PYQ Vault

Playbook

Basic Principles of Organic Chemistry

Naming, isomers, electron effects and reactive intermediates, then purification and the detection and estimation of elements. The heaviest chapter on the recent papers, and the base for every organic chapter.

Questions in the bank
243
q/paper in 2025–26
2.05
Calculation
35%
Notes pages
9

Strand: Structure and recall

When you’ll see it

A compound to name, isomers or stereoisomers to count, species to rank by electron shifts, or a lab method: a purification, a chromatogram, a sodium-fusion test or an element's percentage.

How this chapter is tested

The chapter has two halves. The first is how organic molecules are built: IUPAC names, structural and stereo isomers, the four electronic effects, and the stability of carbocations, radicals and carbanions. Each question there turns on one rule applied in the right order: seniority before numbering, numbering before alphabetical order, resonance before hyperconjugation.

The second half is laboratory method: which distillation suits a mixture, what Rf and the order of elution mean, what colour each sodium-fusion test gives, and the percentage of N, C, H, a halogen, S or P from a mass or a gas volume. Most numeric answers sit here, and the marks go to arithmetic done without a slip: the factor 2 for H₂SO₄ in Kjeldahl, 2/18 for hydrogen, 62/222 for phosphorus.

The counting questions (isomers, stereoisomers, chiral compounds in a list, hyperconjugating hydrogens) have no options to check against, and one missed meso form or one chain counted twice costs −1. Write every structure out in full before counting.

The sub-skills

The distinct skills inside the chapter, in the order to learn them.

  • Naming and counting bonds

    Seniority COOH > SO₃H > COOR > COCl > CONH₂ > CN > CHO > C=O > OH > NH₂; principal group, then multiple bonds, then prefixes take the lowest locants; prefixes go alphabetically, with di- and tri- ignored.

  • Structural isomers

    Name what changed: skeleton (chain), position, functional group, the alkyls around one group (metamerism), or a ring for a double bond. Build counts from the degree of unsaturation, longest chain first.

  • Stereoisomers and conformations

    cis–trans needs two different groups on each C=C carbon; a chiral carbon has four different groups, D counting apart from H; at most 2ⁿ stereoisomers, fewer when a meso form exists; staggered ethane is the stable rotamer.

  • Electronic effects and acidity

    Inductive, resonance and hyperconjugation are permanent, the electromeric effect temporary; rank resonance structures by full octets, no charge separation and negative charge on the electronegative atom; a more stable conjugate base means a stronger acid.

  • Intermediates and reagents

    Carbocations 3° > 2° > 1° > CH₃⁺, with benzylic, allylic and tropylium above; carbanions run the other way; hydride affinity is the reverse of cation stability; 1,2-shifts and ring expansion move the charge.

  • Purification and chromatography

    Simple, fractional, reduced-pressure or steam distillation by boiling-point gap and stability; sublimation, crystallisation and acid–base extraction. Rf is measured from the base line; on silica a more polar compound has a lower Rf and elutes later.

  • Lassaigne's test and nitrogen

    Sodium fusion gives NaCN (only if carbon is present), Na₂S and NaX; boil with HNO₃ before AgNO₃. Dumas collects N₂ (subtract the aqueous tension; 28 g per 22 400 mL); Kjeldahl traps NH₃ in acid and fails for nitro, azo and ring nitrogen.

  • Estimating C, H, halogens, S and P

    C is 12/44 of CO₂, H is 2/18 of water, O by difference; Carius gives AgCl, AgBr or AgI (143.5, 188, 235), BaSO₄ (32/233) and Mg₂P₂O₇ (62/222).

Traps to expect

Distractor shapes this chapter reuses. The Traps page covers the ones that cut across chapters.

  • The dibasic Kjeldahl acid

    One H₂SO₄ neutralises two NH₃. Dropping the factor of 2 gives exactly half the percentage, and that value is always an option.

  • Hydride affinity read as stability

    A more stable carbocation has the LOWER hydride affinity. Options print the stability order and its reverse; pick by what is asked.

  • Prefixes in locant order

    The name is 3-ethyl-1,1-dimethylcyclohexane, not 1,1-dimethyl-3-ethyl: prefixes go alphabetically and di- is ignored. The wrong option obeys every other rule.

  • 2ⁿ without the meso check

    Tartaric acid has two chiral centres but three stereoisomers. Look for identical halves before writing 2ⁿ, and count a stereogenic C=C as a unit too.

  • The paper as the stationary phase

    In paper chromatography the stationary phase is the water held in the pores, and the method is partition. TLC and column chromatography work by adsorption.

Learn it before you drill it

This chapter has full teaching notes — foundations, worked examples, self-checks and a mastery check for each page. Read the notes once, then drill page by page below.

Basic Principles of Organic Chemistry notes

Drill every Basic Principles of Organic Chemistry question

243 questions from the bank, across 9 subtopics.

Drill the 243 questions

Drill one subtopic at a time

The 9 subtopics, in teaching order.

Related playbooks

Often paired with this one — the technique or the trap overlaps. Drill these next.