🧪 Chapter 3.3.11: Amines – The Ammonia Family
Hello future Chemists! Welcome to the fascinating world of Amines. These molecules are essentially organic versions of ammonia, where we swap out hydrogen atoms for hydrocarbon (alkyl or aryl) groups. They are absolutely essential in biochemistry (think DNA and proteins) and industry (dyes and pharmaceuticals).
This chapter focuses on two main roles of amines: first, their property as bases, and second, their power as nucleophiles (electron-donors) in organic synthesis. Don't worry if the mechanisms look complex—we'll break them down step-by-step!
1. Structure and Classification of Amines
What is an Amine?
An amine is a compound derived from ammonia (\(NH_3\)). Ammonia has one nitrogen atom bonded to three hydrogen atoms, and crucially, it possesses one lone pair of electrons. Amines retain this lone pair, which is responsible for almost all their characteristic chemistry.
Classification (Primary, Secondary, Tertiary)
Amines are classified based on how many of the original hydrogen atoms in ammonia have been replaced by R (alkyl) or Ar (aryl) groups:
- Primary Amine (\(RNH_2\)): One H atom replaced. (e.g., methylamine, \(CH_3NH_2\))
- Secondary Amine (\(R_2NH\)): Two H atoms replaced. (e.g., dimethylamine, \((CH_3)_2NH\))
- Tertiary Amine (\(R_3N\)): Three H atoms replaced. (e.g., trimethylamine, \((CH_3)_3N\))
If you add one more R group to a tertiary amine, you form a Quaternary Ammonium Salt (\(R_4N^+X^-\)). This ion is positively charged because the nitrogen atom now carries four covalent bonds.
Quick Review: Naming
Primary aliphatic amines are often named using the prefix "amino-" or by naming the alkyl group followed by "-amine" (e.g., Methylamine or Aminomethane). Aromatic amines derived from benzene are generally called phenylamine (or aniline, though IUPAC prefers phenylamine).
2. Preparation of Amines (Synthesis Routes)
3.3.11.1 Preparation of Primary Aliphatic Amines
There are two key methods for synthesizing primary aliphatic amines:
Method A: Reaction of Ammonia with Halogenoalkanes
This is a nucleophilic substitution reaction where the ammonia molecule attacks the delta-positive carbon atom in a halogenoalkane (RX).
$$RX + NH_3 \rightarrow RNH_3^+ X^-$$
The ammonium salt then reacts with excess ammonia to give the primary amine:
$$RNH_3^+ X^- + NH_3 \rightarrow RNH_2 + NH_4^+ X^-$$
⚠️ Crucial Problem to Note: Over-alkylation
The product, the primary amine (\(RNH_2\)), is itself a nucleophile. It can react with more halogenoalkane, leading to a mixture of secondary, tertiary, and quaternary ammonium salts.
- $$RNH_2 + RX \rightarrow R_2NH$$ (Secondary amine)
- $$R_2NH + RX \rightarrow R_3N$$ (Tertiary amine)
- $$R_3N + RX \rightarrow R_4N^+ X^-$$ (Quaternary salt)
To maximise the yield of the primary amine, you must use a large excess of concentrated ammonia.
Method B: Reduction of Nitriles
This method is often preferred because it produces a pure primary amine and increases the carbon chain length by one atom (C-C bond formed).
A nitrile (RCN) is reduced using hydrogen gas (\(H_2\)) over a nickel catalyst, or by using a strong reducing agent like lithium aluminium hydride (\(LiAlH_4\)) followed by hydrolysis.
$$\text{R-C}\equiv\text{N} + 4[\text{H}] \rightarrow \text{R-CH}_2\text{NH}_2$$ (The notation \([H]\) represents the reducing agent.)
Preparation of Aromatic Amines
Aromatic amines (like phenylamine) are prepared from nitro compounds (like nitrobenzene) via reduction.
1. Reduce the nitro compound using tin (Sn) and concentrated hydrochloric acid (HCl), forming the ammonium salt: $$\text{ArNO}_2 + 6[\text{H}] \rightarrow \text{ArNH}_3^+$$ 2. Neutralise the ammonium salt with sodium hydroxide (NaOH) to release the free amine: $$\text{ArNH}_3^+ + OH^- \rightarrow \text{ArNH}_2 + H_2O$$
3. Base Properties and Explaining Basicity (3.3.11.2)
Amines as Weak Bases
Amines are defined as weak bases because the lone pair of electrons on the nitrogen atom can accept a proton (\(H^+\)) from water or an acid.
$$\text{RNH}_2 (aq) + H_2O (l) \rightleftharpoons \text{RNH}_3^+ (aq) + OH^- (aq)$$
Explaining the Difference in Base Strength
The strength of an amine as a base depends on how readily the lone pair of electrons on the N atom can be donated to bond with a proton. This is called the availability of the lone pair.
