Introductory PowerPoint on stereochemistry

Introductory PowerPoint on stereochemistry

• 1.
  Stereochemistry chemistry in three dimensions
• 2.
  Isomers – differentcompounds with the same molecular formula. Structural Isomers – isomers that differ in which atoms are bonded to which atoms. CH3 eg. C4H10 CH3CH2CH2CH3 CH3CHCH3 n-butane isobutane
• 3.
  Stereoisomers – isomersthat differ in the way the atoms are oriented in space, but not in which atoms are bonded to which atoms. eg. cis-2-butene trans-2-butene H C C H3C CH3 H H C C H3C H CH3
• 4.
  Stereochemistry: importance: biochemicals medicine tool for studyingmechanisms
• 5.
  optical activity –when a substance rotates the plane of plane polarized light. (1815 by Biot) plane polarized light – light that has been passed through a nicol prism or other polarizing medium so that all of the vibrations are in the same plane. non-polarized polarized
• 6.
  polarimeter – aninstrument used to measure optical activity. light source sample tube polarizer analyzer
• 7.
  dextrorotatory – whenthe plane of polarized light is rotated in a clockwise direction when viewed through a polarimeter. (+) or (d) do not confuse with D levorotatory – when the plane of polarized light is rotated in a counter-clockwise direction when viewed through a polarimeter. (-) or (l) do not confuse with L The angle of rotation of plane polarized light by an optically active substance is proportional to the number of atoms in the path of the light.
• 8.
  specific rotation –the angle of rotation of plane polarized light by a 1.00 gram per cm-3 sample in a 1 dm tube. [α ]D (D = sodium lamp, λ = 589 mμ). α [ α ]D = where α = observed rotation l * d l = length (dm) d = concentration (g/cc) (+)-alanine [ α ]D = +8.5 (-)-lactic acid [α ]D = -3.8
• 9.
  Why are somesubstances optically active and others not? Can we predict which ones will be and which ones won’t? Louis Pasteur (1848) recrystallized sodium ammonium tartrate (optically inactive). He noticed that the crystals were of two types which he physically separated. The two types of crystals were optically active, but rotated the plane of polarized light in opposite directions. He proposed that the molecules came in two forms, “left handed” and “right handed”. Together, the mixture of the two forms is optically inactive.
• 10.
  enantiomers - mirror-imagestereoisomers. The physical and chemical properties of enantiomers are identical, except 1) the direction of rotation of the plane of plane polarized light and 2) how they react with optically active reagents. chiral center– is a carbon that is bonded to four different groups of atoms. (do not confuse with “chiral”) * CH3CH2CHBrCH3 (CH3)2CHCH2OH * * CH3CHBrCHBrCH3
• 11.
  configuration – thearrangement in space of the four different groups about a chiral center. How do we show configurations? “wedge” formulas Fischer projections “cross structures” use only for chiral centers! Br F H Cl Br F Cl H
• 12.
  In the Fischerprojection, the horizontal bonds to the chiral center are always above the plane and the vertical bonds to the chiral center are below the plane. (the horizontals are “hugging you.” CH3 H Br Cl CH3 H Cl Br
• 13.
  chiral – notsuperimposeable on the mirror image (“handedness”) achiral – superimposeable on the mirror image; not chiral. Test for optical activity: chiral molecules are optically active. racemic modification – equimolar molar mixture of enantiomers (will be optically inactive) (+).
• 14.
  - compounds withone chiral center will show optical activity - compounds without chiral centers do not normally show optical acitivity - compounds with more than one chiral center may or may not show optical activity depending on whether or not they are non-superimposable on their mirror image (chiral) or superimposable (achiral).
• 15.
  specification of configuration:The R/S system. Cahn, Ingold, Prelog sequence rules: sequence rule 1: the atom attached to the chiral center with the highest atomic number = 1, next = 2, etc. sequence rule 2: if the four atoms attached to the chiral center are not all different, the sequence is determined at the first point of difference. sequence rule 3: =X is equal to two –X, etc.
• 16.
  Br F Cl H 1 2 3 4 OH CH2Br CH3 H 1 2 3 4 CH2CH3 H CH=CH2 Br 1 2 3 4
• 17.
  R/S: 1. Using theCahn, Ingold, Prelog sequence rules, assign numbers to each of the four groups attached to the chiral center. 2. Rotate the number 4 group away from you and observe the sequence 1  2  3 for the remaining groups. 3. If going from 1  2  3 is clockwise, then the configuration is R (rectus). If the sequence 1  2  3 is counter-clockwise, then the configuration is S (sinister).
• 18.
  2 1 3 1 2 3 R S With group#4 rotated away:
• 19.
  Cl Br H F 1 2 3 4 Cl Br F 1 2 3 rotate #4away (S)-configuration
• 20.
