(\sim 16\). (a) Write the acid- dissociation chemical equation for the generic alcohol ROH in water. (b) Which compounds will produce more acidic solutions upon dissolution in water, acids or alcohols? (c) Suggest an explanation for the difference in \(\mathrm{pK}_{\mathrm{a}}\) 's for acids compared to alcohols.

Short answer

(a) The acid-dissociation chemical equation for the generic alcohol ROH in water is: \(ROH \rightleftharpoons RO^- + H_3O^+\) (b) Carboxylic acids will produce more acidic solutions upon dissolution in water compared to alcohols. (c) The difference in pKa values for acids compared to alcohols can be explained by the inherent differences in their functional groups, with carboxylic acids being more acidic due to the presence of a more electron-withdrawing carbonyl group (C=O), which generates a less stable conjugate base upon proton loss.

Step-by-step solution

(a) Acid-dissociation chemical equation for the generic alcohol ROH in water

: In order to write the acid-dissociation chemical equation for the generic alcohol ROH in water, you should first recognize that the alcohol donates a proton (H+) to the water molecule, forming the corresponding alkoxide ion (RO-) and hydronium ion (H3O+). The equation can be written as follows: ROH (in water) <=> RO- + H3O+

(b) Which compounds will produce more acidic solutions upon dissolution in water, acids or alcohols?

: To determine which compounds (acids or alcohols) will produce more acidic solutions upon dissolution in water, we should compare their respective pKa values. Given that carboxylic acids generally have pKa values around -5 and alcohols have pKa values around 16, we can safely conclude that: Carboxylic acids will produce more acidic solutions upon dissolution in water compared to alcohols.

(c) Explanation for the difference in pKa values for acids compared to alcohols

: To explain the difference in pKa values for acids compared to alcohols, you should first understand what pKa represents. pKa is the negative logarithm of the acid dissociation constant (Ka) and indicates the acidity of a compound. A lower pKa value means the compound is a stronger acid, while a higher pKa value means it's a weaker acid. Carboxylic acids have a more acidic functional group (COOH) than alcohols (ROH). The oxygen from the carbonyl group (C=O) of the carboxylic acid is more electron-withdrawing. This means that the carbonyl group will draw electron density towards itself and away from the OH bond, making it easier for the carboxylic acid to lose a proton (H+) compared to the alcohol, thus having a lower pKa value. In sum, the difference in pKa values for carboxylic acids (-5) and alcohols (16) can be explained by the inherent differences in their functional groups, with carboxylic acids being more acidic due to the presence of a more electron-withdrawing carbonyl group (C=O), therefore generating a less stable conjugate base upon proton loss.

Key concepts

Acid-Dissociation Chemical Equation

Understanding the acid-dissociation chemical equation is crucial in grasping why certain compounds like carboxylic acids and alcohols differ in acidity. When a generic alcohol, ROH, is introduced to water, it acts as an acid by donating a proton (H+) to the water molecule. This interaction results in the formation of an alkoxide ion (RO-) and a hydronium ion (H3O+), captured by the equation:

\[\mathrm{ROH} + \mathrm{H_2O} \rightleftharpoons \mathrm{RO^-} + \mathrm{H_3O^+}\]
The direction of the equation emphasizes a reversible reaction, where forward and backward reactions occur. This equation forms the basis for understanding how molecule structures influence their ability to donate protons, and ultimately their acidity.

pKa Values Comparison

Diving into the pKa values comparison between carboxylic acids and alcohols illuminates the disparity in their acidic strength. pKa, short for perchloric acid constant, measures a substance's tendency to donate protons. Substances with lower pKa values, like carboxylic acids (typically around -5), are stronger acids compared to those with higher pKa values, such as alcohols (~16).

This numerical pKa difference demonstrates that in a solution, carboxylic acids dissociate more readily, releasing more protons, and resulting in a more acidic environment. Understanding the pKa value is fundamental in predicting whether a compound will behave as a strong or a weak acid in aqueous solutions.

Electron-withdrawing Effects on Acidity

The concept of electron-withdrawing effects is critical when discussing the acidity of different organic compounds. Molecules such as carboxylic acids possess a carbonyl group (C=O) that exhibits electron-withdrawing characteristics, pulling electron density toward itself. This action creates a polarization where the O-H bond becomes more susceptible to releasing its proton, enhancing the molecule's acidic nature.

By contrast, in alcohols, the RO group doesn't significantly attract electrons away from the hydroxyl group; thus, the O-H bond is less polarized and retains its proton more firmly, decreasing the acidity. Recognizing how electron-withdrawing groups like carbonyl affect proton dissociation helps explain a molecule's acidity.

Conjugate Base Stability

The stability of a conjugate base plays a pivotal role in defining the strength of its corresponding acid. For example, when carboxylic acids lose a proton, they form a conjugate base that is resonance stabilized. This distribution of negative charge over multiple atoms stabilizes the conjugate base, favoring the forward dissociation reaction.

In the case of alcohols, the alkoxide ion formed as the conjugate base lacks such resonance stabilization, making it less stable. This instability reduces the tendency of alcohols to donate a proton, therefore being less acidic. Comprehending conjugate base stability provides insight into the acid-dissociation equilibrium and the overall acidity of compounds.