This chemistry grades 11-12 lesson on amines and polymers notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on amines and polymers and check yourself with the quiz.
Quick answer: Amines are organic bases because the lone pair on nitrogen accepts a proton, and amino acids contain both an amine and an acid group so they form zwitterions. Condensation polymers such as nylon, polyesters and proteins join monomers through amide or ester links while losing small molecules like water.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.11-3.3.13
Amines and polymers notes: the key ideas
Why are amines basic?
An amine is an organic compound formed when one or more hydrogen atoms of ammonia are replaced by alkyl or aryl groups. It is primary (RNH₂), secondary (R₂NH) or tertiary (R₃N) according to the number of carbon groups on nitrogen.
The nitrogen atom of an amine carries a lone pair of electrons. It accepts a proton from an acid, so an amine is a Brønsted–Lowry base: CH₃NH₂ + H⁺ → CH₃NH₃⁺. Alkyl groups push electron density towards nitrogen, which makes the lone pair more available than in ammonia. In phenylamine the lone pair is partly delocalised into the benzene ring, so it is much less available.
| Compound | pKb at 25 °C | Reason for its basicity |
|---|---|---|
| Dimethylamine | 3.27 | Two electron-donating methyl groups |
| Methylamine | 3.34 | One electron-donating methyl group |
| Ammonia | 4.75 | Reference, no donating groups |
| Phenylamine | 9.13 | Lone pair delocalised into the ring |
Students say that a base with a bigger pKb is stronger. It is the opposite: a smaller pKb means a larger Kb and a stronger base.
How are amines made and what do they do?
A haloalkane reacts with an excess of ammonia in ethanol by nucleophilic substitution: CH₃CH₂Br + 2NH₃ → CH₃CH₂NH₂ + NH₄Br. The excess matters, because the amine formed is itself a nucleophile and would otherwise keep reacting to give secondary and tertiary amines. Nitriles are reduced to primary amines, and nitrobenzene is reduced to phenylamine using tin and concentrated hydrochloric acid. Amines also react with acyl chlorides to give N-substituted amides.
Calculate the pH of 0.10 mol/dm³ ethylamine, Kb = 4.5 × 10⁻⁴ mol/dm³, at 25 °C (Kw = 1.0 × 10⁻¹⁴).
[OH⁻] = √(Kb × c) = √(4.5 × 10⁻⁴ × 0.10) = 6.7 × 10⁻³ mol/dm³. pOH = −log(6.7 × 10⁻³) = 2.17, so pH = 14.00 − 2.17 = 11.83.
What are amino acids and how do they behave?
An α-amino acid has an amino group (–NH₂) and a carboxylic acid group (–COOH) on the same carbon, written RCH(NH₂)COOH. Apart from glycine (R = H), the alpha carbon is a chiral centre.
Because the molecule has both a basic and an acidic group, it is amphoteric. In water and in the solid state the acid group gives H⁺ to the amine group, forming a zwitterion with zero overall charge. This ionic character explains why amino acids are crystalline solids with high melting points and are soluble in water.
| Solution | Species of an amino acid | Overall charge |
|---|---|---|
| Strongly acidic | RCH(NH₃⁺)COOH | +1 |
| Near the isoelectric point | RCH(NH₃⁺)COO⁻ | 0 |
| Strongly alkaline | RCH(NH₂)COO⁻ | −1 |
The pH at which the zwitterion dominates is the isoelectric point. A mixture of amino acids can be separated by electrophoresis or chromatography, and spots are revealed with ninhydrin.
How do condensation polymers form?
In condensation polymerisation, monomers with two functional groups join in repeated reactions, and each join releases a small molecule such as H₂O or HCl.
A polyamide forms from a diamine and a dicarboxylic acid or diacid chloride, and a polyester from a diol and a dicarboxylic acid. Proteins are natural polyamides: amino acids join through peptide links (–CO–NH–). Both amide and ester links are broken by hydrolysis, which is why these polymers can be broken down by acids or alkalis, unlike polyalkenes with a carbon-carbon chain.
| Polymer | Monomers | Link | Small molecule lost |
|---|---|---|---|
| Nylon-6,6 | Hexane-1,6-diamine and hexanedioyl dichloride | Amide | HCl |
| PET | Benzene-1,4-dicarboxylic acid and ethane-1,2-diol | Ester | H₂O |
| Protein | Amino acids | Peptide | H₂O |
Calculate the mass of nylon-6,6 formed from 5.80 g of hexane-1,6-diamine (Mr = 116) and excess hexanedioic acid, with 100 % yield. The repeat unit C₁₂H₂₂N₂O₂ has Mr = 226.
n(diamine) = 5.80 ÷ 116 = 0.0500 mol. One repeat unit forms per diamine, so n(repeat units) = 0.0500 mol and m = 0.0500 × 226 = 11.3 g. Water lost = 0.100 mol × 18 = 1.80 g.
What is DNA made of?
DNA is a polymer of nucleotides. Each nucleotide joins a phosphate group, the sugar deoxyribose and one of four bases: adenine (A), thymine (T), guanine (G) or cytosine (C). Phosphate and sugar alternate in the backbone of each strand. Two strands twist into a double helix, held by hydrogen bonds between complementary bases: A pairs with T and G pairs with C. The anticancer drug cisplatin works by binding to guanine bases in DNA and stopping cell division.
For any condensation polymer, find the link first (amide or ester), cut it, and add back the small molecule you lost. The two monomers appear at once.
- Amine basicity: alkylamines are stronger than ammonia, which is stronger than phenylamine; smaller pKb means a stronger base.
- Amino acid: RCH(NH₂)COOH, exists as a zwitterion near its isoelectric point.
- Amide and ester links form with loss of H₂O or HCl and are broken by hydrolysis.
- DNA base pairs: A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds).
Amines, amino acids and condensation polymers in action: frequently asked questions
Why is phenylamine a weaker base than ethylamine?
In phenylamine the nitrogen lone pair overlaps with the delocalised ring electrons, so it is less available to accept H⁺. In ethylamine the alkyl group pushes electron density onto nitrogen, making the lone pair more available.
What is a zwitterion?
A zwitterion is a molecule carrying a positive and a negative charge on different atoms with zero overall charge. An amino acid forms one when its –COOH group transfers a proton to its –NH₂ group.
How does a condensation polymer differ from an addition polymer?
A condensation polymer forms from monomers with two functional groups and releases a small molecule such as water or HCl at each link. An addition polymer forms from alkenes and loses nothing.
Why can polyesters and polyamides be hydrolysed?
Ester and amide links contain polar C=O bonds that water can attack, especially with acid or alkali. The carbon-carbon backbone of poly(ethene) has no such site, so it resists breakdown.
Which bases pair in DNA?
Adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three. This fixed pairing lets one strand act as a template for the other.
