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4.5Unit 4. Rates, Equilibria and Further Organic Chemistry

Organic Chemistry: Chirality — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 11 parts Lesson 5 of 56 1 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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Lesson summary

4.5 Organic Chemistry: Chirality — optical isomers, racemates and mechanisms — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Identify chiral centres, draw and describe optical isomers, explain optical activity and racemic mixtures, and use optical activity as evidence for SN1, SN2 and ad…

  • Chiral centre (chiral carbon)
  • Racemic mixture (racemate)
  • Enantiopure
  • Stereoisomers
  • Enantiomer
  • Polarimeter

Part 1 of 6Chiral centres and optical isomers

Chiral centres and optical isomers
Chiral centre (chiral carbon)
  • Chiral centre (chiral carbon) a carbon atom bonded to four different atoms or groups;
    • it is asymmetric, with no plane of symmetry.
Chiral comes from the Greek for hand, describing the handedness of these molecules.
A molecule with one chiral centre exists as two optical isomers (enantiomers).
  • They are mirror images and are non-superimposable, like left and right hands.
  • One chiral centre always gives exactly two enantiomers.
Number of stereoisomers = 2n, where n is the number of chiral centres: two centres give four isomers (two pairs) and three give eight.
Butan-2-ol, 2-hydroxypropanoic acid (lactic acid) and alanine are chiral.

Part 2 of 6Drawing optical isomers

1
Look for a carbon with four different groups and mark it with an asterisk.
2
Draw the molecule in 3D with two bonds in the plane, one wedge and one dash.
3
Draw its mirror image across a mirror line, keeping the wedge and dashed bonds.
4
Check that the two cannot be superimposed.

Part 3 of 6Properties of enantiomers

Properties of enantiomers
Enantiomers have identical chemical properties with non-chiral reagents and identical physical properties, with two differences.
They rotate plane-polarised light in opposite directions by equal angles.
  • One enantiomer rotates it clockwise, labelled (+), and the other anticlockwise, labelled (−).
  • Other labels are d and l, D and L, or R and S.
They react differently with other chiral molecules, e.g. biological receptors and enzymes.
  • One enantiomer of carvone smells of spearmint and the other of caraway.
  • One thalidomide enantiomer treated morning sickness and the other caused birth defects.

Part 4 of 6Racemates and drugs

Synthetic drugs made from non-chiral materials are usually racemic mixtures, because producing one enantiomer is harder.
About 56% of drugs are chiral and about 88% of those are sold as racemic mixtures.
Enantiopure
  • Enantiopure a compound containing only one enantiomer.
Separating enantiomers is expensive and time-consuming,
  • so for many drugs it is not worthwhile even though only half the drug is active;
    • ibuprofen is sold as a racemate.

Part 5 of 6Nucleophilic addition to carbonyls

Nucleophilic addition to carbonyls
The carbonyl carbon is trigonal planar (120°).
CN− attacks the C=O of an aldehyde or unsymmetrical ketone from above or below with equal probability.
The product is a racemic hydroxynitrile that is optically inactive.

Part 6 of 6More from the specification

1Optical isomerism and enantiomeric rotation
  • Optical isomerism and enantiomeric rotation analyzing interaction with polarized light beams provides an analytical means to resolve racemic mixtures.
2Halogenoalkane
  • An alkane in which hydrogen atoms are replaced by halogen atoms.

Key terms

TermMeaning
StereoisomersMolecules with the same structural formula but a different arrangement of atoms in space.
Optical isomersStereoisomers that are non-superimposable mirror images of each other.
Chiral centreA carbon atom bonded to four different atoms or groups.
EnantiomerOne of a pair of optical isomers.
Optical activityThe ability to rotate the plane of plane-polarised light.
Racemic mixtureAn equimolar mixture of two enantiomers that is optically inactive.
PolarimeterAn instrument that measures the rotation of plane-polarised light.

Avoid these mistakes

1
Saying enantiomers differ in melting point — their physical properties are the same except for optical rotation.
2
Saying a racemate rotates light — the rotations cancel.
3
Forgetting the mirror image when drawing enantiomers — keep wedge and dash bonds.
4
Saying SN1 gives inversion — it gives a racemate; SN2 gives inversion.
5
Marking a carbon as chiral when two groups are the same — all four must be different.

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