Question

Arrange the following acids in increasing order of their acidic strengths: HCOOH, FCH₂COOH, NO₂CH₂COOH, ClCH₂COOH

• A. HCOOH $<$FCH$_2$COOH$<$ NO$_2$CH$_2$COOH $<$ClCH$_2$COOH
• B. HCOOH $<$ NO$_2$CH$_2$COOH $<$ ClCH$_2$COOH $<$ FCH$_2$COOH
• C. NO$_2$CH$_2$COOH $<$ HCOOH $<$ ClCH$_2$COOH $<$ FCH$_2$COOH
• D. HCOOH $<$ ClCH$_2$COOH $<$ FCH$_2$COOH $<$ NO$_2$CH$_2$COOH

Answer 1

The Correct Option is D

To determine the acidic strength of a given set of acids, it is important to assess their molecular structure, particularly the substituents attached to the carboxylic acid group. The acidity of carboxylic acids is influenced by the electron-withdrawing or electron-donating nature of substituents attached to the alpha carbon (the carbon adjacent to the carboxyl group). Stronger electron-withdrawing groups on the alpha carbon typically increase acidic strength due to stabilization of the carboxylate anion formed after dissociation of a proton (H⁺). 

• HCOOH (Formic Acid): It has no substituents other than hydrogen atoms directly attached to the carboxyl group, making it the weakest acid among the given options.
• ClCH₂COOH (Chloroacetic Acid): Chlorine is an electron-withdrawing group, which increases acidity compared to formic acid.
• FCH₂COOH (Fluoroacetic Acid): Fluorine is a more electronegative electron-withdrawing group than chlorine, thus making fluoroacetic acid stronger than chloroacetic acid.
• NO₂CH₂COOH (Nitroacetic Acid): The nitro group is a very strong electron-withdrawing group due to its resonance and inductive effects, enhancing the acidic strength significantly, more than the other acids listed.

Therefore, the order of increasing acidic strength is determined by the substituents' electron-withdrawing ability:

HCOOH < ClCH₂COOH < FCH₂COOH < NO₂CH₂COOH

Answer 2

To arrange the acids in increasing order of their acidic strengths, we need to consider the inductive effect of the substituents attached to the carboxylic acid group.

The acidic strength of a carboxylic acid is influenced by the electron-withdrawing or electron-donating nature of the substituents. Electron-withdrawing groups stabilize the carboxylate anion by dispersing the negative charge, thus increasing the acidity. Electron-donating groups destabilize the carboxylate anion, decreasing the acidity.

Let's analyze the given acids:

HCOOH (Formic acid): No electron-withdrawing group.

FCH₂COOH (Fluoroacetic acid): Fluorine is a highly electronegative and electron-withdrawing group.

NO₂CH₂COOH (Nitroacetic acid): The nitro group (NO₂) is a strong electron-withdrawing group.

ClCH₂COOH (Chloroacetic acid): Chlorine is an electronegative and electron-withdrawing group, but less so than fluorine or the nitro group.

The electron-withdrawing power of the substituents follows the order:

NO₂ > F > Cl > H

Therefore, the acidic strength will follow the same order:

NO₂CH₂COOH > FCH₂COOH > ClCH₂COOH > HCOOH

So, the increasing order of acidic strength is:

HCOOH < ClCH₂COOH < FCH₂COOH < NO₂CH₂COOH

The correct answer is:

Option 4: HCOOH < ClCH₂COOH < FCH₂COOH < NO₂CH₂COOH