AP | A-Level | IB | NCERT 11 + 12 – FREE NOTES, RESOURCES AND VIDEOS!
Edexcel® A-Level
YouTube Channel
YouTube Channel
@chemistry.student
*Revision Materials and Past Papers* 1 Atomic Structure and the Periodic Table 2 Bonding and Structure 3 Redox I 4 Inorganic Chemistry and the Periodic Table 5 Formulae, Equations and Amounts of Substance 6 Organic Chemistry I 7 Modern Analytical Techniques I 8 Energetics I 9 Kinetics I 10 Equilibrium I 11 Equilibrium II 12 Acid-base Equilibria 13 Energetics II 14 Redox II 15 Transition Metals 16 Kinetics II 17 Organic Chemistry II 18 Organic Chemistry III 19 Modern Analytical Techniques II RP Required Practicals

17 Organic Chemistry II

17A Chirality 17B Carbonyl Compounds 17C Carboxylic Acids, Acyl Chlorides, Esters, and Polyesters

Chirality

Specification Reference Topic 17, points 1–5 (Edexcel A-Level Chemistry)

Quick Notes

  • Chiral centre: Carbon atom bonded to 4 different groups.
  • Two molecules that are non-superimposable mirror images of each other are called enantiomers or optical isomers.
  • Optical Activity: A single enantiomer will rotate plane-polarised light. One rotates light clockwise (+) and the other anticlockwise (–).
  • Racemic Mixture: A 50:50 mixture of two optical isomers. Racemic mixtures are optically inactive because the effect of each optical isomer on plane-polarised light is cancelled out.
  • SN1 vs SN2 Mechanisms:
    • SN1 often forms a racemic mixture because the planar intermediate is attacked equally from both sides.
    • SN2 produces a single enantiomer, leading to inversion of configuration.

Full Notes

Optical Isomers

If a carbon atom is bonded to four different atoms or groups it is called a chiral carbon (giving a chiral centre in a molecule).

Edexcel A-Level Chemistry diagram showing a chiral centre in a hydroxynitrile with four different groups attached.

There are two possible ways the atoms or groups can be arranged giving two possible stereoisomers.

The isomers are non-superimposable mirror images of one another and are called optical isomers (enantiomers).

Edexcel A-Level Chemistry schematic of two non-superimposable mirror image enantiomers.

Example 2-hydroxypropanoic acid (Lactic Acid)

Edexcel A-Level Chemistry structure of lactic acid showing the chiral centre CH₃CH(OH)COOH and the two enantiomers.

Chiral carbon: CH3CH(OH)COOH

Two enantiomers exist as mirror images.

Some molecules can have more than one chiral centre in them, especially biological molecules such as amino acids.

Optical Activity

Optical isomers rotate plane-polarised monochromatic light in opposite directions.

Edexcel A-Level Chemistry diagram of plane-polarised monochromatic light passing through solutions of enantiomers.

Example Plane-polarised monochromatic light

Plane-polarised monochromatic light is light of only one wavelength that vibrates in one direction only

Racemic Mixtures (Racemates)

A racemic mixture (or racemate) contains equal amounts of two enantiomers.

Since each enantiomer rotates plane-polarised light in opposite directions, a racemic mixture is optically inactive because the rotations of each enantiomer cancel each other out.

Racemic mixtures often form when a reaction involves an intermediate that is planar, such as in the SN1 mechanism. This allows the nucleophile to attack from either side with equal probability.

Chirality and Reaction Mechanisms: SN1 vs SN2

The optical activity of product mixtures can give clues about the type of mechanism that a reaction follows, especially for SN1 and SN2 reactions.

SN1 Reactions

SN1 reactions proceed via a carbocation intermediate, which is planar.

Nucleophilic attack can occur from either side with equal probability, forming the two possible optical isomers in equal amounts. This gives a racemic mixture.

Edexcel A-Level Chemistry SN1 mechanism with planar carbocation intermediate leading to racemic mixture.

If a product mixture is optically inactive but formed from an optically active compound, it suggests an SN1 mechanism.

SN2 Reactions

SN2 reactions involve a single-step mechanism where the nucleophile attacks from the opposite side to the leaving group.

The incoming nucleophile ends up bonding in the opposite position to the leaving group, causing an inversion of the configuration.

Edexcel A-Level Chemistry SN2 mechanism showing backside attack and inversion of configuration to give a single enantiomer.

This produces only one optical isomer, meaning the product mixture is optically active.

Summary