Chemistry | Carboxylic acids

Welcome to the World of Carboxylic Acids!

Hello future Chemists! This chapter takes us deeper into the fascinating world of Organic Chemistry. We’ve already looked at alkanes, alkenes, and alcohols, and now we move on to one of the most important classes of compounds: Carboxylic Acids.

You encounter these every day! The sharp taste of vinegar (ethanoic acid) or the tang in citrus fruits is all thanks to these molecules. Understanding this chapter is essential because carboxylic acids are the building blocks for many useful materials, including soaps, plastics, and flavourings.

Don't worry if Organic Chemistry sometimes feels like learning a new language. We will break down every concept step-by-step using clear explanations and relatable examples!

1. Structure and the Carboxyl Group

1.1 What Makes an Acid Carboxylic?

Carboxylic acids are defined by a specific functional group called the Carboxyl Group. This group is attached to an alkyl chain (R).

The Carboxyl Functional Group

The Carboxyl group is written chemically as \( -COOH \). It’s actually a combination of two other groups we've studied:

• The Carbonyl Group (\( C=O \))
• The Hydroxyl Group (\( -OH \))

When these two are bonded together on the same carbon atom, they form the Carboxyl group:

\( R - C ( = O ) - OH \)

Remember this! The presence of the \( -COOH \) group is the defining characteristic of all carboxylic acids.

1.2 The General Formula

Carboxylic acids belong to a homologous series with the general formula:

\( C_n H_{2n} O_2 \)

(Where 'n' is the number of carbon atoms, including the one in the carboxyl group.)

2. Naming Carboxylic Acids (Nomenclature)

Naming these acids is straightforward, similar to naming alcohols and alkanes. We follow the IUPAC naming rules:

Rule: Take the name of the corresponding alkane, remove the -e, and add -oic acid.

Since the carboxyl group (\( -COOH \)) must be at the end of the chain, the carbon in this group is always counted as the first carbon (C1).

Common Examples (Up to C4):

1. 1 Carbon (n=1): Derived from Methane.
   Name: Methanoic Acid
   Formula: \( HCOOH \)
   Did you know? This is the acid found in ant stings!
2. 2 Carbons (n=2): Derived from Ethane.
   Name: Ethanoic Acid
   Formula: \( CH_3COOH \)
   This is the chemical name for vinegar.
3. 3 Carbons (n=3): Derived from Propane.
   Name: Propanoic Acid
   Formula: \( CH_3CH_2COOH \)
4. 4 Carbons (n=4): Derived from Butane.
   Name: Butanoic Acid
   Formula: \( CH_3CH_2CH_2COOH \)
   This acid is notorious for its strong, unpleasant smell (it’s found in rancid butter).

Memory Aid: Use the same prefixes you learned for alkanes and alcohols: Meth, Eth, Prop, But.

3. Physical Properties

3.1 High Boiling Points

Carboxylic acids have significantly higher boiling points than alkanes or even alcohols of similar molecular mass. Why?

This is because of very strong Hydrogen Bonding.

• The hydrogen bonds are formed between the \( -OH \) group of one acid molecule and the \( C=O \) group of another acid molecule.
• In fact, in the liquid and sometimes gaseous state, two carboxylic acid molecules often link together to form a stable pair called a dimer.

The formation of these dimers means you need much more energy (a higher temperature) to separate them and turn them into a gas, resulting in a high boiling point.

3.2 Solubility in Water

Small carboxylic acids (usually those with 1 to 4 carbons) are completely soluble (miscible) in water.

Analogy: Water molecules love to form hydrogen bonds. Since carboxylic acids have both the \( C=O \) and \( -OH \) groups, they are fantastic at forming hydrogen bonds with water. They are highly attracted to water molecules, so they dissolve easily.

However, as the carbon chain (the R group) gets longer (5 carbons or more), the molecule becomes more hydrophobic (water-hating). The long, non-polar carbon chain dominates, and the solubility decreases rapidly.

4. Preparation of Carboxylic Acids: Oxidation

How do we make a carboxylic acid in the lab? We usually start with an alcohol!

This process is a continuation of the oxidation chemistry you learned in the alcohol chapter. Carboxylic acids are the final oxidation product of primary alcohols.

Step-by-Step Oxidation of a Primary Alcohol

For oxidation to go all the way to a carboxylic acid, you need a strong oxidising agent and strong heating (reflux conditions are usually necessary).

Starting Material: A Primary Alcohol (where the \( -OH \) is attached to a carbon that is only attached to one other carbon atom).

Oxidising Agent: Common examples include acidified potassium manganate(VII) (\( KMnO_4 \)) or acidified potassium dichromate(VI) (\( K_2Cr_2O_7 \)).

