Amines (exemplified by ethylamine and phenylamine)

Welcome to the World of Amines!

In this chapter, we are diving into a fascinating group of nitrogen-containing molecules called Amines. Think of amines as the chemical cousins of ammonia \( (NH_3) \). They are everywhere—from the smell of fish to the building blocks of life (DNA and proteins). We will focus on two main stars: ethylamine (an aliphatic amine) and phenylamine (an aromatic amine). By the end of these notes, you’ll understand how to make them, why they act as bases, and how they react with other chemicals.

1. What are Amines?

Amines are organic compounds where one or more hydrogen atoms in ammonia are replaced by carbon-based groups (alkyl or aryl groups).
- Ethylamine \( (CH_3CH_2NH_2) \): Nitrogen is attached to an ethyl group.
- Phenylamine \( (C_6H_5NH_2) \): Nitrogen is attached directly to a benzene ring.

How to Make Amines (Formation)

Making amines is all about "reducing" other nitrogen compounds. In chemistry, reduction often means adding hydrogen or removing oxygen.

A. Making Ethylamine

You can make ethylamine in two main ways:
1. From Nitriles \( (CH_3CN) \): Use Lithium Aluminum Hydride \( (LiAlH_4) \) in dry ether, OR Hydrogen gas \( (H_2) \) with a Nickel catalyst.
Analogy: Imagine a nitrile is a closed fist; adding hydrogen "opens" it into an amine.
2. From Amides \( (CH_3CONH_2) \): Use Lithium Aluminum Hydride \( (LiAlH_4) \) in dry ether. This "strips away" the oxygen atom and replaces it with hydrogens.

B. Making Phenylamine

This is a two-step process starting from nitrobenzene \( (C_6H_5NO_2) \):
- Step 1: Heat nitrobenzene with Tin (Sn) and concentrated Hydrochloric acid (HCl). This reduces the \( -NO_2 \) group to an \( -NH_3^+ \) group.
- Step 2: Add aqueous Sodium Hydroxide \( (NaOH) \). This "frees" the amine from its salt form to give you phenylamine \( (C_6H_5NH_2) \).

Quick Review Box:
- Nitrile \( \rightarrow \) Amine (use \( LiAlH_4 \) or \( H_2/Ni \))
- Amide \( \rightarrow \) Amine (use \( LiAlH_4 \))
- Nitrobenzene \( \rightarrow \) Phenylamine (use \( Sn/conc. HCl \), then \( NaOH \))

Key Takeaway: Amines are formed by reducing nitriles, amides, or nitro compounds. Always remember the specific reagents for each!

2. Basicity: The "Lone Pair" Power

Why are amines basic? It’s all about the lone pair of electrons on the Nitrogen atom. According to the Lewis Theory, a base is an electron pair donor. Because Nitrogen has a lone pair, it can "grab" a proton \( (H^+) \) from an acid.

Basicity in the Gaseous Phase

In the gas phase, the more "electron-pushing" groups (alkyl groups) you attach to the Nitrogen, the more basic the amine becomes.
- Trend: Tertiary amine > Secondary amine > Primary amine > Ammonia.
- Why? Alkyl groups (like methyl or ethyl) are electron-donating. They push electron density toward the Nitrogen, making that lone pair even more attractive to protons.

Basicity in Aqueous Medium (The Big Comparison)

This is a very common exam topic! We compare Ammonia, Ethylamine, and Phenylamine in water.

The Strength Order: Ethylamine > Ammonia > Phenylamine

1. Why is Ethylamine stronger than Ammonia?

Ethylamine has an ethyl group \( (-CH_2CH_3) \). This group is electron-donating (the positive inductive effect). It pushes electron density toward the Nitrogen atom, making the lone pair more available to accept a proton \( (H^+) \).

2. Why is Phenylamine the weakest?

In phenylamine, the Nitrogen is attached to a benzene ring. The lone pair of electrons on the Nitrogen delocalises (spreads out) into the \( \pi \)-electron system of the benzene ring.
Analogy: If the lone pair is a "snack," the benzene ring "steals" it and shares it with the whole ring. Because the lone pair is busy inside the ring, it is less available to be donated to a proton outside.

Don't worry if this seems tricky! Just remember: Alkyl groups are "givers" (make it more basic), and Benzene rings are "takers" (make it less basic).

Key Takeaway: Basicity depends on how "available" the Nitrogen lone pair is. Donating groups increase basicity; delocalisation into a ring decreases it.

3. Reactions of Amines

Amines are reactive because of that same lone pair! Here are the key reactions you need to know.

A. Salt Formation

Since amines are bases, they react with acids to form salts.
Example: \( CH_3CH_2NH_2 + HCl \rightarrow CH_3CH_2NH_3^+Cl^- \) (Ethylammonium chloride)
These salts are usually soluble in water because they are ionic.

B. Reaction of Phenylamine with Aqueous Bromine

This is a spectacular reaction! When you add aqueous bromine \( (Br_2(aq)) \) to phenylamine:
- The bromine is decolourised (orange to colourless).
- A white precipitate of 2,4,6-tribromophenylamine is formed.
Did you know? The \( -NH_2 \) group is so "activating" that the reaction happens at room temperature without any catalyst!

C. Formation of Amides (Condensation)

Amines react with acyl chlorides \( (R'COCl) \) to form amides.
Equation: \( RNH_2 + R'COCl \rightarrow R'CONHR + HCl \)
This is a condensation reaction because a small molecule (\( HCl \)) is eliminated. This is how proteins are essentially built!

Common Mistake to Avoid: Don't confuse amines with amides. Amines are basic \( (R-NH_2) \); amides are neutral \( (R-CONH_2) \) because the lone pair is pulled away by the oxygen atom!

Key Takeaway: Amines form salts with acids, phenylamine reacts easily with bromine, and amines react with acyl chlorides to make amides.

Summary Checklist

Can you:
- State the reagents to turn a nitrile or amide into an amine? (Hint: \( LiAlH_4 \))
- Explain why phenylamine is a weaker base than ammonia? (Hint: Delocalisation)
- Describe the visual observation when phenylamine reacts with \( Br_2(aq) \)? (Hint: White ppt)
- Identify the product of an amine reacting with an acyl chloride? (Hint: Amide)

You've got this! Keep practicing the structures, and the logic will follow.