Lecture 9 - Stereochemistry 2/4
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Name the two types of configurational isomers
geometric and optical isomers
We know that we change conformations/conformational isomers by rotating bonds. However, how do we change configurational isomers?
BREAKING BONDS
Configurational isomers are…
DIFFERENT COMPOUNDS
Draw the configurational isomers of but-2-ene. Label them cis/trans and E/Z
You can’t go from cis but-2-ene to trans without…
BREAKING A BOND
Optical isomerism occurs between isomers that are…
chiral
What is a chiral compound?
A compound with no plane of symmetry within it. They contain an stereoogenic centre - an atom bonded to four different groups.
What do we call molecules with a plane of symmetry?
achiral
Name and draw the only achiral amino acid. Why is it achiral?
• glycine/Gly/G • has a plane of symmetry so it is achiral • no chiral atom -central atom is bonded to two H atoms (therefore it isn’t bonded to all different groups)
Alanine is an amino acid but unlike glycine it is ….. Why? Draw it?
chiral. It does not have ap lane of symmetry It contains a carbon atom bonded to 4 different groups- stereogenic centre. The central carbon is bonded to amine group, carboxylic acid group, methyl group AND a hydrogen.
What is another name for an optical isomer?
enantiomer
Define enantiomers
Two non-superimposable mirror image structures
Optical isomers/enantiomers have the same….
physcial properties- melting point, boiling point, NMR
BUT what is the one difference between optical isomers
They rotate plane polarised light in equal but opposite directions.
Draw the optical isomers of alanine
Give the name and symbol for light that is moved anticlockwise
levorotatory - L- negative
Give the name and symbol for light that is moved clockwise
dextrorotatory - D - positive
When we label stereoisomers with +/d and -/l it has nothing to do with how we draw the molecule. It is something you have to…
measure. You cannot draw a molecule based upon how it rotates plane polarised light.
The alpha D equation
How do you get from cm to dm?
divide by 10
Rotation of light only occurs in compounds not with achiral. Therefore, the alpha D of an achiral compound is…?
0
What is a racemic mixture?
a 50:50 mixture of two enantiomers.
What is the alpha D for racemic mixture?
• This is because the two enantiomers rotate PPL in opposite direction. The movement of light in opposite directions cancel each other out.
Calculate the Alpha D of pure cocaine when…. - 1.2g of cocaine in 7.5ml of CHCl3 solvent. - the sample tube is 5cm - the observed rotation is -1.3 degrees.
