Chirality: A carbon with four different substituents can form enantiomers. Use wedge-dash notation and look for the absence of a plane of symmetry (R/S naming is not required in the SEAB syllabus).
Optical activity: Enantiomers rotate plane-polarised light in opposite directions; racemates have no net rotation.
Geometric isomerism: Occurs in alkenes with restricted rotation when distinct substituents are present. Describe as cis/trans in H2 Chemistry (E/Z nomenclature is not required in the SEAB syllabus).
State stereochemical outcomes in mechanisms (e.g. SN2 inversion of configuration).
Butan-2-ol: chiral-centre reference with stereochemical notation.
But-2-ene: framework for discussing geometric isomerism (cis/trans).
3 Mechanism Templates
3.1 Electrophilic Addition to Alkenes
Electron-rich double bond attacks electrophile EX+.
Carbocation intermediate forms (consider rearrangements).
Nucleophile attacks carbocation.
Include regioselectivity explanation (Markovnikov). Provide energy profile diagram if asked.
3.2 SN1 vs SN2
Feature
SN1
SN2
Substrate
Tertiary > secondary.
Primary > methyl.
Rate law
rate = k[RX]
rate = k[RX][NuX−]
Mechanism
Carbocation intermediate, racemisation.
One-step backside attack, inversion.
Solvent
Polar protic stabilises carbocation.
Polar aprotic favours nucleophile.
3.3 Electrophilic Aromatic Substitution
Generation of electrophile (e.g. NOX2X+ from HNOX3+HX2SOX4).
Electrophile attacks pi electrons, forming arenium ion.
Deprotonation restores aromaticity.
Remember directing effects: activating (ortho/para) vs deactivating (meta).
Keep propene and methylbenzene as anchor substrates for this section: they let you practise Markovnikov addition outcomes and ortho/para directing effects using concrete structures instead of abstract arrows.
Propene: common alkene substrate for electrophilic addition questions.
Methylbenzene: activated arene for ortho/para directing effects.
4 Oxidation and Reduction Summary
Conversion
Reagent/conditions
Observation
Primary alcohol → aldehyde
KX2CrX2OX7/HX+, heat and distil.
Orange to green solution; collect distillate.
Primary alcohol → carboxylic acid
KMnOX4/HX+, reflux.
Purple to colourless/brown precipitate.
Aldehyde → carboxylic acid
Tollens' or Fehling's.
Silver mirror or brick-red ppt.
Carbonyl → alcohol
NaBHX4 (aqueous) or LiAlHX4
Carboxylic acid → alcohol
LiAlHX4, dry ether, followed by water.
Powerful reducing agent (dangerous, exothermic).
Tie observations to qualitative analysis (Paper 4).
5 Spectroscopy Checklist
5.1 Infrared (IR)
The SEAB Chemistry Data Booklet provides a quick IR table (use this unless the question paper provides its own table):
Bond
Typical functional groups
Absorption range / cm−1
Peak
C–Cl
chloroalkanes
700−800
strong
C–O
alcohol
970−1260
strong
C–O
ether
1000−1310
strong
C–O
ester
1050−1330
strong
C–O
carboxylic acids
1210−1440
strong
C=C
aromatic
1475−1625
strong
C=C
alkenes
1635−1690
weak
C=O
amides
1640−1690
strong
C=O
ketones and aldehydes
1670−1740
strong
C=O
carboxylic acids
1680−1730
strong
C=O
esters
1710−1750
strong
C≡C
alkynes
2150−2250
weak unless conjugated
C≡N
nitriles
2200−2250
weak
C–H (alkanes)
CHX3/CHX2
=C–H (alkenes/arenes)
=CH
3000−3100
weak
N–H
amines/amides
3300−3500
weak
O–H
carboxylic acid, RCOX2H
2500−3000
strong and very broad
O–H
H-bonded alcohol/phenol, ROH
3200−3600
strong and broad
O–H
free alcohol, ROH
3580−3650
strong and sharp
5.2 Proton NMR
Typical 1H chemical-shift ranges in the SEAB Chemistry Data Booklet (δ, relative to TMS = 0):
alkane 0.9−1.7
alkyl next to C=O2.2−3.0
alkyl next to aromatic ring 2.3−3.0
alkyl next to electronegative atom (e.g. CHX3OX−, CHX2Cl) 3.2−4.0
attached to alkyne 1.8−3.1
attached to alkene 4.5−6.0
attached to aromatic ring 6.0−9.0
aldehyde 9.3−10.5
carboxylic acid 9.0−13.0
δ values for O−H and N−H protons can vary depending on solvent and concentration.
Integration ratio reveals relative number of protons.
Splitting pattern obeys n + 1 rule (ignore exchangeable protons in acids/alcohols).
5.3 Carbon-13 NMR
Supports identification of carbon environments (no splitting). Use along with proton NMR.
5.4 Mass Spectrometry
Molecular ion peak MX+ indicates molar mass.
Fragmentation patterns suggest functional groups (e.g. M−15 implies loss of CHX3).
