Malonic acid and succinic acid each have two different pKa values.
Malonic Acid
pKas: 2.85 and 5.70.
Succinic acid pKas: 4.20 and 5.60.
Explain clearly why the first pKa of malonic acid is lower than the first pKa of succinic acid, and why the second pKa of malonic acid is higher than the second pKa of succinic acid.
I have the same question! ONLY ITS 2013, SOMEONE HELP
To understand why the first pKa of malonic acid is lower than the first pKa of succinic acid, and why the second pKa of malonic acid is higher than the second pKa of succinic acid, we need to consider the molecular structure and functional groups present in these compounds.
Both malonic acid and succinic acid are dicarboxylic acids, meaning they have two carboxylic acid functional groups (-COOH). These functional groups have an acidic hydrogen atom that can dissociate, releasing a proton (H+). The pKa value represents the acidity of the compound, specifically the pH at which half of the compound is in its acidic form (protonated) and the other half is in its conjugate base form (deprotonated).
Now, let's analyze malonic acid first:
The first pKa of malonic acid (2.85) is lower than the first pKa of succinic acid (4.20). This is because malonic acid has an additional electron-withdrawing group (a carbonyl group) in its structure compared to succinic acid. This carbonyl group (-C=O) can stabilize the negative charge on the conjugate base by delocalizing it through resonance. This delocalization of charge makes the carboxylic acid group more acidic and easier to deprotonate. As a result, the first pKa of malonic acid is lower than that of succinic acid.
Next, let's examine the second pKa values:
The second pKa of malonic acid (5.70) is higher than the second pKa of succinic acid (5.60). This is because the presence of the second carboxylic acid group in malonic acid makes the conjugate base less stable compared to succinic acid. The negative charge on the malonic acid conjugate base is distributed between two carboxylate groups, reducing the overall stability of the deprotonated species. Thus, malonic acid requires a higher pH (alkaline conditions) to achieve complete deprotonation compared to succinic acid.
In summary, the first pKa of malonic acid is lower than that of succinic acid due to the electron-withdrawing carbonyl group that stabilizes the negative charge, making it more acidic. On the other hand, the second pKa of malonic acid is higher than that of succinic acid because the second carboxylic acid group reduces the stability of the deprotonated species, making deprotonation more difficult.