DSE Chemistry | Organic reaction types lesson plan (for tutors)

DSE Chemistry | Organic reaction types lesson plan (for tutors) | Telling addition from substitution

2026-11-03

Tutor lesson-prep guides · Secondary Chemistry

Students can recite that alkenes add and alkanes substitute, then freeze when asked which test tells them apart. This lesson pins the three families down with bromine water and acidified dichromate, then finishes on isomer counting.

Objectives — Students can: (1) write the addition products of ethene with H2, Br2 and HBr and use bromine water to tell an alkene from an alkane; (2) write the UV-initiated substitution of methane with chlorine and give the three radical steps; (3) explain that distilling ethanol with acidified dichromate gives ethanal while refluxing gives ethanoic acid, with orange turning green; (4) count 2 isomers for C4H10, 3 for C5H12, 4 for C4H9Br and 3 for C4H8.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Functional groups	What family does a C=C belong to?	The alkenes
Homologous series	What is the alkane general formula?	CnH2n+2
Redox	How is oxidation spotted in carbon compounds?	Gain of oxygen or loss of hydrogen

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Two tests opener	Two colourless liquids, one alkene one alkane: how do you tell?	Propose bromine water
10-24 min	Addition	Ethene + Br2 gives 1,2-dibromoethane, Mr 188	Write three products
24-36 min	Radical substitution	Methane + Cl2 under UV: initiation, propagation, termination	Name all three steps
36-48 min	Oxidising alcohols	Distil for the aldehyde, reflux for the acid	Tell the two set-ups apart
48-56 min	Counting isomers	C4H10 2, C5H12 3, C4H9Br 4	Draw before counting
56-60 min	Exit check	Mark on the spot	Three correct

03 Teaching script (read it out)

Scene 1: Classify first, then write the product

Alkenes and alkynes carry C=C or C triple bond C, so they add and decolourise bromine water; alkanes have only single bonds, react only by substitution under UV, and leave bromine water orange. That one sentence is the whole logic of the test. Addition opens the double bond and puts one group on each carbon, so ethene with H2 gives ethane, with Br2 gives 1,2-dibromoethane and with HBr gives bromoethane, taking Mr from 28 to 109. Always write the structure, not just the name.

Ask them to: draw the structure from ethene plus HBr and mark which carbon takes the H.

Scene 2: Substitution runs as a chain, not in one step

Methane with chlorine under UV goes in three stages. Initiation: Cl2 absorbs UV and splits homolytically into two Cl radicals. Propagation: a Cl radical takes a hydrogen from methane, giving HCl and a methyl radical, which then hits Cl2 to give chloromethane and a fresh Cl radical. Termination: two radicals combine into a stable molecule. Because chloromethane can be attacked again, the experiment yields a mixture from mono to tetrachloro, which is why industry controls the ratio. Writing that UV breaks the Cl-Cl bond scores; writing “heat it” does not.

Follow up: why is the product a mixture rather than pure chloromethane?

Scene 3: Read oxidation by colour, isomers by skeleton

Ethanol with acidified dichromate gives ethanal on distillation, because the aldehyde boils off before it can be oxidised again, and ethanoic acid on reflux, where it stays in contact. The only colour fact to memorise is dichromate orange going to chromium(III) green. For isomers, draw the carbon skeletons first: C4H10 has 2, C5H12 has 3, C4H9Br has 4 because bromine can sit on different carbons of two skeletons, and C4H8 has 3 alkene isomers. Missing isomers almost always means only straight chains were drawn.

Ask them: where does the fourth C4H9Br isomer put its bromine?

04 Question types and worked steps

Type	Example	Steps	Answer
Identification	Telling an alkene from an alkane	Add bromine water and watch the colour	Alkene decolourises, alkane does not
Drawing	Addition product structure	Open the double bond, one group each side	Ethene + HBr gives bromoethane
Explanation	The three radical steps	Initiation, propagation, termination	UV breaks Cl-Cl
Counting	Number of structural isomers	Draw the skeletons, then place the group	C5H12 has 3

Sanity check (teach this): Check the masses: ethene C2H4 is 28, adding Br2 (160) gives 1,2-dibromoethane at 188. Adding HBr takes 28 to 109, a difference of 81, which is exactly the Mr of HBr. Oxidation check: ethanol at 46 loses two hydrogens to ethanal and gains oxygen to ethanoic acid at 60; both come from the same test tube, separated only by distilling or refluxing. Isomer check: 2, 3 and 5 for C4, C5 and C6 alkanes, which is why exam questions stop at C5 or C6.

05 Independent practice (3 questions)

Question	Answer	What to watch
Mr of the product of ethene and Br2	188	28 + 160
Product of refluxing ethanol with acidified dichromate	Ethanoic acid	Keep it in contact, do not distil
Number of structural isomers of C4H9Br	4	Two butane skeletons, then place Br

06 Common mistakes and what to say

Mistake	Student writes	Say this
Writing substitution as addition	“Methane plus Br2 gives bromomethane”	“An alkane has no double bond; it substitutes, and it needs UV.”
Ignoring the conditions	“Ethanol oxidises to ethanoic acid”	“Distilling gives the aldehyde, refluxing gives the acid.”
Counting only straight chains	“C5H12 has one isomer”	“Ask how many skeletons there are: pentane, isopentane, neopentane, so three.”
Mixing up the colour test	“Bromine water clearing means alkane”	“Clearing means a C=C is present, so it is an alkene or alkyne.”

07 Homework (paper, section and question numbers)

Chemistry chapter index — All chapters
Chemistry Paper 1 Section B — Questions 5-8: organic reactions and isomers

08 Exit check (last 5 minutes)

Question	Answer	Decision
Test that separates hexene from hexane	Bromine water, decolourised by the alkene	Wrong: return to script one
Product of refluxing ethanol with acidified dichromate	Ethanoic acid	Wrong: reteach distillation versus reflux
Number of isomers of C5H12	3	Two or more wrong: add skeleton practice next lesson

09 Tutor FAQ

Must every reaction be memorised?

Learn one or two per family; the rest follows from the functional group and the conditions.

Why does distillation stop at the aldehyde?

The aldehyde boils off as it forms, so it escapes before further oxidation.

Do cis and trans count?

Structural isomer questions exclude them; stereoisomer questions add them separately.

Find a tutor — free Tutor Registration →

A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.