Carboxylic Acid & its Derivatives Notes for Class 12, IIT JEE & NEET

Carboxylic acids are in the NCERT chapter Aldehydes, Ketones and Carboxylic Acids, which is Chapter 8 of the current Class 12 Chemistry book (Chapter 12 in older books). The textbook covers the acids in detail, and it meets the derivatives through the reactions of the acids, while problems in JEE and NEET also ask about the relative reactivity and the reactions of the derivatives. These notes put both parts in one place.

Carboxylic Acid and Its Derivatives: Chapter Overview and Syllabus

Check the latest syllabi of CBSE, NTA (JEE Main) and NMC (NEET) for the exact lists.

Tip from the eSaral Chemistry team: Make one table with the four derivatives in the order of their reactivity, and write for each the reagent that makes it from the acid, and the reaction that turns it back into the acid. Most questions in this chapter come from that table.

What Are Carboxylic Acids? Structure and Nomenclature

A carboxylic acid contains the carboxyl group, $\mathrm{{-}COOH}$, in which one carbon is joined to a carbonyl group ($\mathrm{C{=}O}$) and to a hydroxyl group ($\mathrm{{-}OH}$). The general formula is $\mathrm{R{-}COOH}$, where $\mathrm{R}$ is hydrogen, an alkyl group or an aryl group. The carboxyl carbon is $sp^2$ hybridised, and the group is planar.

Naming the derivatives:

Carboxylic Acid & its Derivatives Notes

Preparation of Carboxylic Acids

A Grignard route adds one carbon to the chain, and it comes from a haloalkane, as in eSaral’s Halogen Derivatives notes.

Physical Properties of Carboxylic Acids

• Carboxylic acids form strong hydrogen bonds, so they have higher boiling points than alcohols, aldehydes and ketones of similar molar mass.
• In the vapour state and in non-polar solvents, two molecules of an acid join through two hydrogen bonds to form a dimer, so the molar mass found from the vapour density or from the freezing point is about double the real value.
• The first four acids are miscible with water, and the solubility falls as the carbon chain grows. Benzoic acid is only slightly soluble in cold water.
• Pure ethanoic acid freezes at about 290 K, and it is called glacial acetic acid.

Acidity of Carboxylic Acids

A carboxylic acid ionises in water:

$$\mathrm{R{-}COOH + H_2O \rightleftharpoons R{-}COO^{-} + H_3O^{+}}$$

It is a weak acid (pK$_a$ about 4 to 5), but it is more acidic than phenol and much more than an alcohol, because the carboxylate ion is stabilised by resonance: the negative charge is shared equally between the two oxygen atoms, and both C–O bonds have the same length.

Factors that affect the acidity:

• Electron-withdrawing groups ($\mathrm{{-}F}$, $\mathrm{{-}Cl}$, $\mathrm{{-}NO_2}$, $\mathrm{{-}CN}$) increase the acidity by the $-I$ effect, and the effect grows with the number of the groups and as they come closer to the carboxyl group.
• Electron-donating groups (alkyl groups) decrease the acidity, so $\mathrm{HCOOH} > \mathrm{CH_3COOH} > \mathrm{CH_3CH_2COOH}$.
• Benzoic acid is stronger than acetic acid, because the $sp^2$ carbon of the ring is more electronegative than an $sp^3$ carbon. An electron-withdrawing group on the ring increases the acidity, and an electron-donating group decreases it. An ortho substituent makes the acid stronger than benzoic acid in almost all cases (the ortho effect).

Because $K_a = 10^{-\mathrm{p}K_a}$, a difference of one unit of pK$_a$ means a tenfold difference in $K_a$.

Chemical Reactions of Carboxylic Acids

Reactions that Involve the O–H Bond (Acidity)

$$\mathrm{2R{-}COOH + 2Na \rightarrow 2R{-}COONa + H_2}$$ $$\mathrm{R{-}COOH + NaOH \rightarrow R{-}COONa + H_2O}$$ $$\mathrm{R{-}COOH + NaHCO_3 \rightarrow R{-}COONa + H_2O + CO_2\uparrow}$$

The reaction with sodium hydrogencarbonate, with the release of carbon dioxide, is the test for the carboxyl group. Phenols do not give it.

Reactions that Involve the C–OH Bond (Formation of Derivatives)

Esterification is reversible. To get a good yield, use an excess of the alcohol or remove the water.

Other Reactions of Carboxylic Acids

Acid Derivatives: Relative Reactivity and Nucleophilic Acyl Substitution

The four derivatives have the general formula $\mathrm{R{-}CO{-}Z}$, with $\mathrm{Z = Cl,\ OCOR,\ OR'\ \text{or}\ NH_2}$. They react by nucleophilic acyl substitution, in two steps (addition and elimination):

$$\mathrm{R{-}CO{-}Z + Nu^{-} \rightarrow \big[R{-}C(O^{-})(Z)(Nu)\big] \rightarrow R{-}CO{-}Nu + Z^{-}}$$

A nucleophile adds to the carbonyl carbon and forms a tetrahedral intermediate, which then expels the leaving group $\mathrm{Z}$. The more stable the leaving group, and the less the group $\mathrm{Z}$ donates electrons to the carbonyl group, the more reactive the derivative.

$$\text{acid chloride} > \text{anhydride} > \text{ester} > \text{amide}$$

In an amide, the lone pair on nitrogen is delocalised into the carbonyl group, so the amide is the most stable and the least reactive. A more reactive derivative can be changed into a less reactive one, but not the other way, without special reagents.

