4.5Unit 4. Rates, Equilibria and Further Organic Chemistry
Organic Chemistry: Chirality — IAL Edexcel Modular (Chemistry) Revision Notes & Topical Questions
5 min read 11 parts Lesson 5 of 8 1 quiz
Lesson summary
4.5 Organic Chemistry: Chirality — IAL EDEXCEL MODULAR (CHEMISTRY) notes: Identify chiral centres and optical isomers, explain optical activity and racemic mixtures, and use optical activity of products as evidence for reaction mechanisms.Chiral carbon (chiral centre) = a carbon …
- Optical isomers (enantiomers)
- Number of isomers
- Examples of chiral molecules
- Biological differences
- Plane-polarised light
- Racemic mixture (racemate)
Section 1 of 11Chirality and optical isomers
1Chiral carbon (chiral centre)
Chiral carbon (chiral centre) a carbon atom with four different atoms or groups attached
- It is asymmetric, with no plane of symmetry, and is often marked C*.
- Chiral comes from the Greek word for hand.
2Optical isomers (enantiomers)
- Optical isomers (enantiomers) the two non-superimposable mirror images of a chiral molecule
- Like left and right hands, they cannot be superimposed.
3Number of isomers
- Number of isomers one chiral centre always gives two enantiomers, and n chiral centres give 2n stereoisomers
4Examples of chiral molecules
- Examples of chiral molecules butan-2-ol, 2-hydroxypropanoic acid (lactic acid) and alanine
Section 2 of 11Drawing optical isomers
1
Mark the chiral carbon, which has four different groups.
2
Draw the molecule in three dimensions with two bonds in the plane, one wedge and one dashed bond.
3
Draw its mirror image across a mirror line, including the wedge and dashed bonds.
4
Check that the two cannot be superimposed.
Section 3 of 11Properties of optical isomers
Physical and chemical properties are identical, with exceptions
- Enantiomers differ in how they rotate plane-polarised light.
- They differ in how they react with other chiral molecules and biological sensors.
Biological differences
- Biological differences enzymes and receptors interact with one enantiomer specifically
- One enantiomer of carvone smells of spearmint and the other of caraway.
- Thalidomideone enantiomer treated morning sickness and the other caused birth defects.
Section 4 of 11Optical activity
Plane-polarised light
- Plane-polarised light light that vibrates in one plane only, produced by a polariser
- A polarimeter measures the rotation.
Rotation
- Rotation each enantiomer rotates the plane by the same angle in opposite directions
- One rotates it clockwise and the other anticlockwise.
Labels
- Labels the isomers can be labelled (+) and (−), d and l, D and L, or R and S
Section 5 of 11Racemic mixtures
Racemic mixtures
Racemic mixture (racemate)
Racemic mixture (racemate) a mixture containing equal amounts of each enantiomer
- One enantiomer rotates light clockwise and the other anticlockwise, so the net rotation is zero.
- A racemate is optically inactive.
Synthesis
- Synthesis reactions from non-chiral starting materials usually give racemates
Drugs
- Drugs about 56% of drugs are chiral and 88% of those are sold as racemic mixtures
- Making a single enantiomer (enantiopure) is expensive and time consuming.
- Ibuprofen is sold as a racemic mixture.
Section 6 of 11Optical activity and the SN1 mechanism
Optical activity and the SN1 mechanism
The C–X bond breaks heterolytically in the slow step, leaving a trigonal planar carbocation.
The nucleophile attacks the carbocation from either side with equal probability.
A 50 : 50 mixture of enantiomers forms.
- Evidence
- an SN1 reaction of a chiral halogenoalkane gives a racemic, optically inactive product
Section 7 of 11Optical activity and the SN2 mechanism
1
SN2 is a one-step reaction in which the nucleophile attacks from the side opposite the leaving group
- The halogen causes steric hindrance on its own side, so frontal attack does not occur.
- The C–Nu bond forms as the C–X bond breaks.
2Inversion of configuration
- Inversion of configuration the molecule turns inside out like an umbrella in the wind
- An optically active reactant gives an optically active product with the opposite arrangement.
Section 8 of 11Nucleophilic addition to carbonyls
Nucleophilic addition to carbonyls
Cyanide attacks the planar C=O from above or below with equal probability
- An aldehyde or unsymmetrical ketone gives a racemic hydroxynitrile.
Planar groups
- Planar groups the carbonyl carbon and a carbocation are trigonal planar, with 120° angles
Section 9 of 11More from the specification
More from the specification
Polarimetric rotation of optical isomers
- Polarimetric rotation of optical isomers measuring plane-polarized light rotation angles provides a physical means to distinguish enantiomers.
Substitution
- One atom or group is replaced by another.
Nucleophilic substitution
- Replacement of an atom or group by a nucleophile.
Section 10 of 11Key terms
| Term | Meaning |
|---|---|
| Chiral centre | A carbon atom bonded to four different atoms or groups. |
| Enantiomers | Non-superimposable mirror image isomers of a chiral molecule. |
| Optical activity | The ability to rotate the plane of plane-polarised light. |
| Racemic mixture | An equal mixture of two enantiomers, which is optically inactive. |
| Enantiopure | Containing only one enantiomer. |
| Inversion of configuration | The reversal of the arrangement of groups around a carbon in an SN2 reaction. |
Section 11 of 11Exam technique
1
Calling a carbon chiral when two of its groups are the same
2
Saying a racemic mixture rotates light — the equal and opposite rotations cancel
3
Saying SN1 gives inversion — SN1 gives a racemic mixture and SN2 gives inversion
4
Drawing the mirror image without reflecting the wedge and dashed bonds
5
Saying enantiomers differ in melting point — their physical properties are identical apart from rotation of light
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