Chemistry · Aldehydes, Ketones And Carboxylic Acid · NEET
Both have an O-H bond, but the difference is what happens AFTER the H+ leaves. When an alcohol loses H+ it makes an alkoxide (R-O-) where the negative charge sits on just one oxygen. When a carboxylic acid loses H+ it makes a carboxylate (R-COO-), and here the negative charge is shared equally over TWO oxygen atoms by resonance. Sharing the charge over two atoms makes the ion much more stable. A stable ion means the acid releases H+ easily, so R-COOH is acidic while R-OH is only very weakly acidic.
In the carboxylate ion (R-COO-), the two C-O bonds become identical. Instead of one C=O double bond and one C-O(-) single bond, the electrons are spread out so both oxygens are equivalent and each carries half of the negative charge. We draw this as two resonance structures. Spreading (delocalising) the charge lowers the energy of the ion. The more you can spread a negative charge, the more stable the ion, and the stronger the acid. This is the single most important reason carboxylic acids are acidic. NEET loves this point.
An electron-withdrawing group (EWG) INCREASES acidity. Groups like -Cl, -F, -NO2, -CN pull electron density AWAY from the -COO- through the sigma bonds. This is the negative inductive effect (-I). By pulling charge away, they help spread out the negative charge on the carboxylate, making it more stable. A more stable carboxylate = a stronger acid. So chloroacetic acid (ClCH2COOH) is stronger than acetic acid (CH3COOH).
Alkyl groups (-CH3, -C2H5) PUSH electron density TOWARD the carboxylate. This is the positive inductive effect (+I). Pushing more negative charge onto an ion that is already negative makes it LESS stable. A less stable carboxylate means the acid holds onto its H+ more tightly, so it is a weaker acid. That is why more alkyl groups near -COOH lower the acidity.
In formic acid the group attached to -COOH is just H, which has no electron-pushing effect. In acetic acid the -CH3 group pushes electrons in (+I effect) and destabilises the carboxylate. So acetic acid's ion is less stable and it is the weaker acid. Formic acid (Ka higher, pKa 3.75) beats acetic acid (pKa 4.76). This exact idea was tested in NEET 2025.
The inductive effect works through the sigma bonds and gets WEAKER with every extra bond it has to pass through. So a -Cl or -O right next to the -COOH gives a big boost to acidity, but the same group two or three carbons away gives almost no boost. Distance kills the -I effect. This distance rule was the key to the NEET 2016 tetrahydropyran question.
More electron-withdrawing groups = stronger acid. Each extra group pulls more charge away and stabilises the carboxylate more. That is why the order is CH3COOH < ClCH2COOH < Cl2CHCOOH < Cl3CCOOH. Trichloroacetic acid (three Cl) is a very strong acid, almost as strong as some mineral acids, because three -Cl groups strongly spread the charge.
On an aromatic acid, an electron-withdrawing group like -NO2 pulls charge away by both -I and -M (resonance) effects, so p-nitrobenzoic acid is STRONGER than benzoic acid. An electron-donating group like -OCH3 or -CH3 pushes charge in and makes the acid WEAKER than benzoic acid. NEET (ReNEET 2026) asked exactly this: p-nitrobenzoic acid is stronger than benzoic acid.
The correct order of decreasing acidity of the following aliphatic acids is:
The correct order of acid strength of: (I) cyclohexanecarboxylic acid; (II) a tetrahydropyran-carboxylic acid with the ring oxygen CLOSE (beta) to the -COOH carbon; (III) a tetrahydropyran-carboxylic acid with the ring oxygen FAR (across the ring) from the -COOH carbon.
Statement-I: Oxidation of p-nitrotoluene with acidic KMnO4 gives an acid that is stronger than benzoic acid. Statement-II: Reduction of p-nitrotoluene with Sn/HCl followed by neutralization gives an amine that is more basic than aniline. Choose the correct option.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Because losing the O-H proton gives a carboxylate ion whose negative charge is spread equally over two oxygen atoms by resonance, making it stable and easy to form.
Yes. Electron-withdrawing groups (-Cl, -NO2, -F, -CN) pull charge away by the -I effect, stabilise the carboxylate, and make the acid stronger.
They decrease acidity. Alkyl groups (+I effect) push charge onto the carboxylate, make it less stable, and give a weaker acid.
HCOOH is the strongest because it has no electron-pushing alkyl group. (CH3)3CCOOH is the weakest because it has the biggest +I push. Order: HCOOH > CH3COOH > (CH3)3CCOOH.
The inductive effect passes through sigma bonds and dies down with distance. A group next to -COOH has a big effect; the same group a few carbons away has almost none.
Yes. Carboxylic acids are more acidic than both phenols and alcohols because carboxylate resonance spreads the charge over two oxygens, giving the most stable anion. See the comparison page for details.
