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4.6.3Unit 4. Rates, Equilibria and Further Organic Chemistry

Mechanisms and Optical Activity — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 4 parts Lesson 44 of 56 Unit quiz

📚 IAL Edexcel Linear (Chemistry) Hub / Topical Revision
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Curriculum Equation Key معادلة تفاعل تكوين الإستر (الأسترة) - Esterification Reaction EquationFast Exam Solution
R-COOH + R2-OH ⇌ (conc. H₂SO₄) R-COO-R2 + H₂O
Word Equation & Conditions: حمض كربوكسيلي + كحول (في وجود حمض الكبريتيك المركز كعامل حفاز ونازع للماء) ⇌ إستر + ماء
Location & Specifications: الكيمياء العضوية للثانوية العامة والتحصيلي و Edexcel Chemistry.
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Part 1 of 4Substitution Mechanisms

The route by which a nucleophile attacks decides the optical activity of the product.

SN2
  • SN2 A nucleophile attacks from the side opposite the leaving group in a single step, so a single enantiomer gives a single, inverted product.
SN1
  • SN1 A tertiary halogenoalkane first loses the halide to form a planar carbocation intermediate.
Attack on SN1
  • Attack on SN1 The nucleophile can attack the planar carbocation from either side with equal probability.
SN1 result
  • SN1 result Equal amounts of both enantiomers form, so the product is a racemic mixture with no optical activity.

Part 2 of 4Nucleophilic Addition to Carbonyls

The carbonyl carbon is planar with bond angles of about 120°.

1
The cyanide ion, CN⁻, attacks the δ⁺ carbon of the C=O group in an aldehyde or unsymmetrical ketone.
2
Attack is equally likely from above or below the plane of the carbonyl group.
3
Each face of attack gives a different enantiomer of the hydroxynitrile after protonation.
4
The product is therefore a racemate and shows no optical activity.

Part 3 of 4Predicting the Outcome

Predicting the Outcome
Ethanal and HCN
  • Ethanal and HCN The product, 2-hydroxypropanenitrile, has four different groups on one carbon and forms as a racemate.
Propanone and HCN
  • Propanone and HCN The product carbon carries two identical methyl groups, so no chiral centre forms and no optical isomers exist.
Starting material
  • Starting material If the starting halogenoalkane is itself a single enantiomer, SN2 gives a single enantiomer and SN1 gives racemate.
Exam approach
  • Exam approach Draw the mechanism and mark the planar region to justify why both enantiomers are produced.

Part 4 of 4Exam Checklist & Tips

4.6.3 – Key Points for the Exam

  • Planar intermediate A flat species that can be attacked from either side.
  • Carbocation A positive carbon ion with three bonds.
  • Nucleophile Species that donates a lone pair to form a new bond.
  • Racemic mixture Equal amounts of both enantiomers.

Common Mistakes

  • Stating that SN1 gives a single product enantiomer.
  • Forgetting that propanone gives no optical isomers.
  • Not mentioning equal probability of attack from either side.
  • Confusing nucleophilic addition with electrophilic addition.

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