Due to the lone pair of electrons on the nitrogen atom, amines behave as effective nucleophiles. They can attack electron deficient carbon atoms, leading to substitution or addition-elimination reactions that allow chemists to build complex nitrogen-containing molecules.
🔑 Key Principle
When an amine acts as a nucleophile, it donates its nitrogen lone pair to form a new covalent bond. Because the nitrogen atom initially becomes positively charged upon bonding, the mechanism typically involves the loss of a proton (\( \text{H}^+ \)) in a subsequent step to restore the nitrogen neutral charge state.
Consecutive Substitution with Halogenoalkanes
Reacting a primary amine with a halogenoalkane results in nucleophilic substitution. Because the resulting secondary amine product still has a lone pair of electrons on the nitrogen atom, it can react again with another halogenoalkane molecule. This leads to a series of consecutive reactions:
- Formation of a Secondary Amine:
\[ \text{RNH}_2 + \text{R'X} \rightarrow \text{RNHR'} + \text{HX} \] (In solution, excess amine acts as a base to mop up the HX acid product, yielding a secondary amine and an ammonium salt). - Formation of a Tertiary Amine:
The secondary amine reacts with more halogenoalkane: \[ \text{RNHR'} + \text{R'X} \rightarrow \text{RNR'}_2 + \text{HX} \] - Formation of a Quaternary Ammonium Salt:
The tertiary amine reacts with a final halogenoalkane molecule: \[ \text{RNR'}_2 + \text{R'X} \rightarrow \text{RNR'}_3^+ \text{X}^- \]
Quaternary ammonium salts represent the final product of this consecutive reaction sequence. Because the nitrogen atom is now bonded to four carbon atoms, it has used its lone pair of electrons to form the fourth dative covalent bond. The nitrogen atom carries a permanent positive charge. Because it has no lone pair left, a quaternary ammonium ion cannot act as a nucleophile and cannot undergo any further substitution reactions.
- Dimethylamine
- Trimethylamine
- Tetramethylammonium bromide
1. Dimethylamine (Secondary Amine):
\[ \text{CH}_3\text{NH}_2 + \text{CH}_3\text{Br} \rightarrow (\text{CH}_3)_2\text{NH} + \text{HBr} \]
2. Trimethylamine (Tertiary Amine):
\[ \text{CH}_3\text{NH}_2 + 2\text{CH}_3\text{Br} \rightarrow (\text{CH}_3)_3\text{N} + 2\text{HBr} \]
3. Tetramethylammonium bromide (Quaternary Salt):
\[ \text{CH}_3\text{NH}_2 + 3\text{CH}_3\text{Br} \rightarrow (\text{CH}_3)_4\text{N}^+ \text{Br}^- + 2\text{HBr} \]
Quaternary Ammonium Salts as Cationic Surfactants
If a quaternary ammonium salt contains at least one long chain alkyl group (often 12 to 18 carbons long), it can function as a cationic surfactant.
A surface active agent in which the active molecule carries a positive charge. It consists of a hydrophilic (water loving) positively charged head group and a hydrophobic (water hating) non-polar hydrocarbon tail.
How Cationic Surfactants Work
The hydrophobic tails dissolve in non-polar droplets of grease or oil. The hydrophilic, positively charged head groups face outwards into the water. This allows the grease to be suspended in water as an emulsion and washed away.
Furthermore, because wet fabrics and hair carry negative charges on their surfaces, the positively charged ammonium head groups bind electrostatically to them. This leaves the non-polar tails pointing outwards, creating a smooth, lubricating coating that reduces static electricity and friction.
Common applications include:
- Fabric Softeners: Coat fibres to prevent static cling and make clothes feel soft.
- Hair Conditioners: Bind to hair shafts to flatten hair cuticles, reduce tangling, and improve shine.
Preparation of Amides
Amides contain the functional group \\( \text{-CONH-} \\). They can be prepared by reacting ammonia or amines with either acyl chlorides or acid anhydrides. These reactions are classified as nucleophilic addition-elimination reactions.
A carbonyl group directly bonded to a nitrogen atom (represented as \\( \text{-CO-N-} \\)).
A reaction mechanism in which a nucleophile attacks a carbonyl group (addition), followed by the loss of a leaving group (elimination), restoring the carbonyl double bond.
Reaction 1: Amines with Acyl Chlorides
Acyl chlorides are highly reactive. Reacting an acyl chloride with an amine at room temperature yields an N-substituted amide and an acid product. A second equivalent of the amine reacts with the acid byproduct to form a salt:
\( \text{R-COCl} + 2\text{R'-NH}_2 \rightarrow \text{R-CONHR'} + \text{R'-NH}_3^+ \text{Cl}^- \)
- Reagents: Acyl chloride and primary/secondary amine (or ammonia).
- Conditions: Room temperature, anhydrous (acyl chlorides react violently with water).
- Observation: Steamy white fumes of HCl are evolved (which react with excess amine to form a white solid ammonium salt precipitate).
Reaction 2: Amines with Acid Anhydrides
Acid anhydrides react similarly but are less reactive and safer to handle than acyl chlorides. The reaction is less vigorous and produces a carboxylic acid byproduct instead of toxic hydrogen chloride fumes:
\( (\text{R-CO})_2\text{O} + \text{R'-NH}_2 \rightarrow \text{R-CONHR'} + \text{R-COOH} \)
- Reagents: Acid anhydride and primary/secondary amine.
- Conditions: Warm, anhydrous.
- Product: N-substituted amide and a carboxylic acid.
When naming amides made from amines, the prefix "N-" is used to indicate that an alkyl group is attached directly to the nitrogen atom rather than the main carbon chain. For example, the reaction of ethanoyl chloride with methylamine produces N-methylethanamide: the "ethanamide" part comes from the acyl chloride (2 carbons) and the "N-methyl" part comes from the methyl group on the amine.
- Ethanoyl chloride with excess ethylamine
- Ethanoic anhydride with propylamine
1. Ethanoyl chloride + ethylamine:
\[ \text{CH}_3\text{COCl} + 2\text{CH}_3\text{CH}_2\text{NH}_2 \rightarrow \text{CH}_3\text{CONHCH}_2\text{CH}_3 + \text{CH}_3\text{CH}_2\text{NH}_3^+ \text{Cl}^- \]
The product is N-ethylethanamide and ethylammonium chloride.
2. Ethanoic anhydride + propylamine:
\[ (\text{CH}_3\text{CO})_2\text{O} + \text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2 \rightarrow \text{CH}_3\text{CONHCH}_2\text{CH}_2\text{CH}_3 + \text{CH}_3\text{COOH} \]
The product is N-propylethanamide and ethanoic acid.
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