DSE Chemistry | Organic reactions and isomers - tutor prep guide

DSE Chemistry | Organic reactions and isomers – tutor prep guide | Four-reaction map, isomer counting

2026-11-05

Tutor lesson-prep guides · Secondary Chemistry

Organic questions are marked on reagent and condition pairs, not on product names, and isomer counts are marked on a systematic walk of the carbon skeleton rather than on drawing whatever comes to mind.

Objectives — Students can: (1) match four reaction types to their reagents and conditions; (2) predict the product of an addition, substitution, oxidation or esterification from the functional group; (3) count structural isomers of a given formula with a repeatable skeleton walk.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Functional groups	Name the group in ethanoic acid and in ethanol	Carboxylic acid; hydroxyl
Bromine water test	What does it detect?	C=C, decolourised
Mole arithmetic	Moles from mass and molar mass	n = m / M

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-6 min	Group recall	Four structures on the board, name the functional group in each	Write the four names
6-12 min	The map outline	Draw four arrows out of one alkane and one alkene	Copy the empty map
12-16 min	Addition and substitution	Fill two arrows with reagents and conditions	Fill the reagent boxes
16-30 min	Oxidation and esterification	Fill the last two, then run the ester yield example	Compute the yield
30-48 min	Isomer counting	Walk skeletons for C4H10, C5H12, C6H14, C3H7Cl	Count and list
48-58 min	Exam items	One identification, one yield, one isomer count	Self-mark
58-60 min	Exit check	Three items	Work alone

03 Teaching script (read it out)

Scene 1: Four arrows, reagent and condition on every one

Build the map on one page: alkane to haloalkane is substitution with bromine under UV light, alkene to haloalkane is addition with bromine in the dark, alcohol to carboxylic acid is oxidation with acidified dichromate under reflux, and alcohol plus acid to ester is esterification with a concentrated acid catalyst. Each arrow carries two labels, reagent and condition, because that pair is the mark.
The contrast to say out loud: the same bromine does two different jobs depending on the starting material and the light, and that single sentence answers a whole class of multiple-choice items.

Ask the student to write reagent and condition on all four arrows without looking at notes.

Scene 2: Run one esterification end to end

Ethanol and ethanoic acid with a concentrated acid catalyst give ethyl ethanoate, molar mass 88. With 0.20 mol of ethanol and 0.15 mol of acid, the acid is limiting. Theoretical mass is 0.15 x 88 = 13.2 g. If 9.9 g is collected the yield is 75 per cent. That is a complete calculation chain from moles to mass to percentage, and it is where students lose marks by using the larger mole quantity.
Ask why the yield is never 100 per cent and take three answers: reversible reaction, loss on transfer, side reactions.

Ask the student to name the limiting reagent first and say why before any multiplication.

Scene 3: Count isomers by walking skeletons, not by inspiration

Fix the longest chain first, then shorten it and branch, and stop when the branches cannot be moved without repeating a structure. That gives 2 for C4H10, 3 for C5H12, 5 for C6H14, 2 for C3H7Cl and 4 for C4H8 including the cis and trans pair. The routine is the answer: a student who follows it can show their working and earn method marks even when the final count is off by one.
Close with the trap: but-2-ene drawn twice with the same name is not two isomers, and 2-methylpropane is not a third isomer of C4H10.

Ask the student to draw all isomers of C5H12 with the chain length written beside each one.

04 Question types and worked steps

Type	Example	Steps	Answer
Functional group identification	Which test distinguishes an alkene from an alkane	Bromine water, decolourised, dark	Alkene decolourises it
Reaction prediction	Ethanol with acidified dichromate under reflux	Identify group, choose reagent pair, name product	Ethanoic acid
Yield calculation	0.20 mol alcohol, 0.15 mol acid, 9.9 g collected	Limiting reagent, theoretical mass, percentage	75 per cent
Isomer count	Structural isomers of C6H14	Walk the longest chain down from six	5

Sanity check (teach this): Sanity check: every reaction claim names a reagent and a condition; percentage yield cannot exceed 100; a limiting reagent is never the one in excess; isomer counts must be justified by a written skeleton walk, not by a number alone.

05 Independent practice (3 questions)

Question	Answer	What to watch
Map recall	Fill a blank four-arrow map	Whether conditions are written, not just reagents
Yield set	Three limiting reagent cases, one with excess alcohol	Whether limiting reagent is identified first
Isomer walk	C4H8 and C3H7Cl with chain lengths labelled	Whether duplicates are weeded out

06 Common mistakes and what to say

Mistake	Student writes	Say this
Reagent without condition	Bromine added, nothing else said	Bromine with what, and under what light? Two words missing.
Using the excess reagent	0.20 mol alcohol used as limiting	Which reactant runs out first? That is the one that sets the product.
Duplicate isomers counted	But-2-ene drawn twice	Read the name back. Is this the same structure turned around?
Yield above 100 per cent	Actual mass larger than theoretical	Check molar mass and limiting reagent; a yield over 100 means the theoretical mass is too small.

07 Homework (paper, section and question numbers)

Secondary Chemistry index — All chapters for this subject
Prep guides index — Matching chapters in other subjects
Redo this chapter — Draw the four-arrow map from memory and list C5H12 isomers

08 Exit check (last 5 minutes)

Question	Answer	Decision
Four reaction types	Addition, substitution, oxidation, esterification	Missing one = back to act one
Limiting reagent rule	The one that runs out first	Blank = back to act two
Isomer routine	Longest chain, then branch, then weed duplicates	Cannot state = repeat act three

09 Tutor FAQ

Do students need mechanisms?

Not for this chapter; reagent and condition pairs carry the marks, and mechanisms belong to a separate lesson if the syllabus demands them.

How many isomers should be memorised?

None; the routine is worth more than the list, though C4 and C5 counts are worth knowing as a check.

Is cis-trans needed at this level?

Yes for C4H8, since it turns a count of three into four and examiners use exactly that difference.

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.