Carboxylic Acids- Properties

Carboxylic acids

Carboxylic acids are organic compounds that contain a carboxyl functional group (also known as the carboxylate group). These compounds are widely found in nature and are also synthesized by humans. When a carbonyl carbon (C=O) is also bonded to a hydroxyl group (–OH), we get a carboxylic acid. The general formula for a carboxylic acid is R-COOH, where R represents the rest of the molecule to which the carboxyl group is attached. COOH denotes the carboxyl group, which consists of a carbonyl carbon (C=O) and a hydroxyl group (–OH). The carboxyl group looks like this: –COOH. In this article we will see acidity of carboxylic acids, effect of substituents on acidity, inductive effect and qualitative tests for carboxylic acids, amide and ester.

Table of Contents

Here are some common examples of carboxylic acids.

Acetic acid (CH₃COOH): You might recognize this as a component of vinegar.
Formic acid (HCOOH): It has a pungent odor and is found in ant venom.
Butyric acid (C₄H₇COOH): This one smells like rancid butter.

Properties

Acidity: Carboxylic acids are acidic due to the presence of the carboxyl group. When they lose a proton (H⁺), they form carboxylate ions (R-COO⁻).
Boiling Points: Carboxylic acids have relatively high boiling points compared to other compounds of similar molar mass. Boiling points increase with increasing molar mass.
Solubility: Carboxylic acids with a low number of carbon atoms (up to about 4) are soluble in water. They can form hydrogen bonds with water molecules. However, solubility decreases with increasing molar mass.

Acidity of Carboxylic Acids

pKa Scale and Acid Strength

The acidity of carboxylic acids (and other weak acids) is measured using the pKa scale. Smaller pKa values indicate stronger acids.

For example, consider three compounds:

Acetic acid (CH₃COOH): pKa ≈ 5 (stronger acid)
Phenol (C₆H₅OH): pKa ≈ 10 (weaker acid)
Ethanol (CH₃CH₂OH): pKa ≈ 16 (very weak acid)

Notice the significant difference in pKa values between acetic acid and ethanol—over 12 units!

Conjugate Bases and Resonance

To understand this difference, let’s look at their conjugate bases (the ions formed after donating a proton):

Acetate ion (CH₃COO⁻): The negative charge is delocalized between both oxygen atoms due to resonance. This delocalization stabilizes the ion.
Ethoxide ion (CH₃CH₂O⁻): The negative charge is localized on a single oxygen atom.

Resonance delocalization plays a crucial role. In the acetate ion, the charge is spread out over both oxygens, making it more stable. In contrast, the ethoxide ion lacks this resonance stabilization.

Why Resonance Matters

Resonance allows charge to be distributed across multiple atoms. Stable charges are “spread out” rather than confined to one atom.
In acetic acid, the resonance forms show that the negative charge is shared between both oxygen atoms. This delocalization makes the acetate ion more stable.
In ethanol, the negative charge remains localized on a single oxygen atom.
The delocalization effect accounts for the significant difference in acidity between acetic acid and ethanol.

Carboxylic acids are acidic because the hydrogen in the –COOH group can be donated to form a carboxylate ion. Resonance delocalization and inductive effects significantly impact their acidity. Phenols, while also containing –OH groups, lack the resonance stabilization seen in carboxylic acids, making them weaker acids.

Effect of substituents on acidity

Inductive Effects

The inductive effect is an experimentally observed phenomenon where the transmission of charge occurs through a chain of atoms within a molecule. It results in the creation of a permanent dipole within a chemical bond. Essentially, substituents (groups of atoms) attached to a molecule can influence its properties by either withdrawing or donating electron density. Carboxylic acids (like acetic acid, formic acid, and benzoic acid) exhibit fascinating acidity variations due to inductive effects.

Electron-Withdrawing Substituents: When a substituent withdraws electron density from the carboxyl group, it increases the acidity of the carboxylic acid. Why? Because it stabilizes the conjugate carboxylate anion (the negatively charged form after donating a proton).

Examples of electron-withdrawing substituents include halogens (F, Cl, Br, I) and nitro groups (–NO₂).
The more electronegative the substituent, the greater its effect on acidity.

Electron-Donating Substituents: Conversely, substituents that donate electron density to the carboxyl group decrease acidity. They destabilize the carboxylate anion.

