Compound with Maximum pKa: 4-cyanoacetic acid



1. Phenol:
- Phenol is an aromatic compound with a hydroxyl group (-OH) attached to a benzene ring.
- The pKa value of phenol is around 10, making it a weak acid compared to other compounds.
- The hydroxyl group in phenol can donate its proton (H+) to a base, but it is not as acidic as other compounds in the given options.

2. Ethyl Alcohol:
- Ethyl alcohol, also known as ethanol, has the molecular formula C2H5OH.
- The hydroxyl group in ethanol can donate its proton (H+) to a base, making it a weak acid.
- The pKa value of ethanol is around 16, indicating that it is a weaker acid compared to phenol and formic acid.

3. Formic Acid:
- Formic acid has the molecular formula HCOOH and is commonly found in nature, such as in the venom of ants.
- The carboxylic acid group (-COOH) in formic acid is more acidic than the hydroxyl group in phenol and ethanol.
- The pKa value of formic acid is around 3.75, making it a stronger acid compared to phenol and ethanol.

4. Cyanoacetic Acid:
- Cyanoacetic acid has the molecular formula CH2(CN)COOH and contains both a carboxylic acid group and a cyano group (-CN).
- The presence of the cyano group enhances the acidity of cyanoacetic acid compared to formic acid.
- The pKa value of cyanoacetic acid is around 9.5, making it the most acidic compound among the given options.
- The cyano group stabilizes the negative charge on the conjugate base, making it easier to lose a proton and increasing the acidity.

Conclusion:
Among the given compounds, 4-cyanoacetic acid has the highest pKa value, indicating that it is the most acidic compound. The presence of both a carboxylic acid group and a cyano group contributes to its enhanced acidity compared to phenol, ethanol, and formic acid.

(2) ethyl alcohol, because of the presence of methyl group on ethyl alcohol which shows +I effect and destabilises its corresponding conjugate base. or we can say that methyl group increases the electron density on CH3---CH2--- O- ion and makes it more basic. so if the corresponding conjugate base of the acid is more basic the acid is weaker. so if the acid is weaker then it's Ka value will be less and it's pKa value will be more.