Rank these acids according to their expected pKa values. Highest pKa HCOOH CH3COOH CH3CH2COOH CH3CH2CH2COOH Lowest pKa
The Correct Answer and Explanation is :
To rank the acids HCOOH (formic acid), CH3COOH (acetic acid), CH3CH2COOH (propionic acid), and CH3CH2CH2COOH (butyric acid) according to their expected pKa values from highest to lowest, we need to consider the structure of these carboxylic acids and their ability to donate a proton.
Ranking from Highest pKa to Lowest pKa:
- HCOOH (Formic acid)
- CH3COOH (Acetic acid)
- CH3CH2COOH (Propionic acid)
- CH3CH2CH2COOH (Butyric acid)
Explanation:
The pKa value of an acid indicates the strength of the acid; the lower the pKa, the stronger the acid. The pKa of carboxylic acids depends on the electron-donating or electron-withdrawing effects of the substituents attached to the carboxyl group (-COOH).
- Formic acid (HCOOH): This is the simplest carboxylic acid with only a hydrogen attached to the α-carbon. Since there are no alkyl groups to donate electron density to the carboxyl group, formic acid is a relatively stronger acid. This leads to a higher degree of proton dissociation and a lower pKa (around 3.75).
- Acetic acid (CH3COOH): In acetic acid, a methyl group (-CH3) is attached to the α-carbon. Methyl groups are electron-donating through inductive effects, which makes the carboxyl group slightly less likely to lose a proton than in formic acid. As a result, acetic acid has a slightly higher pKa (around 4.76) than formic acid.
- Propionic acid (CH3CH2COOH): Propionic acid has an ethyl group (-CH2CH3) attached to the α-carbon. This longer alkyl chain donates even more electron density to the carboxyl group than the methyl group in acetic acid, making the carboxyl group less able to lose a proton. Hence, propionic acid is slightly weaker than acetic acid, with a pKa around 4.87.
- Butyric acid (CH3CH2CH2COOH): Butyric acid has an even larger alkyl group, a butyl group (-CH2CH2CH3), attached to the α-carbon. This increased electron-donating effect makes the carboxyl group even less able to release a proton, leading to a higher pKa value around 4.82.
Thus, as the length of the alkyl chain increases, the electron-donating effect increases, making the acid weaker and raising its pKa.