Detailed notes on organic chemistry II - goes into detail for mechanisms and reaction conditions. Notes written using Edexcel Chemistry textbooks, past papers and more. Written by a student with all A*s at GCSE, 3A* predictions at A Level and with an offer for Natural Sciences at Cambridge.
Topic 17: Organic Chemistry II
Topic 17A: Chirality
1. know that optical isomerism is a result of chirality in molecules with a
single chiral centre
2. understand that optical isomerism results from chiral centre(s) in a
molecule with asymmetric carbon atom(s) and that optical isomers are
object and nonsuperimposable mirror images
Optical isomerism
Optical isomerism is a type of stereoisomerism (same
structural formula but the atoms occupy a different
position in space).
Chiral carbon: a carbon atom with 4 different groups attached to it
Organic molecules with chiral carbons show optical isomerism. This is
because the groups can be arranged around that chiral carbon in two
different ways – they are asymmetric.
The two optical isomers, or enantiomers, are non-superimposable
mirror images of each other. If you rotate the molecules, they can
never look exactly the same. Enantiomers should be drawn in
tetrahedral structure mirrored like the image to the right.
Enantiomers have the same chemical properties and similar physical properties.
3. know that optical activity is the ability of a single optical isomer to rotate
the plane of polarisation of plane-polarised monochromatic light in
molecules containing a single chiral centre
Optical Activity
Ordinary light is made of a range of wavelengths and vibrates in all
directions. Light can be polarised to monochromatic, plane-polarised
light, which has a single wavelength and vibrates in one direction.
Racemic mixture: a mixture of equal amounts of each enantiomer of an optical isomer.
One enantiomer of a pair of optical isomers rotates the
plane of polarisation clockwise (this is the + isomer). The
other isomer (- isomer) rotates the plane of polarisation
anticlockwise. If there is only one of a pair of optical
isomers, the solution is optically active i.e. the plane of
polarisation is rotated. In a racemic mixture, the rotations
cancel each other out so the solution is optically inactive.
When plane-polarised light is passed through an optically active solution, the plane of
polarisation is rotated.
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