Topic 13: An Introduction to AS Level Organic Chemistry

Question 1

Definitions I:
(a) Hydrocarbon
(b) Homologous series
(c) Functional group

Answer 1

Definitions I:
(a) Compound containing only carbon and hydrogen atoms.
(b) Series of compounds with same functional group, differing by CH₂.
(c) Group of atoms dictating chemical properties, e.g., -OH.

Question 2

Definitions II:
(a) Nucleophile
(b) Electrophile
(c) Structural isomerism

Answer 2

Definitions II:
(a) Electron-pair donor attacking electron-deficient sites, e.g., OH⁻.
(b) Electron-pair acceptor attacked by nucleophiles, e.g., H⁺.
(c) Isomers with different structural arrangements of atoms.

Question 3

Definitions III:
(a) Geometrical isomerism
(b) Chiral centre
(c) Free radical

Answer 3

Definitions III:
(a) Isomerism from restricted rotation, e.g., cis/trans in alkenes.
(b) Carbon with four different groups, causing optical isomerism.
(c) Reactive species with an unpaired electron, e.g., Cl*.

Question 4

Name these compounds: CH₃CH₂OH and CH₃COOCH₂CH₃ using systematic nomenclature. [2 Marks]

Answer 4

CH₃CH₂OH: ethanol (1 mark)
CH₃COOCH₂CH₃: ethyl ethanoate (1 mark)

Question 5

Draw the displayed formula for pent-2-ene and deduce its empirical formula. [2 Marks]

Answer 5

Displayed: CH₃-CH=CH-CH₂-CH₃ (1 mark)
Empirical: CH₂ (C₅H₁₀ ÷ 5) (1 mark)

Question 6

In ethene (C₂H₄), describe the shape and bonding around the carbon atoms. [3 Marks]

Answer 6

Planar around C, 120° bond angles (1 mark)
sp² hybridisation, one σ (sp²-sp²), one π (p-p) (1 mark)
Double bond restricts rotation (1 mark)

Question 7

What type of fission occurs in:
(a) Cl₂ → 2Cl* under UV light? [1 Mark]
(b) HBr → H⁺ + Br⁻ in water? [1 Mark]

Answer 7

(a) Homolytic, bond splits evenly to radicals (1 mark)
(b) Heterolytic, bond splits unevenly to ions (1 mark)

Question 8

Identify the isomers of C₄H₈ and classify them as structural or stereoisomers. [3 Marks]

Answer 8

But-1-ene, but-2-ene (cis/trans), methylpropene (1 mark)
But-1-ene vs. methylpropene: structural (positional) (1 mark)
But-2-ene cis vs. trans: geometrical (1 mark)

Question 8

Why does but-2-ene exhibit cis/trans isomerism but but-1-ene does not? [2 Marks]

Answer 8

But-2-ene: C=C mid-chain, restricted π rotation (1 mark)
But-1-ene: C=C at end, no cis/trans possible (1 mark)

Question 9

In a lab, propene reacts with HBr. Show the mechanism using curly arrows. [2 Marks]

Answer 9

CH₃CH=CH₂ + HBr → CH₃CH⁺CH₃ (electrophilic H⁺, arrow from π) (1 mark)
CH₃CH⁺CH₃ + Br⁻ → CH₃CHBrCH₃ (nucleophilic Br⁻, arrow from lone pair) (1 mark)

Question 10

Predict the hybridisation and bond angles in CH₃CHO (ethanal). [2 Marks]

Answer 10

C in CH₃: sp³, 109.5°; C in CHO: sp², 120° (1 mark)
O in C=O: sp², planar carbonyl (1 mark)

Question 11

Define the reaction type for CH₃CH₂Cl + OH⁻ → CH₃CH₂OH + Cl⁻ and name the mechanism. [2 Marks]

Answer 11

Substitution, replaces Cl with OH (1 mark)
Nucleophilic substitution mechanism (1 mark)