Question

Which is most acidic compound? (A) \(\mathrm{CH}_{3}-\mathrm{COOH}\) (B) \(\mathrm{CH}_{3}-\stackrel{\Theta}{\mathrm{N}} \mathrm{H}_{3}\) (C) \(\mathrm{CH}_{2}-\mathrm{COOH}\) (D) \(\mathrm{CH}_{2}-\mathrm{COOH}\)

Short answer

The most acidic compounds are (A) \(\mathrm{CH}_{3}-\mathrm{COOH}\), (C) \(\mathrm{CH}_{2}-\mathrm{COOH}\), and (D) \(\mathrm{CH}_{2}-\mathrm{COOH}\) as they have the same pKa values of around 4.76, which are lower than the pKa value of 9.25 for compound (B) \(\mathrm{CH}_{3}-\stackrel{\Theta}{\mathrm{N}} \mathrm{H}_{3}\).

Step-by-step solution

Identify the acidic groups in each compound.

In each given compound, we need to identify the acidic group that can donate a proton (H+). (A) \(\mathrm{CH}_{3}-\mathrm{COOH}\): The carboxylic acid group (-COOH) makes this compound acidic. (B) \(\mathrm{CH}_{3}-\stackrel{\Theta}{\mathrm{N}} \mathrm{H}_{3}\): The amine group (-NH3+), which is protonated here, makes this compound acidic. (C) \(\mathrm{CH}_{2}-\mathrm{COOH}\): The carboxylic acid group (-COOH) makes this compound acidic. (D) \(\mathrm{CH}_{2}-\mathrm{COOH}\): The carboxylic acid group (-COOH) makes this compound acidic.

Compare the pKa values of the acidic groups

The pKa values are a measure of the acidity of a given compound, with lower values indicating a stronger acid. Let's compare the pKa values of the major acidic groups in the given compounds. (A) \(\mathrm{CH}_{3}-\mathrm{COOH}\): pKa value of -COOH group is around 4.76. (B) \(\mathrm{CH}_{3}-\stackrel{\Theta}{\mathrm{N}} \mathrm{H}_{3}\): pKa value of -NH3+ group is around 9.25. (C) \(\mathrm{CH}_{2}-\mathrm{COOH}\): pKa value of -COOH group is around 4.76. (D) \(\mathrm{CH}_{2}-\mathrm{COOH}\): pKa value of -COOH group is around 4.76.

Identify the most acidic compound based on pKa values

Based on the pKa values, we can see that the carboxylic acid groups (-COOH) have the lowest pKa values, indicating that they are stronger acids compared to the amine group (-NH3+). Among the given options, compounds (A), (C), and (D) have the same pKa values, so they have the same acidity. Therefore, it would be inaccurate to single out one compound as the most acidic among the given options. The correct answer should be that (A), (C), and (D) are equally acidic, and are more acidic than compound (B).

Key concepts

Acidity and pKa

Understanding the relationship between acidity and pKa is fundamental in chemistry. Acidity refers to a substance's ability to donate a proton (H+ ion) in an aqueous solution, which is a key aspect of many reactions. The pKa value is an expression of the strength of an acid, defined as the negative logarithm of the acid dissociation constant (Ka) of a compound. In simpler terms, it indicates how easily an acid can donate its proton.

Compounds with lower pKa values are stronger acids because they are more willing to give up their proton. When comparing acids, the one with the lowest pKa value is generally the most acidic. For students grappling with acidity in homework, remember that a lower pKa means a higher acidity, making this a crucial checkpoint in identifying acidic compounds in exercises.

Carboxylic Acid

Carboxylic acids are a group of organic compounds that are characterized by the presence of a carboxyl group (-COOH). This group consists of a carbonyl (a carbon double-bonded to an oxygen) and a hydroxyl group (an oxygen single-bonded to a hydrogen). This structure makes carboxylic acids more acidic than alcohols and many other organic compounds.

Carboxylic acids typically have pKa values ranging from about 3 to 5, qualifying them as weak acids in the context of all possible acids, but still stronger than many other organic compounds. In the context of the given exercise, knowing that carboxylic acids tend to be more acidic helps to quickly eliminate less acidic groups when comparing options.

Amine Group

The amine group consists of a nitrogen atom attached to hydrogen atoms, alkyl groups, or aryl groups. In its protonated form (-NH3+), it can act as an acid by donating a proton. However, amines are generally not as acidic as carboxylic acids. The pKa value of an amine group is usually higher, indicating a lower tendency to release a proton. A common stumbling block for students is to realize that the presence of nitrogen in an amine doesn't necessarily make it a strong acid. In fact, amines are considered weak bases in their unprotonated forms, which means they are more likely to accept a proton than to donate one.

Proton Donation

Proton donation is the act of an acid releasing a hydrogen ion (H+). This process is at the heart of what defines an acid. The ease with which a compound donates a proton determines its strength as an acid, and this concept is central to acid-base chemistry. It's important for learners to understand that the process of proton donation is influenced by the stability of the resulting anion. In our exercise, compounds with a carboxylic acid group are more stable after losing a proton, hence they are more acidic compared to an amine group which produces a less stable cation after proton donation.

Students should also be aware that molecular structure and neighboring atoms can affect the ease of proton donation. Factors such as resonance and electronegativity can either stabilize or destabilize the compound, impacting the acidity. This is why, despite all compounds featuring carboxylic acid groups in the exercise, the answer doesn't single out one as the most acidic, as they share similar stabilizing factors.