Reaction: The primary alcohol is fully oxidised, usually passing through an aldehyde intermediate, to form the carboxylic acid.

$$ \text{Primary Alcohol} \xrightarrow{\text{[O] (Strong Oxidising Agent)}} \text{Carboxylic Acid} + \text{Water} $$

Example (Ethanol to Ethanoic Acid):

$$ CH_3CH_2OH + 2[O] \longrightarrow CH_3COOH + H_2O $$

Visual Check: If you use acidified potassium dichromate(VI), the colour change confirms the reaction: Orange solution turns Green. If using potassium manganate(VII), the colour changes from Purple to Colourless.

5. Chemical Properties: The 'Acid' Reactions

The most important chemical properties come from the fact that they are acids—but they are weak acids.

Prerequisite Concept Check: Remember that an acid is a substance that dissociates (ionises) in water to release hydrogen ions (\( H^+ \)).

5.1 Weak Acids

Carboxylic acids are considered weak acids because they only partially dissociate in water. They do not release all their \( H^+ \) ions into solution.

Example (Ethanoic Acid):

$$ CH_3COOH (aq) \rightleftharpoons CH_3COO^- (aq) + H^+ (aq) $$

The double arrow (\( \rightleftharpoons \)) indicates that the reaction is reversible, meaning most of the acid remains as undissociated \( CH_3COOH \) molecules.

Don't worry if this seems tricky at first: Even though they are weak, they still show the classic acid properties you learned about earlier!

5.2 Characteristic Acid Reactions

Carboxylic acids react readily with metals, bases, and carbonates/hydrogencarbonates. In all these reactions, the product is a salt (called a carboxylate salt) and water (and sometimes a gas).

A) Reaction with Reactive Metals

Acids react with reactive metals (like Mg, Zn, Na) to produce a salt and hydrogen gas.

$$ \text{Acid} + \text{Metal} \longrightarrow \text{Salt} + \text{Hydrogen gas} $$

Example (Ethanoic Acid with Magnesium):

$$ 2CH_3COOH (aq) + Mg (s) \longrightarrow (CH_3COO)_2Mg (aq) + H_2 (g) $$

B) Reaction with Bases/Alkalis (Neutralisation)

Neutralisation is the reaction between an acid and a base (like metal oxides or hydroxides) to produce a salt and water.

$$ \text{Acid} + \text{Base} \longrightarrow \text{Salt} + \text{Water} $$

Example (Ethanoic Acid with Sodium Hydroxide):

$$ CH_3COOH (aq) + NaOH (aq) \longrightarrow CH_3COONa (aq) + H_2O (l) $$

C) Reaction with Carbonates and Hydrogencarbonates

This is the classic test for an acid. They react to produce a salt, water, and carbon dioxide gas (\( CO_2 \)).

$$ \text{Acid} + \text{Carbonate} \longrightarrow \text{Salt} + \text{Water} + \text{Carbon Dioxide} $$

The carbon dioxide produced can be detected using limewater (it turns cloudy).

Example (Ethanoic Acid with Sodium Carbonate):

$$ 2CH_3COOH (aq) + Na_2CO_3 (s) \longrightarrow 2CH_3COONa (aq) + H_2O (l) + CO_2 (g) $$

6. Esterification: Making Esters

The final crucial reaction for carboxylic acids is Esterification, which is the reaction between a carboxylic acid and an alcohol.

6.1 The Esterification Reaction

Esterification is a condensation reaction where two molecules combine to form a larger molecule, eliminating a small molecule (in this case, water).

$$ \text{Carboxylic Acid} + \text{Alcohol} \rightleftharpoons \text{Ester} + \text{Water} $$

Conditions Required:

This reaction is reversible and requires specific conditions:

• Catalyst: Concentrated Sulfuric Acid (\( H_2SO_4 \)) - this is also a dehydrating agent, helping to remove the water formed.
• Heating (usually gentle heating).

6.2 The Products: Esters

Esters are important organic compounds known for their pleasant, fruity smells (like bananas, apples, or pears). They are used widely in flavourings and perfumes.

What happens chemically?

The carboxylic acid loses its \( -OH \) group, and the alcohol loses its \( -H \) from the \( -OH \) group. These combine to form water, and the remaining parts bond together to form the ester.

Example: Making Ethyl Ethanoate (a pleasant-smelling ester)

$$ \text{Ethanoic Acid} + \text{Ethanol} \xrightarrow{H_2SO_4, heat} \text{Ethyl Ethanoate} + \text{Water} $$

$$ CH_3COOH + CH_3CH_2OH \xrightarrow{H_2SO_4, heat} CH_3COOCH_2CH_3 + H_2O $$

Common Mistake to Avoid: Always remember the double arrow (\( \rightleftharpoons \)) in esterification. It signifies that the reaction is reversible. If you heat the ester with water and acid/base, you can split it back into the original acid and alcohol!