Condensation Polymerisation, Amino Acids, and DNA and Other Naturally Occurring Polymers [HT & chem-only for condensation and amino acids]

SPEC 4.7.3.2–4.7.3.4 • PAPER 2

⬢ CHEMISTRY ONLY ⬢

▲ MOSTLY HIGHER TIER ▲

Condensation Polymerisation, Amino Acids & Natural Polymers

A different mechanism: monomers with two functional groups, water lost at each link. Plus the polymers nature already makes — proteins, starch, cellulose, and DNA — and the monomer-naming traps that two-thirds of students fell for in 2023.

Two ways to make a polymer

Last lesson covered addition polymerisation. This lesson is about its alternative: condensation polymerisation. The key difference is what happens at each link.

Addition (recap)

One type of monomer (with C=C)

C=C opens to single C–C bond

No atoms lost — no by-product

Repeat unit has same atoms as monomer

Condensation (this lesson)

Monomers with two functional groups

Functional groups react and link

A small molecule is released at each link (usually water)

New ester or peptide bonds formed

The defining difference

If a polymerisation equation shows only the monomer becoming the polymer (no by-product) → addition. If it also shows water (or another small molecule) being released → condensation.

The condensation principle HIGHER TIER

For condensation polymerisation, each monomer carries two reactive functional groups — typically one at each end of the molecule.

What "condensation" means here

A reactive group on one monomer joins to a reactive group on another, and a small molecule (often water) is squeezed out as the new bond forms. Repeat that thousands of times and you get a long polymer chain — with thousands of small molecules of by-product released along the way.

The simplest case — two different monomers

The spec describes the simplest condensation polymer being made from two different monomers, each with two of the same functional group. For example:

Monomer 1 — a diol: a molecule with two –OH groups, one at each end. Example: ethanediol, HO–CH₂–CH₂–OH .

Monomer 2 — a dicarboxylic acid: a molecule with two –COOH groups, one at each end. Example: hexanedioic acid, HOOC–(CH₂)₄–COOH .

Vocabulary

"Di-" means "two". A diol has two –OH groups; a dicarboxylic acid has two –COOH groups. Two functional groups per monomer is the requirement for condensation polymerisation.

Worked example — making a polyester HIGHER TIER

An –OH group on the diol meets a –COOH group on the dicarboxylic acid. They react: the –OH loses an H, the –COOH loses an OH, and these combine to form water. The remaining atoms join into an ester linkage.

What just happened

One ester linkage (the –O–C(=O)– group) replaced the original –OH and –COOH ends. One water molecule is released. Now repeat at every junction along thousands of monomers — the chain grows, and a stream of water molecules is produced.

General polyester equation HIGHER TIER

Putting many diol molecules with many dicarboxylic acid molecules: every –OH reacts with every –COOH along the chain, releasing water at each link.

In words

n × diol + n × dicarboxylic acid → polyester chain (with ester linkages) + 2n × H₂O

This is the standard AQA simplification: each diol contributes 2 –OH ends and each dicarboxylic acid contributes 2 –COOH ends, and almost every end reacts (releasing one water per ester linkage). The coefficient "2n" assumes every functional group reacts — which is approximately true for long polymer chains.

The repeat unit of a polyester

The repeating section of the polymer is: –O–R–O–C(=O)–R'–C(=O)– where R is the chain from the diol and R' is the chain from the dicarboxylic acid. Two ester bonds per repeat unit.

Spec example

The spec uses ethanediol + hexanedioic acid → polyester + water. Same principle: an –OH from ethanediol meets a –COOH from hexanedioic acid, releasing water and forming an ester linkage. The repeating unit alternates: diol-section, ester, dicarboxylic-acid-section, ester, diol-section, ester... and so on.

Examiner alert — circling the right group HIGHER TIER

One of the most consistently lost marks on this topic comes from confusing the –OH of an alcohol with the –OH inside a –COOH group. When a question asks you to circle the alcohol functional group, you must circle ONLY the –OH attached to a regular carbon — not the –OH inside a carboxyl group.

Verbatim — P2 Nov21 Q04.5 examiner report

"Some students seem to think that any -OH is an alcohol group even if it is part of a carboxylic acid group. Other students circled whole molecules rather than functional groups."

The two contexts of –OH

Alcohol –OH: attached to a plain carbon (C–OH). Example: ethanol CH₃CH₂OH. Found on diols.

Carboxyl –OH: the H of the –COOH group, where the O is also bonded to a C=O. Example: ethanoic acid CH₃COOH. Found on dicarboxylic acids.

Both contain –OH but the chemistry is different. When circling, look at what the –OH is attached to.

Amino acids — two functional groups HIGHER TIER

Amino acids are nature's condensation monomers. Each one carries two different functional groups in the same molecule:

An amine group, –NH₂

A carboxyl group, –COOH

Two ends, two reactions

Because each amino acid has both an –NH₂ at one end and a –COOH at the other, every amino acid can link to two other amino acids — one to its amine, one to its carboxyl. That's how a chain forms.

