Chemistry (9701) | Primary amines

Primary Amines: Your Essential Study Guide (9701)

Welcome to the fascinating world of Nitrogen Chemistry! This chapter focuses on Amines, which are essentially organic derivatives of ammonia (\(\text{NH}_3\)). They are incredibly important molecules, acting as building blocks for proteins (amino acids), DNA, and many pharmaceuticals and synthetic materials.

Don't worry if organic chemistry sometimes feels like a maze of reactions. We'll break down the structure, synthesis, and key chemical property (basicity) of primary amines into clear, manageable steps!

1. Structure, Classification, and Nomenclature

1.1 What is an Amine?

An amine is a compound where one or more hydrogen atoms in the ammonia molecule (\(\text{NH}_3\)) have been replaced by an alkyl group (R).

The nitrogen atom in all amines possesses a crucial feature: a lone pair of electrons. This lone pair makes amines:

• Strong Nucleophiles (electron-pair donors, seeking positive centres).
• Weak Bases (proton acceptors, key to their chemical behaviour).

1.2 Classification of Amines (Primary, Secondary, Tertiary)

Amines are classified based on how many alkyl groups (R) are attached directly to the nitrogen atom:

Primary Amines (1°):
The nitrogen atom is attached to one alkyl group and two hydrogen atoms.
Formula: \(\text{R}-\text{NH}_2\)
Example: Ethylamine (\(\text{CH}_3\text{CH}_2\text{NH}_2\))

Secondary Amines (2°):
The nitrogen atom is attached to two alkyl groups and one hydrogen atom.
Formula: \(\text{R}_2\text{NH}\) or \(\text{R}-\text{NH}-\text{R}'\)
Example: Dimethylamine (\(\text{CH}_3\text{NHCH}_3\))

Tertiary Amines (3°):
The nitrogen atom is attached to three alkyl groups and no hydrogen atoms.
Formula: \(\text{R}_3\text{N}\)
Example: Trimethylamine (\((\text{CH}_3)_3\text{N}\))

Memory Aid for Classification:

Think about how many C-N bonds there are, not how many C atoms are in total!

• 1° Amine = 1 C-N bond
• 2° Amine = 2 C-N bonds
• 3° Amine = 3 C-N bonds

1.3 Nomenclature (Naming Primary Amines)

Primary aliphatic amines are usually named by taking the name of the alkyl group (R) and adding the suffix -amine.

• \(\text{CH}_3\text{CH}_2\text{NH}_2\) is Ethylamine.
• \(\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2\) is Propylamine or Propan-1-amine.

2. Synthesis of Primary Amines

The syllabus requires you to know three main methods for producing primary amines, typically starting from halogenoalkanes, nitriles, or amides.

2.1 Method 1: Reaction of Halogenoalkanes with Ammonia

This is a standard nucleophilic substitution reaction where the nucleophile, ammonia (\(\text{NH}_3\)), attacks the partially positive carbon atom (\(\delta+\text{C}\)) bonded to the halogen.

Reagents: Halogenoalkane (\(\text{R-X}\)) and Ammonia (\(\text{NH}_3\)).
Conditions: In ethanol (solvent), heated under pressure (in a sealed tube).

Step-by-Step Reaction:

1. The ammonia nucleophile attacks the carbon:
\(\text{R}-\text{X} + \text{NH}_3 \rightarrow \text{R}-\text{NH}_3^+ \text{X}^-\) (an alkylammonium salt)

2. Excess ammonia then removes a proton from the salt to form the amine:
\(\text{R}-\text{NH}_3^+ \text{X}^- + \text{NH}_3 \rightarrow \text{R}-\text{NH}_2 + \text{NH}_4^+ \text{X}^-\)

⚠️ Common Mistake to Avoid: Polysubstitution

The product, the primary amine (\(\text{RNH}_2\)), is also a nucleophile and often reacts faster than the original ammonia! If you don't use a massive excess of ammonia, the primary amine attacks more halogenoalkane molecules, forming secondary, tertiary, and eventually quaternary ammonium salts.

\(\text{RNH}_2 + \text{R-X} \rightarrow \text{Secondary Amine} + \dots\)

2.2 Method 2: Reduction of Nitriles

This method is excellent because it reliably produces only the primary amine and also increases the carbon chain length by one carbon atom (the carbon from the nitrile group, \(\text{-CN}\)).

Starting Material: Nitrile (\(\text{R}-\text{C}\equiv\text{N}\)).
Reagents/Conditions (Two options):

• Option A (Strong Reducing Agent): Lithium aluminium hydride (\(\text{LiAlH}_4\)) in dry ether, followed by aqueous acid.
• Option B (Catalytic Hydrogenation): Hydrogen gas (\(\text{H}_2\)) over a nickel (\(\text{Ni}\)) catalyst (or Platinum/Palladium) and heat.

