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Chapter 15: Problem 54

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

Short Answer

Expert verified
Carboxylic acids are acidic because their carboxylate ion is resonance stabilized, whereas alcohols are not because their alkoxide ion lacks such stability.

Step by step solution

01

Identify the Functional Groups

Carboxylic acids contain a carboxyl group (–COOH), while alcohols contain a hydroxyl group (–OH). Both have an –OH group but their overall structures affect their properties.
02

Assess the Acidic Nature

To understand why carboxylic acids are acidic and alcohols are not, consider their ability to donate a proton (H⁺) in water. Acidity is determined by the ease of this proton donation.
03

Analyze Proton Donation of Carboxylic Acids

Carboxylic acids can readily donate a proton from the –COOH group to water, forming a carboxylate anion (RCOO⁻) and a hydronium ion (H₃O⁺). This is facilitated by resonance stabilization of the carboxylate anion.
04

Analyze Proton Donation of Alcohols

Alcohols do not readily donate a proton from the –OH group to water. Unlike carboxylate anions, the resulting alkoxide ion (RO⁻) is not as stabilized, making proton donation less favorable.
05

Compare Resonance Stability

The key difference lies in the resonance stability. The carboxylate ion formed when carboxylic acids donate a proton is resonance stabilized, while the alkoxide ion from alcohols is not.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Functional Groups
In chemistry, **functional groups** are specific groups of atoms within molecules that determine the characteristics of the molecules. Carboxylic acids and alcohols both contain the –OH group, but have different overall functional groups causing them to behave differently.
Carboxylic acids contain a carboxyl group (–COOH).
Alcohols contain a hydroxyl group (–OH).
The functional group of a molecule is crucial because it dictates how the molecule interacts with others. This is especially important when discussing acidity.
Proton Donation
Acidity is all about the ability to donate a proton (H⁺). A substance is considered an acid if it can easily donate a proton to water.
Carboxylic acids are good at donating protons because when they lose a proton, they form a stable carboxylate ion (RCOO⁻).
In contrast, alcohols do not donate protons as readily. When an alcohol loses a proton, it forms an alkoxide ion (RO⁻), which is less stable.
This difference in stability is key to understanding why carboxylic acids are acidic, while alcohols are not.
Resonance Stabilization
One reason carboxylic acids are more acidic than alcohols is due to resonance stabilization. Resonance occurs when electrons can be distributed over multiple atoms, increasing stability.
When a carboxylic acid donates a proton, it forms a carboxylate ion, which is resonance stabilized. This means the negative charge is delocalized over several atoms, making the ion more stable.
Alcohols do not have the same resonance stabilization. When they lose a proton, the resulting alkoxide ion lacks the ability to delocalize the negative charge effectively, leading to lower stability and thus lower acidity.
Carboxyl Group
The carboxyl group (–COOH) is the hallmark of carboxylic acids. It includes both a carbonyl group (C=O) and a hydroxyl group (–OH) attached to the same carbon atom.
Due to this structure, carboxylic acids can donate a proton easily and achieve resonance stabilization in the carboxylate ion. This ability to stabilize the negative charge after losing a proton is what makes carboxylic acids acidic.
The presence of both a carbonyl and a hydroxyl group in close proximity allows for delocalization of electrons, aiding stability and increasing acidity.
Hydroxyl Group
The hydroxyl group (–OH) characterizes alcohols. Unlike the carboxyl group, it does not have a carbonyl component nearby.
While the –OH group in alcohols can potentially lose a proton, the resulting alkoxide ion (RO⁻) does not benefit from resonance stabilization. This lack of stabilization makes the ion less stable and not as willing to form.
This is why alcohols do not donate protons as readily as carboxylic acids and thus are not considered acidic in water.

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Most popular questions from this chapter

The polypeptide chain in proteins does not exhibit free rotation because of the partial double-bond character of the peptide bond. Explain this fact with resonance structures.

Explain each statement in terms of atomic properties: (a) Carbon engages in covalent rather than ionic bonding. (b) Carbon has four bonds in all its organic compounds. (c) Carbon forms neither stable cations, like many metals, nor stable anions, like many nonmetals. (d) Carbon atoms bond to other carbon atoms more extensively than do the atoms of any other element. (e) Carbon forms stable multiple bonds.

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Compound \(\mathrm{A}\) is branched and optically active and contains \(\mathrm{C}, \mathrm{H},\) and \(\mathrm{O} .\) (a) \(\mathrm{A} 0.500-\mathrm{g}\) sample burns in excess \(\mathrm{O}_{2}\) to yield \(1.25 \mathrm{~g}\) of \(\mathrm{CO}_{2}\) and \(0.613 \mathrm{~g}\) of \(\mathrm{H}_{2} \mathrm{O} .\) Determine the empirical formula. (b) When \(0.225 \mathrm{~g}\) of compound A vaporizes at 755 torr and \(97^{\circ} \mathrm{C},\) the vapor occupies \(78.0 \mathrm{~mL} .\) Determine the molecular formula. (c) Careful oxidation of the compound yields a ketone. Name and draw compound A, and circle the chiral center.
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