Summary A LEVEL CHEMISTRY NOTES - Organic Chemistry II
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Course
Unit 17 - Organic Chemistry II
Institution
PEARSON (PEARSON)
- Detailed A- Level Chemistry Notes
- Clear, detailed notes with diagrams for ease of understanding
- Covers all specification points to ensure all content is covered
Included :
- chirality and optical isomerism
- carboxylic acids, acyl chlorides, carbonyl reactions
- aldehydes and ketones...
Must have chiral centre in order to form an optical isomer
Has 4 different groups attached to the central carbon atom
atoms can be arranged in 2 different ways which form the enantiomers (isomers that are mirror
images of each other and non super imposable)
How to nd the chiral centre?
Draw the molecule
Identify the carbon that has four different groups bonded to it. This is the chiral centre.
Draw the molecule in a tetrahedral shape with the chiral centre being the carbon in the middle
Draw the mirror image of this optical isomer you’ve just drawn- these are the 2 enantiomers
Non super imposable (mirror images) means that no matter which way you rotate them, they will
not overlap eg our hands are mirror images of each other but they’re non super imposable.
Optical isomerism is a result of chirality in molecules with a single chiral centre. It’s a type of
stereoisomerism in which the molecular formula is the same but the atoms are arranged
differently.
Two compounds that are optical isomers of each other are called enantiomers.
They have similar chemical and physical properties and rotate the plane of plane polarised light
equally but in different directions - one clockwise and the other anti-clockwise.
Optical activity
Occurs in carbon compounds with a chiral centre
Isomers that are optically active rotate the plane of plane polarised (monochromatic) light.
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, We can distinguish between pure samples of each optical isomer by
using a polarimeter measure the rotation of plane polarised light in opposite directions
Racemic mixture
An equimolar mixture of the two enantiomers
(Equal concentrations/proportions/50:50)
don’t rotate plane polarised light and so show no optical activity because the 2 enantiomers rotate
light in opposite directions so the rotations cancel out.
Forming a racemic mixture via an SN1 reaction
Forming a racemic mixture via an SN₂ reaction
, Aldehydes and ketones
Aldehyde - RCOH
All end in -al
C=O bond is always on the rst carbon
Ketone - RCOR
All end in -one
When ketones have 5 carbons or more in a chain, it needs a number to show the position of the
double bond. E.g. pentan-2-one
If there’s two ketone groups then di is put before –one and an “e” is added to the stem eg
heptane-2,4-dione
The pre x oxo- should be added if there’s a ketone group as well as a carboxylic acid/aldehyde
Aldehyde and ketones have low boiling points in comparison to alcohols with the same carbon chain
length.
Intermolecular forces between aldehyde and ketones
London forces
Permanent dipole- permanent dipole interactions as there’s polarity within the molecules
Solubility
Aldehydes/ ketones can hydrogen bond with water so some can dissolve in water
Lone pair of electrons on oxygen on the carbonyl (C=O) can form a hydrogen bond with the
slightly positive hydrogen atom on a water molecule
As the carbon chain length increases, solubility decreases. Why?
London forces between the non polar hydrocarbon chain will be stronger than the hydrogen
bonding between the slightly negative oxygen on the carbonyl and slightly positive hydrogen on
water molecules. So aldehydes/ketones with long carbon chains don’t dissolve.
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