Question

Which among the given acids has lowest pKa value? (a) Chloroacetic acid (b) Bromoacetic acid (c) Nitroacetic acid (d) Cyanoacetic acid

Short answer

Nitroacetic acid (c) has the lowest pKa value.

Step-by-step solution

Understanding pKa

pKa is a measure of the strength of an acid in solution. The lower the pKa value, the stronger the acid is, as it indicates a greater tendency to donate protons. In this problem, we need to identify the compound with the strongest acidic character by finding the lowest pKa value.

Evaluate the Electron-withdrawing Groups

Each compound given has different substituents: chloro (-Cl), bromo (-Br), nitro (-NO2), and cyano (-CN). Electron-withdrawing groups increase the acidity of an acid by stabilizing the negative charge on the conjugate base, thus lowering the pKa value. Stronger electron-withdrawing groups will lead to a lower pKa.

Analyze the Substituents

Among chloro, bromo, nitro, and cyano groups, the nitro group (-NO2) and cyano group (-CN) are known for having strong electron-withdrawing effects. The nitro group is generally considered to be a stronger electron-withdrawing group than the cyano group, as nitro is a resonance and inductive electron-withdrawing group.

Determine the Lowest pKa Value

Since the nitro group is the strongest electron-withdrawing group, nitroacetic acid will have the most stabilized conjugate base, resulting in the lowest pKa value among the given options. Thus, it is expected that nitroacetic acid will be the strongest acid with the lowest pKa value.

Key concepts

pKa Value

The pKa value is an essential indicator of an acid's strength. When we talk about pKa, we're referring to the measure of how easily an acid gives up a proton. The lower the pKa, the more willing the acid is to donate a proton, meaning it's stronger. This is because a lower pKa value suggests that the acid dissociates more completely in solution.
In any competitive analysis of acid strength, identifying the compound with the lowest pKa value will help spot the strongest acid. So, in our exercise, finding the acid with the lowest pKa implies identifying the one most ready to release a proton.

Electron-withdrawing Groups

Electron-withdrawing groups (EWGs) have a significant impact on the acidity of a compound. These groups attract electrons towards themselves, generally by the inductive effect or resonance, which in turn affects how easily the acid's hydrogen atom is released.
In the context of the given acids, each functional group attached to the acetic acids either enhances or diminishes its acidity. The chloro (-Cl), bromo (-Br), nitro (-NO2), and cyano (-CN) groups all function as EWGs.

• The nitro (-NO2) group is particularly influential because it can stabilize the conjugate base through both resonance and inductive effects. This leads to a stronger acidity.
• The cyano group also provides electron-withdrawing effects, but not as strongly as nitro.

These effects modulate the acidity by stabilizing the conjugate base that forms after the acid donates a proton, leading to different pKa values.

Conjugate Base Stability

Conjugate base stability is crucial when discussing acid strength. When an acid donates a proton, the remaining species is known as the conjugate base. The more stable this conjugate base, the stronger the acid.
The stability of a conjugate base is greatly enhanced by electron-withdrawing groups. By pulling electron density away from the negatively charged oxygen in the conjugate base, these groups help disperse the negative charge, stabilizing the conjugate base.

• For example, the nitro group (-NO2) not only withdraws electrons but also allows for resonance, which distributes the negative charge over several atoms, proving it to be a strong acid due to its stable conjugate base.
• This makes nitroacetic acid a frontrunner in our exercise, thanks to the powerful stabilization provided by the nitro group.

Cyano groups also provide stabilization, but not as effectively as nitro groups, leading to a higher pKa and lower acid strength.