Alkyl groups (such as methyl, ethyl, tert-butyl) are electron-donating.
The tert-butyl group, for instance, is electron-donating and decreases acidity.

Resonance Effects

The exceptional acidity of carboxylic acids is largely due to resonance effects. The carboxyl group (–COOH) can resonate between two forms:

Contributing Structure 1: The negative charge is shared between both oxygen atoms (delocalized). This stabilizes the carboxylate ion.
Contributing Structure 2: The negative charge is localized on a single oxygen atom.

Electron-withdrawing substituents enhance this resonance effect, making the carboxylate ion even more stable and the acid more acidic.

Examples

Formic acid (HCOOH) vs. Acetic acid (CH₃COOH):

Formic acid has a hydrogen directly attached to the carbonyl carbon (no alkyl group). It’s more acidic due to the absence of electron-donating alkyl groups.
Acetic acid has a methyl group (CH₃) attached to the carbonyl carbon. The alkyl group donates electron density, making acetic acid less acidic than formic acid.

Benzoic acid (C₆H₅COOH):

The phenyl ring (C₆H₅) stabilizes the carboxylate ion through resonance.
Substituents on the phenyl ring (e.g., –Cl, –NO₂) can further affect acidity. Electron-withdrawing groups increase acidity, while electron-donating groups decrease it.

Overall Rule

Electron-Withdrawing Substituents: Increase acidity.
Electron-Donating Substituents: Decrease acidity.

Qualitative tests

Let’s explore the qualitative tests for carboxylic acids, amides, and esters. These tests help identify these functional groups based on their unique reactivity.

Carboxylic Acids

Litmus Test

Add a small amount of your unknown compound to a piece of blue or red litmus paper.
Carboxylic acids turn blue litmus paper red because they are acidic.
The litmus paper will change color from blue to red.

Sodium Bicarbonate Test (or Sodium Hydrogen Carbonate Test)

Add a pinch of sodium bicarbonate (baking soda) to your unknown compound.
Effervescence (bubbling) indicates the presence of a carboxylic acid.
The reaction produces carbon dioxide gas.

Ester Test

Mix your unknown with a few drops of concentrated sulfuric acid (H₂SO₄).
Heat the mixture gently.
If an ester is present, a fruity smell (resembling fruits or flowers) will be released.
This fruity odor is characteristic of esters.

Fluorescein Test

Dissolve a small amount of your unknown in ethanol.
Add a few drops of fluorescein solution (a yellow-green dye).
If a carboxylic acid is present, the solution will turn pink or red due to the formation of a complex between the dye and the acid.

Amides

Hofmann Test

Treat your unknown with chloroform (CHCl₃) and silver nitrate (AgNO₃) in the presence of sodium hydroxide (NaOH).
An amide reacts to form an isocyanide (also called a carbylamine).
The isocyanide has a foul, pungent odor.
If you smell something unpleasant, it indicates the presence of an amide.

Acid Hydrolysis

Heat your unknown with concentrated hydrochloric acid (HCl) or sulfuric acid (H₂SO₄).
Amides undergo hydrolysis to form carboxylic acids and ammonia (NH₃).
Test the evolved gas with damp red litmus paper; it will turn blue due to the ammonia released.

Esters

Hydrolysis Test

Heat your unknown with dilute hydrochloric acid (HCl) or dilute sulfuric acid (H₂SO₄).
Esters undergo hydrolysis to form carboxylic acids and alcohols.
The liberated carboxylic acid can be tested using the litmus paper or other acid tests.

Silver Nitrate Test

Mix your unknown with silver nitrate (AgNO₃) solution.
Esters do not react with silver nitrate.
No precipitate forms, distinguishing them from carboxylic acids.

Summary

Carboxylic acids, with their characteristic carboxyl group (–COOH), exhibit acidity due to proton donation. Resonance stabilization and inductive effects play key roles. Electron-withdrawing substituents (like halogens) enhance acidity, while electron-donating groups (such as alkyls) reduce it. In qualitative tests, carboxylic acids turn litmus paper red, effervesce with sodium bicarbonate, and emit fruity odors upon esterification. Amides, on the other hand, yield foul-smelling isocyanides in the Hofmann test and release ammonia during acid hydrolysis. Esters, when hydrolyzed, produce carboxylic acids and alcohols.

Carboxylic acids- Properties