Glycine — the simplest amino acid HIGHER TIER

The spec gives glycine as the worked example. Glycine has the formula H₂N–CH₂–COOH — i.e. an amino acid where the side group "R" is just one H atom.

Glycine polymerises to a polypeptide

The –COOH of one glycine reacts with the –NH₂ of the next. The –OH from the carboxyl combines with one H from the amine to give water, and the remaining atoms form a new bond called a peptide bond (also written as the amide bond, –C(=O)–NH–). The mechanism is exactly analogous to polyester formation — the same condensation principle.

Equation in words

n × glycine → polypeptide + (n − 1) × water

The "n − 1" is because joining n monomers in a single chain produces (n − 1) links — and one water molecule is released per link.

Verbatim — P2 Jun23 Q08.4 examiner report

"A wide range of responses were seen to this question. Common incorrect answers included alkene, hydrogen, named alkanes and alkenes, protein and nitrogen. Around one-half of students gained this mark." When asked "what small molecule is released when an amino acid polymerises?", the answer is water (or H₂O). Not "protein" — the protein is the polymer, not the by-product.

Amino acid M_r calculations HIGHER TIER

A reliable exam question asks you to calculate the mass of part of a molecule. The trick is consistent: add the M_r values of every atom in the named section. Don't subtract functional groups from each other.

Worked example: M_r of the central section in glycine

Glycine has M_r = 75. Calculate the M_r of the –CH₂– central section between the –NH₂ and the –COOH groups.

Note on terminology

The "side group" R in an amino acid is what differs between different amino acids. In glycine, R = H (just one hydrogen atom). The –CH₂– shown here is the alpha carbon plus its bonded H atoms — not the side group itself. AQA past papers ask for the M_r of this central section.

Step-by-step

Find M_r of –NH₂: N + 2H = 14 + 2 = 16

Find M_r of –COOH: C + 2O + H = 12 + 32 + 1 = 45

Sum of functional groups: 16 + 45 = 61

M_r of the central section: 75 − 61 = 14 (which matches CH₂ = 12 + 2 = 14 ✓)

Atomic masses: H = 1, C = 12, N = 14, O = 16.

Verbatim — P2 Jun23 Q08.5 examiner report

"For those who didn't gain any marks a common mistake was subtracting the M_r of NH₂ and COOH from each other." Always add the atom counts for each functional group, then subtract from the whole-molecule M_r. Subtracting one group from the other is the standard trap.

Proteins — chains of many amino acids HIGHER TIER

The spec is precise: glycine on its own polymerises to a polypeptide. Different amino acids combined in the same chain produce proteins.

Polypeptide

A chain of amino acid monomers linked by peptide bonds. If only one type of amino acid is used, this is technically a "polypeptide" but not yet a "protein".

Protein

A chain made of different amino acid monomers in a specific sequence. The variety of side groups (different "R" groups on each amino acid) is what gives different proteins their different functions.

In one sentence

Amino acids → polypeptide (by condensation polymerisation, with water released at each peptide bond). Different amino acids in a specific order → a specific protein.

Naturally occurring polymers from glucose

Many biologically important polymers are made by condensation in living systems. The spec specifically names starch and cellulose as natural polymers built from glucose monomers. AQA mark schemes also accept glycogen (and the umbrella term polysaccharide) as correct answers.

PolymerMonomerFound in / roleStatus

StarchglucosePlants — energy storageSpec-named ✓

CelluloseglucosePlants — cell wall structureSpec-named ✓

GlycogenglucoseAnimals/fungi — energy storageMark-scheme accepted

Verbatim — P2 Jun23 Q08.7 examiner report

"The most common correct answers seen were starch, cellulose or glycogen. Polysaccharide was less commonly seen. Incorrect responses included alcohol, amino acids, proteins and a whole range of polymer names including polyester. Fewer than one-third of students gained this mark." When asked "name a natural polymer made from glucose", give a specific name: starch or cellulose are the safest (spec-named); glycogen and polysaccharide are also accepted by the mark scheme.

DNA — deoxyribonucleic acid

DNA is the molecule that encodes genetic instructions for the development and functioning of living organisms — and viruses. The spec lists four facts you must know about its structure:

DNA in four points

DNA is a large molecule essential for life.

DNA encodes the genetic instructions for the development and functioning of living organisms and viruses.

Most DNA molecules are two polymer chains, made from four different monomers.

The two chains form a double helix.

The monomers of DNA

The four monomers of DNA are called nucleotides. Each nucleotide contains a sugar, a phosphate, and one of four bases (A, T, C, G) — but the monomer is the whole nucleotide, not just the base. This is the next slide's examiner trap.

The biggest DNA exam trap

In June 2023, two-thirds of students dropped this mark by confusing "base" with "nucleotide". The exam asks for the monomer — and the monomer is the whole nucleotide.

