Introduction to Making Nylon
Welcome to this study guide on making nylon for CCEA A2 2 Organic Chemistry. Whether you love organic chemistry or find reaction mechanisms a bit intimidating, do not worry! In this guide, we will break down how nylon is formed step-by-step, explore the famous "nylon rope trick", and look closely at why this versatile material behaves the way it does.
Nylon is one of the most famous synthetic polymers in human history. From toothbrush bristles and climbing ropes to clothing and surgical sutures, understanding how to synthesise and manipulate polyamides is a core part of your A2 specification.
---1. Condensation Polymerisation & Polyamides
What is Condensation Polymerisation?
In AS Chemistry, you learned about addition polymerisation, where alkene monomers join by breaking double bonds without losing any atoms. Condensation polymerisation works differently:
• Condensation polymerisation is a chemical reaction where monomer molecules join together with the simultaneous loss or elimination of a small molecule (such as water, \(\text{H}_2\text{O}\), or hydrogen chloride, \(\text{HCl}\)) at each newly formed linkage.
• Each monomer must have at least two functional groups (one at each end of the molecule) so the chain can keep growing in both directions.
What is a Polyamide?
A polyamide is a condensation polymer formed when amine groups (\(\text{—NH}_2\)) react with carboxylic acid groups (\(\text{—COOH}\)) or acyl chloride groups (\(\text{—COCl}\)).
The repeating functional group holding the polymer chain together is the amide linkage (also known as a peptide bond in biochemistry):
\(\text{—CONH—}\) or \(\text{—C(=O)—NH—}\)
Quick Review: Addition polymers have an all-carbon continuous backbone (\(\text{—C—C—C—C—}\)) with no atoms lost during synthesis. Polyamides have nitrogen atoms built right into the main chain backbone along with carbonyl carbons (\(\text{—C(=O)—NH—}\)), releasing a small molecule every time a link is forged.
---2. The Monomers & Naming System of Nylon
Understanding the Name: Why "Nylon-6,6"?
Nylons are named with numbers that tell you exactly how many carbon atoms are in each monomer unit:
• The first number represents the number of carbon atoms in the diamine.
• The second number represents the number of carbon atoms in the dicarboxylic acid (or diacyl chloride).
Key Monomers You Need to Know
1. Hexane-1,6-diamine (also called 1,6-diaminohexane):
Structure: \(\text{H}_2\text{N—}(\text{CH}_2)_6\text{—NH}_2\)
This provides the \(6\)-carbon diamine component.
2. Hexanedioic acid (adipic acid):
Structure: \(\text{HOOC—}(\text{CH}_2)_4\text{—COOH}\)
This provides a \(6\)-carbon dicarboxylic acid (4 carbons in the middle plus 2 carbonyl carbons = 6 carbons in total).
3. Hexanedioyl dichloride (adipoyl chloride):
Structure: \(\text{ClOC—}(\text{CH}_2)_4\text{—COCl}\)
The acyl dichloride equivalent of hexanedioic acid, used in laboratory preparations.
4. Decanedioyl dichloride (sebacoyl chloride):
Structure: \(\text{ClOC—}(\text{CH}_2)_8\text{—COCl}\)
A \(10\)-carbon diacyl chloride used to make Nylon-6,10 when reacted with hexane-1,6-diamine.
Memory Tip: Always count all the carbons in the diacid! In hexanedioic acid, the formula is written \(\text{HOOC}(\text{CH}_2)_4\text{COOH}\). That is \(1 + 4 + 1 = 6\) carbons, giving Nylon-6,6.
---3. Industrial Synthesis vs. Laboratory Preparation
A. Industrial Synthesis of Nylon-6,6
In industry, costs and environmental impact are critical. Manufacturers react the diamine directly with the dicarboxylic acid under high heat:
• Reactants: Hexane-1,6-diamine + Hexanedioic acid
• Byproduct: Water (\(\text{H}_2\text{O}\))
• Equation:
\(n\,\text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2 + n\,\text{HOOC}(\text{CH}_2)_4\text{COOH} \longrightarrow \text{—[—NH—}(\text{CH}_2)_6\text{—NH—CO—}(\text{CH}_2)_4\text{—CO—]}_n\text{—} + 2n\,\text{H}_2\text{O}\)
B. Laboratory Preparation: The "Nylon Rope Trick"
Carboxylic acids react very slowly with amines at room temperature. In the school laboratory, we use a diacyl dichloride instead because acyl chlorides are significantly more reactive and react rapidly and irreversibly at room temperature.
• Reactants: Hexane-1,6-diamine + Hexanedioyl dichloride (or Decanedioyl dichloride for Nylon-6,10)
• Byproduct: Hydrogen chloride (\(\text{HCl}\))
• Equation for Nylon-6,6 in the lab:
\(n\,\text{H}_2\text{N}(\text{CH}_2)_6\text{NH}_2 + n\,\text{ClOC}(\text{CH}_2)_4\text{COCl} \longrightarrow \text{—[—NH—}(\text{CH}_2)_6\text{—NH—CO—}(\text{CH}_2)_4\text{—CO—]}_n\text{—} + 2n\,\text{HCl}\)
---4. Interfacial Polymerisation: Step-by-Step
How the Demonstration Works
The laboratory preparation is carried out using an ingenious method called interfacial polymerisation:
Step 1: Preparing Layer 1 (Aqueous Phase)
Hexane-1,6-diamine is dissolved in water containing aqueous sodium hydroxide (\(\text{NaOH}\)). This forms the denser, lower layer in a beaker.
Step 2: Preparing Layer 2 (Organic Phase)
Hexanedioyl dichloride (or decanedioyl dichloride) is dissolved in a non-polar organic solvent (such as cyclohexane, hexane, or dichloromethane). This layer is gently poured on top, forming an upper layer that does not mix with the water.