General Equation (using \(\text{H}\) for reduction):
\(\text{R}-\text{C}\equiv\text{N} + 4[\text{H}] \rightarrow \text{R}-\text{CH}_2\text{NH}_2\)

Example: The reduction of propanenitrile yields propylamine (or propan-1-amine). Notice the new \(\text{CH}_2\) group is formed where the triple bond was.

2.3 Method 3: Reduction of Amides

Amides contain the \(\text{RCONH}_2\) functional group. They can be reduced to primary amines using a very powerful reducing agent.

Starting Material: Primary Amide (\(\text{RCONH}_2\)).
Reagent: Lithium aluminium hydride (\(\text{LiAlH}_4\)).

Analogy: Imagine the oxygen atom (\(\text{C=O}\)) is simply swapped for two hydrogen atoms (\(\text{CH}_2\)).

General Equation:
\(\text{RCONH}_2 + 4[\text{H}] \rightarrow \text{RCH}_2\text{NH}_2 + \text{H}_2\text{O}\)

Key Takeaway from Synthesis: The method starting from nitriles and amides gives pure primary amines, avoiding the mixture of products seen when using halogenoalkanes and ammonia.

3. Chemical Properties of Primary Amines: Basicity

Primary amines are known for their basic nature. This is a crucial concept, and you must be able to describe and explain the basicity of aqueous amine solutions.

3.1 Amines as Brønsted-Lowry Bases

A Brønsted-Lowry Base is a proton (\(\text{H}^+\)) acceptor. The nitrogen lone pair enables the amine to accept a proton, forming an alkylammonium ion (\(\text{RNH}_3^+\)).

When dissolved in water, an amine establishes an equilibrium, reacting with water to produce hydroxide ions (\(\text{OH}^-\)). This presence of hydroxide ions makes the solution alkaline.

Equilibrium Reaction:
\[\text{RNH}_2 (aq) + \text{H}_2\text{O} (l) \rightleftharpoons \text{RNH}_3^+ (aq) + \text{OH}^- (aq)\]

The position of this equilibrium determines the strength of the base. The further the equilibrium shifts to the right (producing more \(\text{OH}^-\)), the stronger the base.

3.2 Explaining the Relative Basicities (Alkyl Amines vs. Ammonia)

In general, Primary Alkyl Amines are stronger bases than ammonia.

Explanation: The Inductive Effect

The difference in basicity is explained by the inductive effect of the alkyl group (R).

1. Alkyl groups (like methyl, ethyl) are electron-releasing groups. We say they have a positive inductive effect (+I).
2. When the alkyl group is attached to the nitrogen, it pushes electron density towards the nitrogen atom.
3. This increase in electron density makes the nitrogen's lone pair more available to accept a proton (\(\text{H}^+\)) from water.
4. Therefore, the equilibrium shifts further to the right, producing a higher concentration of \(\text{OH}^-\), resulting in a stronger base (higher pH).

Did You Know?

Secondary amines (with two alkyl groups) are generally stronger bases than primary amines, and both are stronger than ammonia, due to the cumulative positive inductive effect of multiple alkyl groups pushing electron density onto the nitrogen. (Tertiary amines can be slightly weaker than secondary amines due to solubility/steric hindrance effects, but for A-Level, the trend 1° > \(\text{NH}_3\) based on the inductive effect is key.)

4. Reaction with Acyl Chlorides: Formation of Amides

Primary amines are highly reactive nucleophiles and react readily with acyl chlorides (like ethanoyl chloride) in an example of a condensation reaction (or nucleophilic addition-elimination).

4.1 Formation of N-Substituted Amides

When an amine reacts with an acyl chloride, an amide is formed, specifically an N-substituted amide, along with hydrogen chloride gas (\(\text{HCl}\)).

Reagents: Primary amine (\(\text{RNH}_2\)) and Acyl chloride (\(\text{R}'\text{COCl}\)).
Conditions: Room temperature.

General Equation:
\[\text{R}'\text{COCl} + \text{RNH}_2 \rightarrow \text{R}'\text{CONHR} + \text{HCl}\]

Example: Reaction of ethylamine with ethanoyl chloride.
\(\text{CH}_3\text{COCl} + \text{CH}_3\text{CH}_2\text{NH}_2 \rightarrow \text{CH}_3\text{CONHCH}_2\text{CH}_3 + \text{HCl}\)

The product, \(\text{CH}_3\text{CONHCH}_2\text{CH}_3\), is named N-ethylethanamide. (The N- prefix indicates the ethyl group is attached to the nitrogen atom).

Step-by-Step Explanation (Mechanism Context):

1. The primary amine (\(\text{RNH}_2\)), acting as a nucleophile via its lone pair, attacks the electron-deficient carbonyl carbon (\(\text{C=O}\)) of the acyl chloride.
2. A short-lived intermediate forms.
3. The chloride ion (\(\text{Cl}^-\)) is eliminated, followed by the loss of a proton (\(\text{H}^+\)) from the nitrogen, resulting in the formation of the N-substituted amide and \(\text{HCl}\).

This reaction is quick and vigorous at room temperature because acyl chlorides are highly reactive compounds.