Question

The most acidic compound is (A) \(\mathrm{HCOOH}\) (B) \(\mathrm{Ph}-\mathrm{COOH}\) (C) \(\mathrm{CH}_{3}-\mathrm{COOH}\) (D) \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\)

Short answer

The most acidic compound among the given options is (B) \(\mathrm{Ph}-\mathrm{COOH}\) (benzoic acid) due to the resonance stabilization of its benzoate ion.

Step-by-step solution

Identify the acidic functional groups

The compounds contain the following acidic functional groups: (A) Carboxylic acid: \(\mathrm{HCOOH}\) (formic acid) (B) Carboxylic acid: \(\mathrm{Ph}-\mathrm{COOH}\) (benzoic acid) (C) Carboxylic acid: \(\mathrm{CH}_{3}-\mathrm{COOH}\) (acetic acid) (D) Alcohol: \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) (ethanol)

Analyze the stability of the conjugate base

After losing a proton (H+), the conjugate base of each compound is: (A) Formate ion: \(\mathrm{HCOO}^{-}\) (B) Benzoate ion: \(\mathrm{Ph}-\mathrm{COO}^{-}\) (C) Acetate ion: \(\mathrm{CH}_{3}-\mathrm{COO}^{-}\) (D) Ethoxide ion: \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{O}^{-}\) We need to assess the stability of these conjugate bases. The more stable the conjugate base, the stronger the acid.

Compare the effect of resonance and inductive effect on conjugate bases' stability

In benzoate ion (\(\mathrm{Ph}-\mathrm{COO}^{-}\)), the negative charge is delocalized throughout the benzene ring due to resonance, making the conjugate base more stable compared to formate ion (\(\mathrm{HCOO}^{-}\)) and acetate ion (\(\mathrm{CH}_{3}-\mathrm{COO}^{-}\)), which do not have any resonance stabilization. Compared to formate ion (\(\mathrm{HCOO}^{-}\)), the acetate ion (\(\mathrm{CH}_{3}-\mathrm{COO}^{-}\)) has an additional methyl group (-CH3), which exerts a weak inductive effect. This inductive effect makes the negative charge on \(\mathrm{CH}_{3}-\mathrm{COO}^{-}\) slightly more stable compared to \(\mathrm{HCOO}^{-}\). The ethoxide ion (\(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{O}^{-}\)) does not have any significant resonance or inductive stabilization. Hence, among carboxylic acids and alcohols, carboxylic acids are stronger acids because the conjugate base of an alcohol (alkoxide ion) is less stable.

Rank the compounds based on acidic strength

Now, we can rank the acidic strength of these compounds based on the analysis above: 1. \(\mathrm{Ph}-\mathrm{COOH}\) (benzoic acid) - strongest acid due to the resonance stabilization of its benzoate ion. 2. \(\mathrm{CH}_{3}-\mathrm{COOH}\) (acetic acid) - moderately strong acid due to the inductive effect of the methyl group on the acetate ion. 3. \(\mathrm{HCOOH}\) (formic acid) - weaker acid as the formate ion has no resonance or significant inductive stabilization. 4. \(\mathrm{CH}_{3}-\mathrm{CH}_{2}-\mathrm{OH}\) (ethanol) - weakest acid due to the lack of significant stabilization of the ethoxide ion. Conclusion: The most acidic compound among the given options is (B) \(\mathrm{Ph}-\mathrm{COOH}\).

Key concepts

Carboxylic Acids and Acidity

Carboxylic acids, such as formic acid (HCOOH), acetic acid (CH3-COOH), and benzoic acid (Ph-COOH), are known for their high acidity compared to other common organic compounds like alcohols. The reason behind this acidity lies in the structure of carboxylic acids; they possess a carbonyl group (C=O) adjacent to a hydroxyl group (-OH). Upon losing a proton (H+), a negative charge is left on the oxygen, forming a conjugate base called a carboxylate ion. The stability of this carboxylate ion, due to factors discussed in the following sections, is what makes carboxylic acids highly acidic.

It's important to remember that not all carboxylic acids have the same level of acidity. Differences in their molecular structure can influence their ability to donate a proton; hence, understanding the stability of their conjugate bases is crucial in determining the acidity of carboxylic acids.

Conjugate Base Stability

The stability of a conjugate base plays a crucial role in determining the acidity of a molecule. A conjugate base is formed when an acid donates a proton (H+). A more stable conjugate base will more willingly accommodate the negative charge left after the loss of a proton, thus enhancing the parent compound's acidity.

For example, in the molecules (A) formic acid, (B) benzoic acid, and (C) acetic acid, the stability of their conjugate bases varies significantly. The benzoate ion (Ph-COO-) is highly stable due to resonance, which is not present in formate (HCOO-) or acetate (CH3-COO-) ions. This high stability associated with resonance in the benzoate ion is what makes benzoic acid the most acidic among the given compounds.

Resonance Effect in Acidity

Resonance is a concept that refers to the delocalization of electrons in a molecule. This delocalization can result in the stabilization of a structure, particularly for a conjugate base like the benzoate ion, where the negative charge can be spread out over multiple atoms. The resonance effect is a major factor contributing to the acidity of benzoic acid (Ph-COOH), as it allows the charge to be shared across the benzene ring, lowering the energy of the molecule and making it more stable.

On the other hand, while the formate ion (HCOO-) and acetate ion (CH3-COO-) also have resonance structures, they do not benefit from the extensive delocalization seen in the benzoate ion. Therefore, their corresponding acids are less acidic than benzoic acid.

Inductive Effect in Acidity

The inductive effect is a concept where electron density is pulled through sigma bonds due to electronegativity differences between atoms. This effect influences the stability of the conjugate base and, consequently, the acidity of the parent compound. In acetic acid (CH3-COOH), the methyl group (-CH3) attached to the carbonyl (C=O) has a slight inductive effect, pulling electron density away from the carboxylate ion (CH3-COO-), which provides some stabilization of the negative charge.

While the inductive effect of a single methyl group is relatively weak, it still contributes to the overall acidity of the molecule. In comparison, the formate ion does not have any alkyl groups exerting an inductive effect, making formic acid less acidic than acetic acid. In summary, the inductive effect plays a part in dictating the strength of an acid by affecting the electron distribution within its conjugate base.