Aldehydes, Ketones and Carboxylic Acids

Properties and Uses of Carboxylic Acids

Chemical properties:

(ii) Reducing with Tollen's reagent:
only formic acid will give this test
(iii) Fehling test
(iv) Formic acid does not react with NH2OH

 Up to nine carbons of carboxylic acid are liquid in state with unpleasant order. The higher members of carboxylic acids are waxy solids.
Carboxylic acid having higher boiling points than aldehydes, ketones and even alcohols due to their strong inter molecular hydrogen bond. The 1st four members of carboxylic acids are highly soluble in water. They undergo dimerisation in non-polar solvents like benzene or CCl4. They undergo ionisation in polar solvents like water.

Acidic Nature of carboxylic acid.
When carboxylic acids are treated with Na metal liberates dihydrogen.
Similarly when COOH treated with base like NaOH they form salt and water.
When COOH are treated with NaHCO3 (Sodium hydrogen carbonate) they liberate brisk effervescence of CO2. This reaction is used to distinguish COOH from other organic compounds.
Above all reactions inference that COOH has acidic nature.

\tt R-COOH + Na \rightarrow RCOO^{-}Na^{+} + \frac{1}{2}H_{2} \uparrow
\tt RCOOH + NaOH \rightarrow RCOO^{-}Na^{+} + H_{2}O
\tt RCOOH + NaHCO_{3} \rightarrow RCOO^{-}Na^{+} + H_{2}O + CO_{2} \uparrow

Acidic strength
Acidic strength of any COOH acid can be understood with the help of acid dissociation constant value ka or power of acid dissociation constant Pka
This can be illustrate as follows:-
From the above general reaction it is very clear that,

\tt k = \frac{\left[RCOO^{-}\right]\left[H_{3}O^{+}\right]}{\left[RCOOH\right]\left[H_{2}O\right]} Higher the ka value stronger the acid.
\tt k\left[H_{2}O\right] = \frac{\left[RCOO^{-}\right]\left[H_{3}O^{+}\right]}{\left[RCOOH\right]} Pka = −log ka
acid dissociation constant \tt k_{a} = \frac{\left[RCOO^{-}\right]\left[H_{3}O^{+}\right]}{\left[RCOOH\right]} lesser the Pka value stronger the acid
∴ Acidic strength ∝ ka
\tt \propto \frac{1}{P^{k_{a}}}

Carboxylic acids are weaker acids than mineral acids (HCl, HNO3, H2SO4) but stronger acids than alcohols and some phenols.

Carboxylic acids are stronger acids than phenol due to identical resonance structure in carboxylic acids whereas phenoxide ion having non-identical resonance structures

Acidic strength can also be understood by the type of group attached to the carbon of carboxylic acid.

(i) F — CH2 — COOH
(ii) Cl — CH2 — COOH
(iii) Br — CH2 — COOH
Acidic Strength order
(i) > (ii) > (iii)

When electron withdrawing group is present on the carboxylic carbon, it will increase the polarity of OH bond and increases the acidic strength. As a result Pka value decreases.
If electron releasing group is present, polarity of OH bond decreases, acidic strength decreases. Consequently Pka value increases.

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 value)

As distance increases from the carboxylic group to electron withdrawing group, acidic strength decreases.

Arrange the following in decreasing order of acidic strength:-

Formic acid is more acidic than benzoic acid. Benzoic acid is more acidic than any other carboxylic acid.

When electron withdrawing group is present at the para position in benzoic acid increases the acidic strength than benzoic acid.
If electron releasing or donating group is present at the para position decreases the acidic strength of benzoic acid.


Whether an electron releasing group or withdrawing group present at ortho position in benzoic acid it makes more acidic than benzoic acid. This effect is known as ortho effect.

Special Case:-

In case of para fluro benzoic acid and para chloro benzoic acid, para chloro benzoic acid is more acidic than para fluro benzoic acid. This is due to more effective back donation of fluorine than chlorine as shown above.

Carboxylic acids are more acidic than alcohol but picric acid is more acidic than benzoic acid.

Part1: View the Topic in this Video from 37:41 to 45:10

Part2: View the Topic in this Video from 0:40 to 53:50

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