Step 3: Reaction at the Interface
Because the two solvents are immiscible (they do not mix), the monomers can only meet at the interface (the boundary between the two liquid layers). A thin film of solid nylon forms immediately at this boundary.
Step 4: Pulling the Rope
Using a pair of tweezers or a glass rod, the film at the interface is carefully caught and lifted upwards. As the nylon rope is pulled out continuously, fresh diamine and diacyl chloride molecules come into contact at the boundary, regenerating the film instantaneously!
Crucial Exam Detail: The Role of Sodium Hydroxide (\(\text{NaOH}\))
Why do we add \(\text{NaOH}\) to the aqueous diamine layer?
• During the reaction, acidic hydrogen chloride (\(\text{HCl}\)) is produced as a byproduct.
• If \(\text{HCl}\) accumulates, it reacts with the unreacted amine groups of hexane-1,6-diamine, converting them into protonated ammonium salts (\(\text{—NH}_3^+\text{Cl}^-\)).
• Protonated ammonium groups lack a lone pair of electrons on the nitrogen and cannot act as nucleophiles to continue the polymerisation.
• \(\text{NaOH}\) neutralises the \(\text{HCl}\) as it forms, preventing the diamine from being deactivated and allowing the polymerisation to continue smoothly.
5. Structure, Properties, and Uses of Nylon
Intermolecular Forces in Polyamides
Why are nylon fibres so tough, elastic, and resistant to abrasion?
• Polyamide chains contain highly polar groups: the carbonyl group has an electronegative oxygen (\(\text{C=O}^{\delta-}\)) and the amine group has an electropositive hydrogen bonded to nitrogen (\(\text{N—H}^{\delta+}\)).
• Adjacent polymer chains line up parallel to one another and form extensive networks of hydrogen bonds between the \(\text{N—H}^{\delta+}\) of one chain and the \(\text{C=O}^{\delta-}\) of a neighbouring chain.
• These strong intermolecular hydrogen bonds give nylon exceptional tensile strength, high melting point, elasticity, and resistance to wear and tear.
Everyday Applications
Thanks to these properties, polyamides are widely used in:
• Textiles & Fabrics: Clothing, stockings, carpets, and sportswear.
• High-strength equipment: Ropes, climbing cords, and parachute fabrics.
• Engineering components: Low-friction gears, mechanical bearings, and casings.
• Medical applications: Non-absorbable surgical sutures.
6. Degradation and Hydrolysis of Polyamides
Why Can Nylon Degrade?
A major environmental contrast between addition polymers and condensation polymers lies in their degradation:
• Addition polymers (like polyethene) have non-polar \(\text{C—C}\) single-bond backbones. They are chemically inert and non-biodegradable.
• Polyamides contain polar amide bonds (\(\text{—CONH—}\)). The carbon atom in the carbonyl group has a partial positive charge (\(\text{C}^{\delta+}\)), making it susceptible to nucleophilic attack.
Hydrolysis Conditions
Polyamides can be broken down by hydrolysis (splitting the bond using water):
• Acid Hydrolysis: Boiling with hot aqueous strong acid (e.g., \(\text{HCl}\)) breaks the amide bond, yielding a dicarboxylic acid and a diammonium salt.
• Alkaline Hydrolysis: Boiling with hot aqueous strong base (e.g., \(\text{NaOH}\)) breaks the amide bond, yielding a dicarboxylate salt and a free diamine.
Because they can be broken down by chemical and biological agents via hydrolysis of the amide linkage, polyamides are considered biodegradable under appropriate conditions over time.
---7. Common Exam Pitfalls & How to Avoid Them
Pitfall 1: Confusing the byproducts
Mistake: Writing \(\text{H}_2\text{O}\) when reacting a diacyl chloride, or writing \(\text{HCl}\) when using a dicarboxylic acid.
Correction: Diacyl chloride + Diamine \(\longrightarrow\) Polyamide + \(\text{HCl}\).
Dicarboxylic acid + Diamine \(\longrightarrow\) Polyamide + \(\text{H}_2\text{O}\).
Pitfall 2: Drawing the repeat unit incorrectly
Mistake: Forgetting the open continuation bonds at either end, or counting the wrong number of \(\text{—CH}_2—\) groups.
Correction: Always ensure you show open extension bonds passing through the brackets:
\(\text{—[—NH—}(\text{CH}_2)_6\text{—NH—CO—}(\text{CH}_2)_4\text{—CO—]}_n\text{—}\)
Check your carbon count: 6 carbons in the diamine section, 4 \(\text{CH}_2\) carbons in the adipoyl section (making 6 carbons total for the diacid part).
Pitfall 3: Incomplete explanation of \(\text{NaOH}\)
Mistake: Stating simply that "\(\text{NaOH}\) makes the solution alkaline" or "\(\text{NaOH}\) acts as a catalyst".
Correction: State clearly that \(\text{NaOH}\) neutralises the \(\text{HCl}\) byproduct to stop the diamine reactant from being protonated into an unreactive ammonium salt.
Summary Checklist: Are You Exam-Ready?
Can you:
• Define condensation polymerisation and identify the amide linkage (\(\text{—CONH—}\))?
• Name and draw the structures of hexane-1,6-diamine and hexanedioic acid / hexanedioyl dichloride?
• Write balanced equations for the industrial and laboratory synthesis of Nylon-6,6?
• Explain why the laboratory method uses an interface and why \(\text{NaOH}\) is added?
• Describe how intermolecular hydrogen bonding gives nylon high tensile strength?
• Explain why polyamides undergo hydrolysis while polyalkenes do not?