Primary and secondary amines

Welcome to the World of Amines!

Hello there! Today, we are diving into a fascinating group of organic compounds called amines. Think of amines as the "chemical cousins" of ammonia (\(NH_3\)). They are incredibly important in the real world—they are the building blocks of proteins in your body, the reason some fish have a distinct smell, and the basis for many life-saving medicines.

Don’t worry if organic chemistry feels like a puzzle sometimes. We are going to break this down piece by piece so you can master the 9701 syllabus with confidence!

1. What exactly is an Amine?

Before we look at the reactions, let's understand what we are looking at.

Prerequisite check: Remember ammonia (\(NH_3\))? It has a nitrogen atom bonded to three hydrogen atoms, with a lone pair of electrons sitting on top of the nitrogen.

An amine is what happens when you take an ammonia molecule and swap one (or more) of those hydrogen atoms for a carbon group (an alkyl group, like \(CH_3\) or \(C_2H_5\)).

Primary Amines

In a primary amine, only one of the hydrogen atoms in ammonia has been replaced by a carbon chain.

General Formula: \(R-NH_2\) (where \(R\) is your carbon chain).

Example: Propylamine (\(CH_3CH_2CH_2NH_2\)).
Analogy: Imagine ammonia is a stool with three legs (hydrogens). To make a primary amine, you saw off one wooden leg and replace it with a shiny metal leg (the carbon group).

Quick Review: Identifying a Primary Amine

Look for the \(-NH_2\) group attached to a carbon atom. If you see that "2" next to the hydrogen, and only one bond going to a carbon, you've found a primary amine!

Key Takeaway: Amines are derivatives of ammonia where carbon chains replace hydrogen atoms. For your AS Level, we focus specifically on primary amines.

2. How do we make Amines? (Preparation)

According to your syllabus, there is one main way you need to know to create a primary amine: reacting a halogenoalkane with ammonia.

The Reaction: Nucleophilic Substitution

When you mix a halogenoalkane (like bromoethane) with ammonia, the nitrogen's lone pair "attacks" the carbon attached to the halogen. The halogen is kicked out, and the nitrogen takes its place.

The Equation:
\(CH_3CH_2Br + 2NH_3 \rightarrow CH_3CH_2NH_2 + NH_4Br\)

Step-by-Step Process:
1. The lone pair on the \(NH_3\) nitrogen is attracted to the \(\delta+\) carbon in the \(C-Br\) bond.
2. The \(C-Br\) bond breaks (Heterolytic fission).
3. An intermediate salt is formed.
4. A second ammonia molecule removes a hydrogen to leave you with the final primary amine.

Important Conditions (Don't miss these!)

To get this reaction to work correctly for your exam, you must remember these three conditions:
1. Ethanolic Ammonia: The ammonia must be dissolved in ethanol, not water. (Using water might give you an alcohol instead!)
2. Heat: The mixture needs to be warmed up.
3. Under Pressure: Because ammonia is a gas, we perform this in a sealed tube to keep the ammonia from flying away.

Memory Aid: The "E.H.P." Rule

To make an amine, you need:
E - Ethanol solvent
H - Heat
P - Pressure

Key Takeaway: Primary amines are made by reacting halogenoalkanes with excess ethanolic ammonia under pressure and heat.

3. Why do Amines act as Bases?

Just like their "cousin" ammonia, amines are bases. Specifically, they are Brønsted–Lowry bases.

What does that mean? A Brønsted–Lowry base is a proton (\(H^+\)) acceptor.

The nitrogen atom in the amine has a lone pair of electrons. This lone pair is like a powerful magnet for a positive \(H^+\) ion. When an amine meets an acid, it uses that lone pair to form a dative covalent bond with the proton.

Reaction with Water:
\(R-NH_2 + H_2O \rightleftharpoons R-NH_3^+ + OH^-\)
Because they produce \(OH^-\) ions in water, amine solutions are alkaline (usually having a pH around 10-12).

Did you know?

The "fishy" smell of decaying fish is actually caused by methylamine and trimethylamine. When you put lemon juice (an acid) on fish, the acid reacts with the amine to form a salt that doesn't smell. Chemistry makes your dinner better!

Key Takeaway: Amines are basic because the lone pair on the nitrogen atom can accept a proton (\(H^+\)).

4. Common Mistakes to Avoid

Even top students can trip up on these details. Keep an eye out for:

• Wrong Solvent: Forgetting to specify ethanol as the solvent for the preparation of amines. If you use aqueous \(NaOH\), you'll make an alcohol instead!
• Formula Confusion: Don't confuse Amine (\(-NH_2\)) with Amide (\(-CONH_2\)). Amides have an extra oxygen atom!
• The "2 NH3" Rule: In the balanced equation for making an amine, you usually need two molecules of ammonia—one to form the amine and one to mop up the \(HBr\) or \(HCl\) produced.

5. Quick Summary Table

Functional Group: Amine (\(-NH_2\))
General Formula: \(C_nH_{2n+1}NH_2\)
Classification: Primary (Nitrogen attached to 1 carbon)
Preparation: Halogenoalkane + Ammonia (Ethanol, Heat, Pressure)
Chemical Nature: Weak Base (Proton acceptor)
Naming Example: \(CH_3NH_2\) is Methylamine; \(CH_3CH_2NH_2\) is Ethylamine.

Don't worry if this seems like a lot of names and formulas. The more you draw the structures, the more natural it will feel. You've got this!