What is the correct order of increasing acidic strength of the given compounds? 

(III) < (IV) < (II) < (I)

The correct answer is option 1. (III) < (IV) < (II) < (I).

To explain the correct order of increasing acidic strength for the given compounds, let's delve into how substituents affect the acidity of carboxylic acids.

Key Concepts:

Acidic Strength and Conjugate Base Stability: The strength of a carboxylic acid depends on the stability of its conjugate base. The more stable the conjugate base, the stronger the acid.

Effect of Substituents:

Electron-Withdrawing Groups (EWGs): Increase acidity by stabilizing the negative charge on the conjugate base. Examples include bromine (-Br) in carboxylic acids.

Electron-Donating Groups (EDGs): Decrease acidity by destabilizing the negative charge on the conjugate base. Examples include methyl groups (-CH₃).

The name sof the given structures are:

1. 2-Bromobutanoic Acid :
 Bromine is a strong EWG. When positioned at the 2nd carbon (near the carboxyl group), it significantly stabilizes the negative charge on the conjugate base (2-bromobutyrate) through induction. This makes 2-bromobutanoic acid the most acidic of the given compounds.

2. 3-Bromobutanoic Acid:

Bromine is still an EWG, but its effect is less pronounced because it's further from the carboxyl group (at the 3rd position). The electron-withdrawing effect is weaker compared to 2-bromobutanoic acid. Thus, 3-bromobutanoic acid is less acidic than 2-bromobutanoic acid but more acidic than compounds with EDGs.

3. 2-Methylpropanoic Acid:

No strong substituent effects here. This is the baseline acidity of a carboxylic acid with no additional groups affecting the carboxyl group.

4. Butanoic Acid:

The methyl group is an EDG. It donates electron density towards the carboxyl group, destabilizing the negative charge on the conjugate base (2-methylpropanoate) and thus decreasing the acidity of the acid. Therefore, 2-methylpropanoic acid is the least acidic among the given compounds.

Conclusion: Based on the effects of the substituents on the acidity:

Least acidic: 2-Methylpropanoic Acid (III) due to the EDG effect.

Intermediate acidity: Butanoic Acid (IV) with no significant substituent effects.

More acidic: 3-Bromobutanoic Acid (II) due to the weaker EWG effect.

Most acidic: 2-Bromobutanoic Acid (I) due to the strong EWG effect of bromine.

Thus, the correct order of increasing acidic strength is: (III) < (IV) < (II) < (I)