Reactions of Acid Chlorides, Anhydrides, Esters and Amides

Acid Chlorides

Acid Anhydrides

An anhydride reacts like an acid chloride, but more slowly. With water, it gives two molecules of the acid, with an alcohol an ester and an acid, and with ammonia an amide and an ammonium salt. Ethanoic anhydride is used to acetylate an $\mathrm{{-}OH}$ or an $\mathrm{{-}NH_2}$ group, for example in the making of aspirin from salicylic acid.

Esters

Saponification is irreversible, because the carboxylate ion that forms is not attacked by the alcohol. The name comes from the use of fats (esters of glycerol) to make soap. Esters have a pleasant fruity smell, and they are used in flavours, perfumes and as solvents.

Amides

In the Hofmann bromamide degradation, the amine has one carbon atom less than the amide, and it is part of the Amines chapter.

Key Facts for Acids and Derivatives

Solved Examples

Example 1 (acidity order): Arrange $\mathrm{CH_3COOH}$, $\mathrm{ClCH_2COOH}$, $\mathrm{Cl_3CCOOH}$ and $\mathrm{Cl_2CHCOOH}$ in the decreasing order of acidity.

$\mathrm{Cl_3CCOOH} > \mathrm{Cl_2CHCOOH} > \mathrm{ClCH_2COOH} > \mathrm{CH_3COOH}$, because each chlorine atom stabilises the carboxylate ion by the $-I$ effect.

Example 2 (names): Name $\mathrm{CH_3CH(CH_3)COOH}$, $\mathrm{C_6H_5COCl}$, $\mathrm{CH_3COOC_2H_5}$ and $\mathrm{CH_3CONH_2}$.

2-Methylpropanoic acid, benzoyl chloride, ethyl ethanoate and ethanamide.

Example 3 (esterification): Write the product of $\mathrm{CH_3COOH}$ with $\mathrm{C_2H_5OH}$ in the presence of a few drops of concentrated $\mathrm{H_2SO_4}$.

$\mathrm{CH_3COOH + C_2H_5OH \rightleftharpoons CH_3COOC_2H_5 + H_2O}$. The product is ethyl ethanoate (ethyl acetate).

Example 4 (Hofmann bromamide): Give the product when ethanamide is treated with $\mathrm{Br_2}$ and $\mathrm{NaOH}$.

$\mathrm{CH_3CONH_2 + Br_2 + 4NaOH \rightarrow CH_3NH_2 + Na_2CO_3 + 2NaBr + 2H_2O}$. The product is methanamine, which has one carbon less than the amide.

Example 5 (decarboxylation): What is formed when sodium ethanoate is heated with soda lime?

$\mathrm{CH_3COONa + NaOH \xrightarrow{CaO,\ \Delta} CH_4 + Na_2CO_3}$. The product is methane.

Example 6 (a distinguishing test): How can you distinguish benzoic acid from phenol?

Add $\mathrm{NaHCO_3}$ solution. Benzoic acid gives effervescence of $\mathrm{CO_2}$, and phenol does not, because phenol is a weaker acid than carbonic acid.

Example 7 (relative reactivity): Arrange $\mathrm{CH_3COCl}$, $\mathrm{(CH_3CO)_2O}$, $\mathrm{CH_3COOC_2H_5}$ and $\mathrm{CH_3CONH_2}$ in the decreasing order of reactivity towards nucleophilic acyl substitution.

$\mathrm{CH_3COCl} > \mathrm{(CH_3CO)_2O} > \mathrm{CH_3COOC_2H_5} > \mathrm{CH_3CONH_2}$.

Example 8 (pK$_a$ and strength): Chloroethanoic acid has pK$_a$ $= 2.86$, and ethanoic acid has pK$_a$ $= 4.76$. How many times is the first acid stronger?

$\dfrac{K_a(\text{chloro})}{K_a(\text{ethanoic})} = 10^{4.76 - 2.86} = 10^{1.9} \approx 79$, so it is about 79 times stronger.

Example 9 (oxidation of a side chain): What is formed when ethylbenzene is heated with alkaline $\mathrm{KMnO_4}$ and then acidified?

The whole side chain is oxidised to the carboxyl group, so the product is benzoic acid, $\mathrm{C_6H_5COOH}$.

Common Mistakes in Carboxylic Acid Problems

How to Study Carboxylic Acid and Its Derivatives

1. Read the NCERT chapter first, and download the textbook from eSaral’s NCERT books for Class 12 page.
2. Learn the pK$_a$ table and the three rules for acidity (the $-I$ effect, the $+I$ effect and the ring effect).
3. Make the derivatives table: for each derivative, write the reagent from the acid, the reactivity order and its hydrolysis product.
4. Write the named reactions (HVZ, Rosenmund, Hofmann bromamide, Kolbe, Friedel–Crafts acylation) with the reagent and the product.
5. Revise with eSaral’s Class 12 Chemistry notes and JEE Main Chemistry revision notes, and test yourself with eSaral’s NEET chapter-wise PYQs and JEE Main chapter-wise PYQs.