Carboxylic acids and derivatives

Welcome to the World of Carboxylic Acids and Esters!

In this chapter, we are going to explore some of the most "famous" smells and tastes in chemistry. From the sharp sting of vinegar to the sweet fragrance of a ripe pineapple, carboxylic acids and esters are everywhere. By the end of these notes, you’ll understand how to make them, how they react, and how to tell them apart. Don't worry if organic chemistry feels like a puzzle sometimes—we’ll piece it together one step at a time!

1. What are Carboxylic Acids?

Carboxylic acids are organic compounds that contain the carboxyl functional group: \( -COOH \). This group is actually a "double team" of two other groups you’ve met before: a carbonyl group (\( C=O \)) and a hydroxyl group (\( -OH \)) both attached to the same carbon atom.

Naming Them (Nomenclature)

To name a carboxylic acid, we look for the longest carbon chain containing the \( -COOH \) group and add the suffix -oic acid. The carbon in the \( -COOH \) group is always counted as carbon number 1.

Examples:
1 Carbon: Methanoic acid (\( HCOOH \))
2 Carbons: Ethanoic acid (\( CH_3COOH \)) - This is the main ingredient in vinegar!
3 Carbons: Propanoic acid (\( CH_3CH_2COOH \))

Quick Review Box:
The functional group is \( -COOH \).
The suffix is -oic acid.
The \( C \) in \( -COOH \) is always Carbon #1.

2. How Do We Make Carboxylic Acids?

There are three main ways you need to know for your AS Level exam:

A. Oxidation of Primary Alcohols or Aldehydes

Think of this as "maxing out" the oxygen. If you take a primary alcohol and heat it under reflux with an oxidising agent, it turns into an aldehyde and then into a carboxylic acid.

Reagents: Acidified potassium dichromate(VI) (\( K_2Cr_2O_7 / H^+ \)) or acidified potassium manganate(VII) (\( KMnO_4 / H^+ \)).
Condition: Heat under reflux (this ensures the reaction goes all the way to the acid and doesn't stop at the aldehyde).

B. Hydrolysis of Nitriles

A nitrile is a compound with a \( -C \equiv N \) group. "Hydrolysis" means splitting a molecule using water. You can do this in two ways:

1. Acid Hydrolysis: Heat with a dilute acid (like \( HCl \)).
Equation: \( RCN + 2H_2O + HCl \rightarrow RCOOH + NH_4Cl \)

2. Alkali Hydrolysis: Heat with a dilute alkali (like \( NaOH \)), then add a strong acid to release the carboxylic acid.
Equation: \( RCN + NaOH + H_2O \rightarrow RCOONa + NH_3 \)
(Then add \( H^+ \) to get \( RCOOH \))

C. Hydrolysis of Esters

If you break an ester apart using dilute acid and heat, you get the carboxylic acid back (along with an alcohol).

Key Takeaway: Carboxylic acids can be made by "adding oxygen" to alcohols/aldehydes or by "adding water" to nitriles and esters.

3. Chemical Reactions of Carboxylic Acids

Carboxylic acids are weak acids. This means they only partially dissociate in water, but they still do all the "acid-y" things you learned about in IGCSE/O-Level!

A. Reaction with Reactive Metals (Redox)

Just like \( HCl \) reacts with magnesium, carboxylic acids react with metals to form a salt and hydrogen gas.

Example: \( 2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2 \)
Observation: You will see effervescence (fizzing) as hydrogen gas is released.

B. Reaction with Alkalis (Neutralisation)

Acid + Alkali \(\rightarrow\) Salt + Water.

Example: \( CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O \)

C. Reaction with Carbonates (The Acid Test)

This is a great way to test for a carboxylic acid in the lab. When you add a carbonate, it fizzes!

Example: \( 2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + H_2O + CO_2 \)
Observation: Effervescence. The gas (\( CO_2 \)) will turn limewater cloudy.

D. Reduction (Turning back to Alcohols)

If oxidation turns an alcohol into an acid, reduction does the opposite. However, carboxylic acids are quite stubborn! You need a very strong reducing agent.

Reagent: \( LiAlH_4 \) (Lithium tetrahydridoaluminate) in dry ether.
Result: The acid is reduced straight back to a primary alcohol.
Note: \( NaBH_4 \) is not strong enough to reduce carboxylic acids!

Common Mistake Alert: Students often try to use \( NaBH_4 \) to reduce carboxylic acids. Remember: "L" for LiAlH4 is Large/stronger, so it works on acids. \( NaBH_4 \) only works on aldehydes and ketones!

4. Esters: The Sweet Side of Chemistry

Esters have the functional group \( -COOR \). They are famous for their fruity smells and are used in perfumes and food flavourings.

Making Esters (Esterification)

This is a condensation reaction where a carboxylic acid and an alcohol join together and spit out a small molecule of water.

Equation: Carboxylic Acid + Alcohol \(\rightleftharpoons\) Ester + Water
Reagents: The acid and alcohol.
Catalyst: A few drops of concentrated sulfuric acid (\( H_2SO_4 \)).
Conditions: Heat gently.

How to Name Esters (The Secret Trick)

Naming esters can be confusing, but try this simple two-part rule:
1. The first part comes from the Alcohol. Change the name to end in -yl.
2. The second part comes from the Carboxylic Acid. Change the name to end in -oate.

Example: If you react methanol with ethanoic acid, you get methyl ethanoate.

Memory Aid: "The Alcohol is the YL (ill) child, the Acid is the OATE (old) parent." The alcohol-part always comes first in the name!

5. Breaking Down Esters (Hydrolysis)

Hydrolysis is the opposite of esterification. We use water to split the ester back into its components. There are two types:

A. Acid Hydrolysis

Reagent: Dilute acid (e.g., \( HCl \)).
Condition: Heat under reflux.
Result: This is a reversible reaction. You get the Carboxylic Acid + Alcohol.

B. Alkaline Hydrolysis (Saponification)

Reagent: Dilute alkali (e.g., \( NaOH \)).
Condition: Heat under reflux.
Result: This is non-reversible (goes to completion). You get the Sodium Salt of the acid + Alcohol.
Example: Methyl ethanoate + \( NaOH \rightarrow \) Sodium ethanoate + Methanol.

Did you know? Alkaline hydrolysis is how soap is made! If you use large fat molecules (which are esters), the salt produced is what we use as soap.

Summary Checklist

Before your exam, make sure you can:
- Draw and name carboxylic acids and esters up to 6 carbons.
- Describe how to make acids from alcohols, nitriles, or esters.
- Recall that \( LiAlH_4 \) is needed to reduce an acid to an alcohol.
- Explain the difference between acid and alkaline hydrolysis of esters.
- Identify the fruity smell and the "fizzing" with carbonates as key properties.

Don't worry if this seems tricky at first! Organic chemistry is all about patterns. Once you recognize that the \( -COOH \) group always behaves like an acid and the ester reaction is just "swapping parts," it becomes much easier!