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Stereochemistry. Stereochemistry refers to the 3-dimensional properties and reactions of molecules. It has its own language and terms that need to be learned in order to fully communicate and understand the concepts. Stereochemistry. Stereoisomer. Constitutional isomers. Conformation.
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Stereochemistry Stereochemistry refers to the 3-dimensional properties and reactions of molecules. It has its own language and terms that need to be learned in order to fully communicate and understand the concepts.
Stereochemistry Stereoisomer Constitutional isomers Conformation Configuration Geometrical Trans Cis Diastereomer Enantiomer
Stereochemistry of Tetrahedral Carbons • Is the chemistry of the molecules in three dimensions 3
Definitions • Configuration isomers - differ from each other only in arrangement of their atoms in space • Chiral compounds( asymmetric carbon) (stereogenic center or stereo center) – sp3 carbon with 4 different atoms or groups attached & don’t have plane of symmetry (that is optically active and are called optical isomers) . • achiralcompound – a compounds have two or more atoms or groups that are the same (that is optically inactive) • Optical activity – the ability to rotate the plane of plane –polarized light • Polarimeter – device that measures the optical rotation of the chiral compound
More Definitions • Stereoisomers – compounds with the same connectivity, different arrangement in space • Enantiomers – stereoisomers that are non- superimposible mirror images; only properties that differ are direction (+ or -) of optical rotation • Diastereomers – stereoisomers that are not mirror images; different compounds with different physical properties • Racemic mixtures – a mixture of enanatiomers with the same amount of each ( are optically inactive and donated by +/-)
Specific Rotation, [α] [α] = α / cl a = observed rotation c = concentration in g/mL l = length of tube in dm Dextrorotary designated as d or(+), clockwise rotation Levorotary designated as l or (-), counter- clockwise rotation
Specific Rotations of some Common Organic Compounds Compound [a] # * centers Penicillin V +233.0 3 Sucrose +66.5 10 Camphor +44.3 2 MSG +25.5 1 Cholesterol -31.3 8 Morphine -132.0 5
NAMING ENANTIOMERS Since two enantiomers are different compounds, we will need to have nomenclature which distinguishes them from each other. The convention which is used is called the (R,S) system (absolute configuration) because one enantiomer is assinged as the R enantiomer and the other as the S enantiomer. What are the rules which govern which is which? The conversion of a perspective drawing to a Fischer projection requires rotating the molecule so that the "top" and "bottom" groups are oriented back, away from you. This places the "right" and "left" groups in a position where they are projecting outward towards you, as in a "bow-tie"
These rules are restated below, with examples: 1.Examine the four atoms directly attached to the chiral center in question. Assign priorities in order of decreasing atomic number; the atom with the highest atomic number is #1, the next is #2, etc. 2.If a decision regarding priority cannot be reached using Rule #1, compare the atomic numbers of the second atoms in each substituent, then the third, etc., until a difference is found.Example: 3-Ethoxy-2-chlorohexane
3. Multiple bonds count twice (or three times) when examining substituents. 3-Ethyl-4-chloro-pentene
Diastereomers Molecules with more than one chirality center have mirror image stereoisomers that are enantiomers . In addition they can have stereoisomeric forms that are not mirror images
Reactions that Generate Chirality Centers Hydrogenation, syn
BrominationTrans is formed exclusivelyNo Meso is formed (cis)
