Identify the most and the least acidic compound in each of the following sets. Leave the remaining answer in each set blank. a) cyclohexanol: most least phenol: most least benzoic acid: most least b) 2-chlorobutanoic acid: most least 4-chlorobutanoic acid: most least 3-chlorobutanoic acid: most least c) acetic acid: most least fluoroacetic acid: most least chloroacetic acid:
The correct answer and explanation is:
a) Cyclohexanol, Phenol, Benzoic Acid:
- Most acidic: Benzoic acid
- Least acidic: Cyclohexanol
Explanation:
- Benzoic acid is the most acidic compound in this set because the carboxyl group (-COOH) attached to the benzene ring can release a proton (H+) easily, making it a strong acid. Additionally, the negative charge formed after deprotonation is stabilized by resonance with the aromatic ring.
- Phenol is less acidic than benzoic acid because its hydroxyl group (-OH) is directly attached to the benzene ring. The negative charge formed after deprotonation is somewhat stabilized by resonance with the ring, but not as effectively as in benzoic acid.
- Cyclohexanol is the least acidic because it lacks the resonance stabilization that the other two compounds have. The alcohol group (-OH) in cyclohexanol does not release a proton as easily as the carboxyl group in benzoic acid or the hydroxyl group in phenol.
b) 2-Chlorobutanoic Acid, 4-Chlorobutanoic Acid, 3-Chlorobutanoic Acid:
- Most acidic: 2-Chlorobutanoic acid
- Least acidic: 4-Chlorobutanoic acid
Explanation:
- 2-Chlorobutanoic acid is the most acidic due to the electron-withdrawing effect of the chlorine atom at the second position. Chlorine’s electronegativity pulls electron density away from the carboxyl group (-COOH), stabilizing the conjugate base and making it more likely to lose a proton.
- 3-Chlorobutanoic acid is slightly less acidic because the chlorine is positioned farther from the carboxyl group. Although it still has some inductive effect, it is not as strong as in 2-chlorobutanoic acid.
- 4-Chlorobutanoic acid is the least acidic because the chlorine atom is at the farthest position from the carboxyl group, meaning its electron-withdrawing effect is weaker compared to the other two.
c) Acetic Acid, Fluoroacetic Acid, Chloroacetic Acid:
- Most acidic: Fluoroacetic acid
- Least acidic: Acetic acid
Explanation:
- Fluoroacetic acid is the most acidic in this group. Fluorine is a very strong electron-withdrawing group due to its high electronegativity, and it significantly stabilizes the conjugate base of the acid. This makes it easier for the proton (H+) to dissociate, increasing the acid strength.
- Chloroacetic acid is less acidic than fluoroacetic acid but more acidic than acetic acid. Chlorine is also an electron-withdrawing group, though not as strong as fluorine, which makes chloroacetic acid more acidic than acetic acid.
- Acetic acid is the least acidic because it lacks any strong electron-withdrawing substituents on the carboxyl group. Without such stabilization, acetic acid has the weakest tendency to lose a proton.