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Organic Chemistry II

Chirality

  • 3 things to know
  • Papers 2 and 3

Overview

Chiral centres and optical isomers, how they rotate plane-polarised light, racemic mixtures, and using the optical activity of the product to tell an Sₙ1 mechanism through a planar carbocation from an Sₙ2 mechanism with inversion.

What you need to know

Answer each one before you reveal it.

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Process

What happens to the optical activity when a single optical isomer undergoes an Sₙ1 reaction?

Key term

What happens to the configuration at the chiral centre during an Sₙ2 reaction?

Key term

What molecular feature gives rise to optical isomerism?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What molecular feature gives rise to optical isomerism?

    Model answer

    Handedness in a molecule carrying one chiral carbon

  2. 2

    Define a chiral centre.

    Model answer

    An asymmetric carbon atom bonded to four different groups

  3. 3

    What is the relationship between two optical isomers?

    Model answer

    They are non-superimposable mirror images of each other

  4. 4

    What is optical activity?

    Model answer

    One optical isomer turns plane-polarised light of a single wavelength clockwise; its mirror image turns it by an equal angle the other way.

  5. 5

    What is a racemic mixture?

    Model answer

    A mixture containing equal amounts of both optical isomers (enantiomers)

  6. 6

    Why does a racemic mixture show no net optical activity?

    Model answer

    Because the rotations of the two enantiomers cancel out

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