stereoisomers - ib chem · 2015-11-12 · stereoisomers molecules with the same molecular and...
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STEREOISOMERS
Types of isomers
Stereoisomers
Molecules with
the same molecular and structural formula,
but with
different spatial arrangements of atoms.
Types of stereoisomers
● Geometrical isomers– Cycloalkanes
– Linear alkenes
cis - trans
● Optical isomers
right (+) - left (-)
Cycloalkanes
Cis
substitutes on the same side of plane
Trans
substitutes on opposite sides of plane
Cyclopropane
Geometry does not allow rotation of single bonds
1,2-dichlorocyclopropane
1,2-dichlorocyclobutane
The same happens with 1,2-dimethylcyclopropane
Substituted alkenes
Cis
substitutes on the same side of double bond
Trans
substitutes on opposite sides of double bond
1,2-dichloroethene
2-butene
Geometrical isomers rule
Trans isomer is more stable
Trans fatty acids => higher b.p.
More linear => more dense packing => stronger V.d.Waals => higher b.p.
Cis alkenes => higher b.p.
Net polarity => dipole-dipole forces => higher b.p.
but-2-ene-1,4-dioic acid
Cis-isomer reacts more easily (at lower temperature)
cis & trans platin
Cis-platin = anti-cancer drug
Binds to DNA of cancer cells and blocks reproduction
Optical isomers
Images cannot get superimposed
Mirror images = enantiomers
Chiral = enantiomers exist
Chiral center =an atom bonded to four different groups of atoms
If there is a chiral center =>
Molecule is chiral=>
Enantiomers exist
Chiral molecules
Non-chiral molecules
Polarized light
Polarimeter
Enantiomers and rotation
Enantiomers rotate plane of polarization in different directions
L
R
Racemic mixtrure
Ratio of
enantiomers
50% - 50%
Chemical properties
Enantiomers react differently with other optically active compounds
Thalidomide
Aspartame
L-aspartame = sweet R-aspartame = bitter
Sn2 – complete inversion
Sn1 – racemic mixture
Practice questions (1)
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