Verbatim — P2 Jun23 Q08.8 examiner report

"Many students gave the answer 'bases' or gave the names of DNA bases. Fewer than one-third responded 'nucleotides'."

Nucleotide

The whole monomer unit of DNA. Each nucleotide is built from three parts: a sugar, a phosphate, and a base. There are four different bases (A, T, C, G), so there are four different nucleotides.

This is the exam-correct answer.

Base ≠ monomer

A base (A, T, C, or G) is just one of the three components of a nucleotide. Naming the base alone misses the sugar and the phosphate. Don't write "bases" or "A, T, C, G" — write nucleotides.

Verbatim — P2 Jun23 Q08.9 examiner report

"The majority of students (9/10) knew the shape of DNA. A common incorrect response was 'DNA'." When asked "name the shape of DNA", the answer is double helix. Writing "DNA" doesn't answer the question (DNA is the molecule, not the shape).

Natural polymers — a summary table

The spec covers four naturally occurring polymers and their monomers — DNA plus three others (proteins, starch, cellulose). Memorise this table.

Natural polymerMonomerBond type / process

Proteinsamino acidspeptide bonds (condensation)

Starchglucosecondensation

Celluloseglucosecondensation

DNAnucleotidestwo strands → double helix

Quick recall

Proteins ← amino acids. Starch and cellulose ← glucose. DNA ← nucleotides. If you can map every named natural polymer to its monomer, you've covered the spec.

Addition vs condensation — side by side

This is the comparison question students lose marks on most often. The differences come down to three points: number of monomer types, what happens to atoms, and whether a by-product is released.

AdditionCondensation

Monomer requirementOne monomer type with a C=C double bondMonomer(s) with two functional groups (–OH, –COOH, –NH₂, etc.)

What changesC=C opens to single C–CFunctional groups react to form ester / peptide bonds

By-productNone — no atoms lostYes — typically water (H₂O) at each link

Repeat unitSame atoms as the monomerAtoms from two monomers, minus the small molecule that was lost

Examplespoly(ethene), poly(propene), poly(chloroethene)polyesters, polypeptides / proteins

Practice question HIGHER TIER

3 MARKS

The amino acid alanine has the structural formula H₂N–CH(CH₃)–COOH, with relative formula mass M_r = 89.

A dipeptide is formed when two alanine molecules join together by condensation polymerisation.

(a) Name the small molecule released when the dipeptide forms. 1 MARK

(b) Calculate the relative formula mass of the dipeptide. 2 MARKS

(a)

Small molecule released: When two amino acids join in a peptide bond, an –OH from one carboxyl and an H from the other amine combine to form water (H₂O). (1 mark)

(b)

Step 1: Two alanine molecules give a starting mass of 2 × 89 = 178. (1 mark)

(b)

Step 2: One water molecule (M_r = 18) is lost when the peptide bond forms. So M_r of dipeptide = 178 − 18 = 160. (1 mark)

Lesson summary

Condensation polymerisation HT

Monomers with two functional groups react to form a polymer chain, releasing a small molecule (usually water) at each link. Two main families: polyesters (from diols + dicarboxylic acids) and polypeptides/proteins (from amino acids).

Amino acids HT

Each carries –NH₂ and –COOH. Glycine (H₂N–CH₂–COOH) polymerises to a polypeptide. Different amino acids in one chain → a protein. M_r of dipeptide = 2 × (M_r of amino acid) − 18.

Naturally occurring polymers (all tiers)

Proteins ← amino acids. Starch / cellulose ← glucose (spec-named) (glycogen also accepted by mark schemes). DNA ← nucleotides (NOT bases). DNA structure: two polymer chains in a double helix.

The three biggest mark-losers

"DNA monomer = bases" → write nucleotides.

"Polymer of glucose" without naming it → write starch or cellulose (or glycogen).

Subtracting –NH₂ from –COOH in M_r calculations → always add functional group masses, then subtract from the whole molecule.

Checkpoint

Before moving on, you should be able to:

Higher tier — condensation

Define condensation polymerisation: monomers with two functional groups join, releasing a small molecule.

Describe polyester formation: diol + dicarboxylic acid → polyester + water (one water per ester linkage).

Distinguish the alcohol –OH from the carboxyl –OH inside –COOH.

Higher tier — amino acids

Identify the two functional groups on an amino acid: –NH₂ and –COOH.

State that glycine polymerises to a polypeptide and that different amino acids combine to form proteins.

Calculate the M_r of a peptide section by adding atom masses, never subtracting one functional group from another.

All tiers — natural polymers

Name the monomer of each natural polymer: proteins ← amino acids; starch and cellulose ← glucose (mark schemes also accept glycogen); DNA ← nucleotides.

State that DNA molecules are usually two chains in a double helix.

Avoid the "bases" trap — DNA's monomer is the whole nucleotide.