Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.
ORGANIC CHEMISTRY II
17A: CHIRALITY
1. know that optical isomerism is a result of chirality in molecules with
a single chiral centre
Optical isomerism:
- Optical isomerism is a type of stereoisomerism.
o Stereoisomerism – molecules with the same molecular and structural
formulae but different arrangement of atoms in 3D space.
- Optical isomers are molecules that are the mirror image of each other and
that cannot be superimposed onto each other.
o Optical isomers are also called enantiomers.
Chirality:
- Optical isomerism is a result of chirality in molecules with a chiral centre.
- Chiral molecules are those that cannot be superimposed onto their mirror
image.
- Common chiral molecules have a saturated carbon atom that is bonded to
four different groups.
o A carbon atom bonded to three different groups is not chiral as it is
planar.
- A chiral centre (= chiral carbon = asymmetrical carbon) is the saturated
carbon atom that is bonded to four different groups.
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 non-superimposable mirror images
The importance of symmetry:
- Asymmetric objects are objects that do not possess a plane of symmetry.
- When an object possessing a plane of symmetry is reflected in a mirror, its
mirror image can be superimposed.
o The mirror image can be placed onto the original and it will look
identical.
- When an asymmetric object that does not possess a plane of symmetry is
reflected in a mirror, its mirror image cannot be superimposed, no matter
how much it is rotated.
o This molecule is therefore chiral.
Properties of enantiomers:
,- Same infrared and mass spectrum – as the bonds present will be the same in
both enantiomers.
- Same physical properties (e.g. boiling point, density).
- Same chemical properties (e.g. reaction with Br 2) – same bonds and
functional groups present.
- Different biological activity (e.g. smell, taste, pharmacological effects).
o Proteins are made up of chiral amino acids.
o Receptors and enzymes will have a binding site or active site with a
specific shape that is only complementary to one enantiomer.
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 – the ability of a single optical isomer to rotate the plane of
polarisation of plane-polarised monochromatic light.
- Chiral molecules demonstrate optical activity (i.e. they are optically active) –
they are able to rotate the plane of plane-polarised light.
- Non-chiral molecules are optically inactive and cannot rotate the plane of
plane-polarised light.
Effect of chiral compounds on plane-
polarised light:
- Passing normal light through a
polariser restricts the light travelling
through so that it oscillates in a single plane, producing plane-polarised light.
- We can then detect the optical rotation using a polarimeter.
o Optical rotation is the angle through which the plane of plane-polarised
light is rotated by an enantiomer.
- When plane-polarised-light travels through a solution containing a single
enantiomer of a chiral compound, the plane of plane-polarised light is rotated
by α degrees.
o A solution containing the other
enantiomer will rotate the
plane of plane-polarised light
by –α degrees.
- In other words, one enantiomer
rotates the plane of plane-polarised
light clockwise (+) whilst the other enantiomer rotates the plane of plane-
polarised light anti-clockwise (-).
o Each enantiomer will rotate the plane of plane-polarised light in equal
and opposite directions (e.g. 30o, –30oC)
4. understand the nature of a racemic mixture
Racemic mixture:
, - A racemic mixture contains equimolar (50:50) quantities of enantiomers of a
chiral compound.
- A solution containing a racemic mixture will be optically inactive.
o The optical rotation of one enantiomer will be cancelled out by the
equal and opposite optical rotation of the other enantiomer.
- Non-chiral molecules will also not rotate the plane of plane-polarised light.
5. be able to use data on optical activity of reactants and products as
evidence for SN1 and SN2 mechanisms
SN1 mechanism: involves one species in the rate-determining step
(unimolecular).
- Involves a tertiary (or secondary) halogenoalkane.
o E.g. 2-chloro-2-methylpropane reacting with aqueous OH - ions.
- The shape of the carbocation intermediate formed is trigonal planar.
- This means that the nucleophile has an equal chance of attacking from above
or below the plane.
- SN1 reactions therefore form a racemic mixture of both enantiomers.
SN2 mechanism: involves two species in the rate-determining step (bimolecular).
- Involves a primary (or secondary) halogenoalkane.
o E.g. 1-chloropropane reacting with aqueous OH - ions.
- The nucleophile can only attack from the back of the C-X bond (from behind
the halogen atom).
- This causes the remaining 3 atoms bonded to the carbon to turn inside
outside like an umbrella.
o This process is described as an inversion.
- SN2 reactions therefore only form one enantiomer.
Optical activity as evidence for organic mechanisms:
- A solution that is optically inactive could be achiral or racemic.
- If an optically pure sample of 2-bromobutane (containing only a single
enantiomer) was reacted with aqueous OH - ions and the resulting product was
optically inactive, we can deduce that the reaction proceeded via the S N1
mechanism.
o In an SN1 reaction, a trigonal planar intermediate is formed.
o The nucleophile has an equal chance of attacking from below or below
the plane.
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