Comparison: Aliphatic > Ammonia > Aromatic
- Primary Aliphatic Amines (Strongest Bases):
Alkyl groups (like \(CH_3\)) are electron-donating groups. They push electron density towards the nitrogen atom. This makes the lone pair on the N atom
more concentrated and thus
more available to accept a proton, making the aliphatic amines stronger bases than ammonia. - Ammonia (\(NH_3\)):
This acts as a neutral benchmark, with no donating or withdrawing groups affecting the lone pair. - Primary Aromatic Amines (Weakest Bases):
In phenylamine, the nitrogen atom is directly attached to the benzene ring. The lone pair of electrons on the N atom becomes delocalised into the ring's \(\pi\)-system. This process spreads out the electron density, making the lone pair
less available to accept a proton. This greatly reduces its basic strength.
🧠 Memory Aid: Electron Flow
Imagine the lone pair is a gold coin.
- Aliphatic: The alkyl group acts like an assistant, giving the N atom extra coins, making it rich and ready to donate. Strong Base.
- Aromatic: The benzene ring acts like a giant magnet, pulling the coin away (delocalization). The N atom is poor and reluctant to donate. Weak Base.
4. Nucleophilic Properties and Reactions (3.3.11.3)
Because the nitrogen atom possesses a lone pair, amines act as excellent nucleophiles—molecules that seek positive centres and donate an electron pair. We study two main types of nucleophilic reaction involving amines.
4.1 Nucleophilic Substitution with Halogenoalkanes
As discussed in preparation (Section 2), this is a multi-step substitution where the amine attacks the halogenoalkane.
Mechanism Outline: Primary Amine Formation
This mechanism is required by the syllabus.
- Attack: The lone pair on the nitrogen atom of ammonia attacks the \(\delta+\) carbon atom of the halogenoalkane. The C-X bond breaks, and the halide ion (\(X^-\)) leaves (forming an intermediate alkylammonium ion).
- Deprotonation: A second ammonia molecule acts as a base and removes a proton (\(H^+\)) from the alkylammonium ion to generate the neutral primary amine and an ammonium ion.
Application: Quaternary Ammonium Salts
When tertiary amines react with halogenoalkanes, they only form Quaternary Ammonium Salts (\(R_4N^+X^-\)). These salts have important real-world uses.
Did you know? Quaternary ammonium salts with long alkyl chains act as cationic surfactants. They are widely used in fabric conditioners because the positive charge on the ion sticks to the slightly negative surface of wet fabric, making the fabric feel soft and smooth. They also function as disinfectants (e.g., in mouthwashes).
4.2 Nucleophilic Addition-Elimination Reactions
Amines react vigorously with very reactive carboxylic acid derivatives, specifically acyl chlorides and acid anhydrides, to produce amides.
These reactions are fast and do not require a catalyst. When ammonia or primary amines react, they produce substituted amides.
Example 1: Primary Amine with Acyl Chloride
$$\text{RNH}_2 + \text{R'COCl} \rightarrow \text{R'CONHR} + \text{HCl}$$ (Amine + Acyl Chloride \(\rightarrow\) Substituted Amide + Hydrogen Chloride)
Note: Since HCl gas is produced, which is acidic, a base like NaOH is often added to "mop up" the acid byproduct.
Mechanism Outline: Addition-Elimination
The syllabus requires you to outline this mechanism for primary amines reacting with acyl chlorides (or acid anhydrides).
- Addition/Attack: The lone pair on the nitrogen atom (the nucleophile) attacks the highly \(\delta+\) carbon atom of the carbonyl group (\(C=O\)). The \(\pi\) bond in the \(C=O\) breaks, and the electrons move onto the oxygen atom, forming a tetrahedral intermediate.
- Elimination: The lone pair electrons on the oxygen atom move back down to reform the \(C=O\) bond. This causes the weakest bond to break, eliminating the halide ion (\(Cl^-\))—the leaving group.
- Deprotonation: A molecule of the amine (or ammonia) acts as a base to remove a proton from the nitrogen atom, neutralizing the positive charge and forming the final, stable amide product.
❌ Common Mechanism Mistake
When drawing the addition-elimination mechanism, remember:
- The first curly arrow must always start from the lone pair on the nucleophile (N atom).
- The intermediate structure is tetrahedral around the carbon atom before the elimination step occurs.
- The \(\text{Cl}^-\) (or the corresponding acid anhydride leaving group) is eliminated, not \(H^+\), in the main elimination step.
Key Takeaway Summary
Amines are characterized by the lone pair of electrons on the nitrogen atom. This lone pair allows them to act as weak bases (accepting \(H^+\)) and powerful nucleophiles (donating the electron pair). Their basic strength is determined by electron-donating alkyl groups (making them stronger bases) or electron-withdrawing aryl groups (making them weaker bases). Crucial reactions include substitution with halogenoalkanes (producing a mixture of products) and addition-elimination with acyl compounds (producing amides).