Answer the question on basis of passage given below:
Organic compounds containing carbon-oxygen double bonds (C=O) are called carbonyl compounds. These are highly polar molecules. Hence, their boiling points are higher than hydrocarbons and ethers. Aldehydes can be prepared from dehydrogenation or oxidation of alcohols and by reduction of acyl halides. Ketones are prepared by oxidation of secondary alcohols or by hydration of alkynes etc. Aldehydes and ketones undergo nucleophilic addition reaction with a number of nucleophiles such as HCN, NaHSO3, ammonia derivatives. Carboxylic acids are prepared by the oxidation of primary alcohols, aldehydes and alkenes, by hydrolysis of nitriles. Carboxylic acids are more acidic than alcohols and phenols.
Arrange the following acids in increasing order of their acidic strengths:
\(HCOOH,\) \(FCH_2COOH,\) \(NO_2CH_2COOH,\) \(ClCH_2COOH\)
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → HCOOH < ClCH2COOH < FCH2COOH < NO2CH2COOH
An electron withdrawing group on the α-carbon pulls electron density away from the carboxylate ion, spreads its negative charge and stabilises it, so the acid becomes stronger. The stronger the electron withdrawal, the stronger the acid.
\( \ce{HCOOH} \): no electron withdrawing group next to –COOH, first (weakest acid)
\( \ce{ClCH2COOH} \): −I effect of chlorine: second
\( \ce{FCH2COOH} \): fluorine is more electronegative, stronger −I effect: third
\( \ce{NO2CH2COOH} \): the nitro group withdraws electrons even more strongly, last (strongest acid)
Shortcut: HCOOH is the weakest and the nitro acid the strongest. Only Option (4) starts with HCOOH and ends with \( \ce{NO2CH2COOH} \).
NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – “… The effect of the following groups in increasing acidity order is Ph < I < Br < Cl < F < CN < NO2 < CF3. Thus, the following acids are arranged in order of increasing acidity (based on pKa values): CF3COOH > CCl3COOH > CHCl2COOH > NO2CH2COOH > NC-CH2COOH > FCH2COOH > ClCH2COOH > BrCH2COOH > HCOOH > ClCH2CH2COOH > C6H5COOH > C6H5CH2COOH > CH3COOH > CH3CH2COOH”
Note: The same/similar question has been asked in different years by NTA as per the details given below:
| 2026 | 2025 | 2024 | 2023 | 2022 |
|---|---|---|---|---|
| – | – | 15-05 S1 | 09-06 S3 07-06 S3 | – |