Topic 29: An Introduction to A Level Organic Chemistry

Question 1

Definitions I:
(a) Functional group
(b) Electrophilic substitution
(c) Optical isomerism

Answer 1

Definitions I:
(a) Group of atoms dictating properties, e.g., -OH in alcohols.
(b) Electrophile replaces an atom on aromatic ring, e.g., Br⁺ on benzene.
(c) Isomerism with mirror-image enantiomers, differing in light rotation.

Question 2

Name these compounds systematically:
(a) CH₃CH₂COOCH₂CH₃ [1 Mark]
(b) C₆H₅NO₂ [1 Mark]

Answer 2

(a) Ethyl propanoate (1 mark)
(b) Nitrobenzene (1 mark)

Question 3

Draw the displayed formula for cyclopentanol and name it. [2 Marks]

Answer 3

Cyclopentane ring with -OH on C1 (1 mark)
Named cyclopentanol (1 mark)

Question 4

Describe the shape of benzene and its bonding. [2 Marks]

Answer 4

Planar, 120° angles, sp² hybridised C (1 mark)
6 σ bonds, delocalised π system (1 mark)

Question 5

Show the mechanism for benzene reacting with Br₂ (AlBr₃ catalyst). [3 Marks]

Answer 5

Br₂ + AlBr₃ → Br⁺ + AlBr₄⁻ (electrophile forms) (1 mark)
Br⁺ attacks ring, forms carbocation intermediate (1 mark)
H⁺ lost, bromobenzene forms (1 mark)

Question 5

In a lab, CH₃CH(OH)CH₃ rotates plane-polarised light. Explain its chirality. [2 Marks]

Answer 5

Chiral C with 4 different groups (CH₃, H, OH, CH₃) (1 mark)
Rotates light, optically active (1 mark)

Question 6

Name this aromatic compound: C₆H₅CH₂OH and predict its reaction with KMnO₄. [2 Marks]

Answer 6

Phenylmethanol (benzyl alcohol) (1 mark)
Oxidised to C₆H₅COOH (benzoic acid) (1 mark)

Question 7

Why do enantiomers of a drug like ibuprofen differ in biological activity? [2 Marks]

Answer 7

Different 3D shapes fit receptors differently (1 mark)
One enantiomer active, other inert (1 mark)

Question 8

In a synthesis, how does a chiral catalyst produce pure CH₃CH(OH)CH₂CH₃? [2 Marks]

Answer 8

Catalyst controls stereochemistry at chiral C (1 mark)
Produces single enantiomer, e.g., R-form (1 mark)

Question 9

Define racemic mixture and describe its effect on plane-polarised light. [2 Marks]

Answer 9

50:50 mix of enantiomers (1 mark)
No net rotation, cancels out (1 mark)

Question 10

Write the skeletal formula for 2,4-dichlorophenol and identify its functional group. [2 Marks]

Answer 10

Benzene ring, Cl on C2/C4, OH on C1 (1 mark)
Functional group: -OH (phenol) (1 mark)

Question 11

Explain why benzene undergoes substitution, not addition, with Cl₂ (AlCl₃). [3 Marks]

Answer 11

Delocalised π electrons stabilise ring (1 mark)
Substitution retains aromaticity (1 mark)
Addition disrupts, less favoured (1 mark)