6 Worked Synthesis Problem
Question:
Propose a synthesis route to convert benzene into 4-nitrobenzoic acid. Include reagents, conditions, and key mechanism justifications.
Exam-ready route (aligned with SEAB’s benzene/methylbenzene conditions):
Friedel-Crafts alkylation (introduce an CHX3 director):
CX6HX6CHX3ClAlClX3CX6HX5CHX3
Methylbenzene is activated and directs electrophilic substitution to the 2- and 4- positions.
Nitration of methylbenzene (keep the mixture at 30∘C):
(SEAB also allows hot acidified KMnOX4 for this side-chain oxidation.)
This sequence avoids the “meta-directing trap”: if you oxidise to COX2H too early, nitration of benzoic acid gives mainly the meta product, not the para target.
Start: benzene.
Intermediate: para-nitrotoluene (major target isomer to isolate).
Target: 4-nitrobenzoic acid.
7 Exam Strategies
Mechanism marks: Include curly arrows, partial charges, and intermediate structures. Label slow step if relevant.
Synthetic planning: Work backwards (retrosynthesis) to identify functional group transformations. Mention reagents, conditions (temperature, catalyst), and by-products.
Spectroscopy interpretation: Combine data types systematically-deduce degree of unsaturation from 22C+2−H; use IR to identify functional groups; match NMR signals to structure.
Paper 4: When planning organic experiments, highlight drying agents, reflux/distillation apparatus, hazard mitigation (e.g. LiAlHX4 reacts violently with water).
8 Common Pitfalls
Forgetting to regenerate catalyst in electrophilic aromatic substitution mechanism.
Confusing reagent choices (e.g. NaBH4 reduces ald/ket but not carboxylic acids).
Ignoring stereochemistry in addition reactions (e.g. hydrogenation produces syn addition).
Misreading NMR integration (ratios must be simplified to whole numbers).
9 Quick Drills
Predict products (with mechanisms) when 2-bromobutane reacts with (a) NaOH(aq) and (b) NaOEt(ethanol) under reflux.
Deduce the structure of compound X (Mr=88) given: IR strong peak at 1715cm−1, 1H NMR shows (i) a quartet at 4.1ppm (integration 2), (ii) a triplet at 1.3ppm (integration 3), and (iii) a singlet at 2.0ppm (integration 3). Suggest identity and justify.
Outline reagents and conditions to convert propanone to 2-methyl-2-propanol.
Common exam mistakes
Mistake: Drawing curly arrows from the wrong atom or bond - arrows must originate from an electron-rich site (lone pair or bond) and point towards the electron-poor site; reversing the direction loses mechanism marks immediately.
Mistake: Omitting reagents or conditions in synthesis questions - writing only a product without specifying the reagent (e.g. "NaBHX4, aqueous") and condition (e.g. "room temperature") typically loses a mark per step.
Mistake: Using NaBHX4 to reduce a carboxylic acid - NaBHX4 reduces aldehydes and ketones but not carboxylic acids; LiAlHX4 in dry ether is required for carboxylic acids.
Mistake: Forgetting to regenerate the catalyst when drawing the electrophilic aromatic substitution mechanism - the halogen carrier (FeClX3 or AlClX3
Mistake: Confusing primary and secondary haloalkane mechanisms - primary substrates favour SN2 with clean inversion; tertiary substrates favour SN1 with racemisation; applying the wrong pathway to the substrate costs marks.
Mistake: Assigning NMR peaks without checking integration - the area under each peak gives the relative number of protons, and students often match chemical shifts to functional groups without confirming the ratio fits the molecular formula.
Mistake: Nitrating benzene at too high a temperature in synthesis planning - keeping the reaction below 55∘C gives mononitration; higher temperatures risk dinitration, which is rarely the target product.
Frequently asked questions
Do I need to know R/S and E/Z nomenclature for H2 Chemistry? No. The SEAB H2 Chemistry (9476) syllabus requires you to identify chiral centres and describe geometric isomerism using cis/trans terminology only. R/S and E/Z nomenclature are not examinable, though understanding inversion of configuration in SN2 reactions is required.
What is the most mark-heavy section in organic chemistry? Multi-step synthesis questions in Paper 3 and mechanism questions (curly arrows, intermediates, products) typically carry the most marks. Spectroscopy interpretation - combining IR, \(^1)H NMR, and mass spectrometry data - is also a common source of structured marks.
How do I avoid the meta-directing trap in synthesis planning? Attach or retain the ortho/para director (e.g. alkyl or amino group) before performing electrophilic substitution to get the desired ring position. If you oxidise a methyl group to −COOH before nitrating, the electron-withdrawing carboxyl group directs mainly to the meta position.
Which reducing agent should I use for each functional group? NaBHX4 (aqueous) reduces aldehydes and ketones to alcohols. LiAlHX4 (dry ether) is required for carboxylic acids, esters, amides, and nitriles. Using NaBHX4 where LiAlHX4 is needed - or vice versa - is a common source of lost marks.
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