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http://chirbase.u-3mrs.fr/chirality/laboratory/en/pres.htm Organic Chemistry 9 Stereoisomers IB Topic 20.6 Reference: Higher Level Chemistry, p. 408-415 1 Isomers - Review Isomers are molecules with the same molecular formula but different structural formulas. molecular formula = C4H10 H butane 2-methylpropane These isomers are called structural isomers. 2 Types of Isomers isomers structural isomers stereoisomers different order of attachment of atoms e.g. linear vs branched molecules e.g. different position of a double bond or functional group C4H8 C4H10 butane H 2-methylpropane but-1-ene but-2-ene 3 Types of Isomers stereoisomers atoms are joined in the same order, but they have a different arrangement in space (i.e. different “spatial” arrangements) 1. geometric isomers 2. optical isomers 4 Geometric Isomers Geometric isomers form when atoms are restricted from rotating around a bond. double C=C bonds restrict free rotation ring structures restrict free rotation 5 Geometric Isomers Geometric isomers are also known as cis-trans isomers. cis isomer = substituents on the same side trans isomer = substituents on opposite sides 6 Geometric Isomers Name these geometric isomers: example 1 trans-1,2-dichloroethene cis-1,2-dichloroethene example 2 cis-but-2-ene trans-but-2-ene 7 Geometric Isomers Name these geometric isomers of C3H4Cl2: Note: An alkane ring includes the prefix “cyclo” in its name. trans-1,2-dichlorocyclopropane cis-1,2-dichlorocyclopropane 8 Geometric Isomers Draw and name the geometric isomers of CH3CH2CH2CHCHCH3. TIP: Start by drawing the C=C bond first. Draw and name one unbranched structural isomer of this compound. 9 Geometric Isomers - Physical Properties 1. The boiling point of geometric isomers depends on the polarity of the molecule. ∂- ∂net dipole moment ∂+ NO net dipole moment ∂+ cis isomers are polar trans isomers are non-polar stronger dipole-dipole intermolecular forces weaker van der Waals intermolecular forces higher BP (60ºC) lower BP (48ºC) 10 Geometric Isomers - Physical Properties 2. The melting point of geometric isomers depends on the shape of the molecule. cis isomers have lower MP trans isomers have higher MP packing of cis isomers is looser - weaker IMFs packing of trans isomers is tighter - stronger IMFs melting point = 139 ˚C melting point = 287 ˚C 11 Geometric Isomers - Chemical Properties Cis isomers are more reactive than trans isomers. Functional groups are close together in cis isomers and can react. Functional groups are further apart in trans isomers and cannot react easily. 12 Optical Isomers Optical isomers are molecules that are mirror images, but which cannot be superimposed on each other. 13 Optical Isomers - Chiral Carbon Atoms Optical isomers must have a carbon atom known as a chiral carbon atom. A chiral carbon atom is asymmetric, and has four different atoms or groups bonded to it. 14 Optical Isomers - Chiral Carbon Atoms Chiral carbon atoms do NOT have a plane of symmetry. non-chiral carbon atom chiral carbon atom 15 Optical Isomers - Chiral Carbon Atoms Examples of Molecules with Chiral Carbon Atoms glycine - none alanine - carbon 2 ketose sugar - carbon 3 and 4 16 Optical Isomers - Chiral Carbon Atoms Examples of Molecules with Chiral Carbon Atoms 17 Optical Isomers Optical - Chiral Isomers Carbon Atoms Mark the chiral atom in these molecules with an asterisk (*). no chiral carbon atom * * 18 Optical Isomers - Chiral Carbon Atoms Mark the chiral atom in these molecules with an asterisk (*). no chiral carbon atom no chiral carbon atom * 19 Optical Isomers - Enantiomers The two non-superimposable forms of an optical isomer are called enantiomers. Note the prefixes used to distinguish between the 2 enantiomers. (Other prefixes are + or -, and S or R.) 20 Optical Isomers - Enantiomers Enantiomers of lactic acid Note the different uses of the enantiomers. This relates to their biological activity. 21 OpticalOptical IsomersIsomers - Enantiomers Dash-wedge diagrams are used for 3D structures. solid lines = in the same plane dashed lines = behind the plane wedge lines = in front of the plane Enantiomers (optical isomers) can be drawn like this: 22 Optical Isomers - Enantiomers Draw these molecules in 3D and show the enantiomers. Put the chiral atom at the centre. butan-2-ol 2-chloropropanoic acid 23 Optical Isomers Optical- Isomers Racemic Mixtures A mixture that contains equal amounts (or equimolar amount) of the two enantiomers of an optical isomer is called a racemic mixture. When an optical isomer is synthesized in the laboratory, a racemic mixture is produced. 24 Optical Isomers - Properties Optical isomers (the enantiomers) have identical physical and chemical properties, with these two important exceptions: 1. optical activity (physical) 2. reactivity with other chiral molecules (chemical) (e.g. many biological molecules are chiral) 25 Optical Isomers - Optical Activity Optical activity refers to how a substance interacts with light. First, you need to know a little bit about light and “polarity” Light from a source travels as a wave. The wave can be in an infinite number of planes and is called “non-polarized” light. The light that passes through the polaroid is called “plane polarized light. A “polaroid” is a filter that only allows light travelling in one plane to pass through it. 26 Optical Isomers - Optical Activity When plane polarized light passes through an optical isomer, the isomer interacts with it in a way that “rotates” the plane of the light. plane polarized light optical isomers light rotates The two different enantiomers rotate the light in different directions. 27 Optical Isomers - Optical Activity A polarimeter is an instrument that measures the rotation of plane polarized light by a substance. Plane polarized light is rotated after passing through a sample of an optical isomer. 28 Optical Isomers - Optical Activity Observations of different mixtures of an optically active substance (e.g. 2-aminopropanoic acid) on the analyzer of a polarimeter: L-2-aminopropanoic acid D-2-aminopropanoic acid racemic mixture plane polarized light rotated counter-clockwise plane polarized light rotated clockwise no rotation of planepolarized light equal proportions of the two isomers cancel out the rotation 29 Optical Isomers - Reactivity Chemical properties of enantiomers are the same, EXCEPT when they react with other chiral molecules (like biological molecules). This means the biological or biochemical reactivity of enantiomers are DIFFERENT. 30 Optical Isomers - Reactivity Thalidomide was a drug given to pregnant women to prevent morning sickness. Where is the chiral carbon? 31 Optical Isomers - Reactivty One enantiomer of thalidomide is therapeutic, acting as an anti-nausea agent. The other enantiomer of thalidomide is teratogenic, causing limb malformations during fetal development. 32