  Using R/S problemson the web: http://chemistry2.csudh.edu/organic/startnewrands.html
• 21.
  Angew. Chem. Int.Ed. Engl. 36, 1057 (1997). absolute configuration for bromochlorofluoromethane: Br Br Cl H H Cl F F (R)-(-)- (S)-(+)-
• 22.
  * * ** aldohexose CH2-CH-CH-CH-CH-CH=O OH OH OH OH OH n chiral centers  2n maximum stereoisomers n = 4  24 = 16 stereoisomers * * 2,3-dichloropentane CH3CHCHCH2CH3 Cl Cl n = 2  22 = 4 stereoisomers
• 23.
  diastereomers – non-mirrorimage stereoisomers. (the physical and chemical properties of diastereomers are different.) CH3 H Cl CH2 H Cl CH3 Cl H CH2 Cl H CH3 H Cl CH2 Cl H CH3 Cl H CH2 H Cl CH3 CH3 CH3 CH3 I II III IV I & II are enantiomers; III & IV are enantiomers; I & III are diastereomers; I & IV are diastereomers…
• 24.
  CH3 CH2 H H Cl Cl CH3 C C H Cl1 4 H,H,H Cl,C,H 2 3 1 3 2 (S)- C C H Cl 1 4 C,H,H 3 2 1 2 3 (R)- Cl,C,H (2S,3R)-2,3-dichloropentane
• 25.
  CH3 H Cl CH2 H Cl CH3 ClH CH2 Cl H CH3 H Cl CH2 Cl H CH3 Cl H CH2 H Cl CH3 CH3 CH3 CH3 (2S,3R)- (2R,3S)- (S,S)- (R,R)-
• 26.
  * * 2,3-dichlorobutane CH3CHCHCH3 ClCl meso-compound – a compound that has chiral centers but is not chiral (optically inactive). CH3 H Cl CH3 H Cl CH3 Cl H CH3 Cl H CH3 H Cl CH3 Cl H CH3 Cl H CH3 H Cl I II III
• 27.
  Reactions involving stereoisomers: (a)the conversion of an achiral molecule into a chiral molecule, with the generation of a chiral center. n-butane + Cl2, hv  sec-butyl chloride + etc. achiral chiral * CH3CH2CHClCH3 CH3 C2H5 H Cl CH3 C2H5 Cl H (S)-(+)-sec-butyl chloride (R)-(-)-sec-butyl chloride product is optically inactive  racemic modification
• 28.
  The synthesis ofchiral compounds from achiral reactants always yields the racemic modification. Why? ‡ R is enatiomeric to ‡S Eact (R) = Eact (S) rate (R) = rate (S)  equimolar amounts racemic modification optically inactive
• 29.
  (b) reaction ofa chiral molecule where bonds to the chiral center are not broken. * * CH3CH2CHClCH3 + Cl2, hv  CH3CH2CHClCH2Cl + etc. A reaction that does not involve the breaking of a bond to a chiral center proceeds with retention of configuration about the chiral center. Can be used to “relate” configurations. If a compound can be synthesized by such a reaction from a compound of known configuration, then the configuration is known in the product.
• 30.
  CH3CH2CHCH2OH + HCl CH3 CH3CH2CHCH2Cl+ H2O CH3 * * It is known from X-ray crystallography that (-)-2-methyl-1-butanol is the (S)-isomer. When pure (-)-2-methyl-1-butanol is reacted with HCl, the product is dextrorotatory. Since no bonds to the chiral center were broken in the reaction, the (+)-1-chloro-2-methyl butane is now known to be the (S)-isomer. CH3 C2H5 H CH2OH + HCl CH3 C2H5 CH2-Cl H
• 31.
  (c) reactions like(b) in which a second chiral center is generated: * * * CH3CH2CHClCH3 + Cl2, hv  CH3CHClCHClCH3 + isomers CH3 CH2 H Cl CH3 H Cl CH3 H Cl CH3 H Cl CH3 Cl H CH3 + (S)- (R,S)- (S,S)- diastereomers in unequal amounts The transition states are “diastereomeric”, the Eact’s are not equal, the rates are different.
• 32.
  (d) reactions ofchiral compounds with optically active reagents. Enantiomers have the same physical properties and cannot be separated by normal separation techniques like distillation, etc. Enantiomers differ in reaction with optically active reagents. Enantiomeric acids or bases can be reacted with an optically active base or acid to form salts that are diastereomers. Since diastereomers have different physical properties they can be separated by physical methods. The salts can then be converted back into the free acids or bases. Resolution – the separation of enantiomers.
• 33.
  (+)-HA + (-)-Base [(-)-baseH+ ,(+)A- ] + (-)-HA [(-)-baseH+ ,(-)A- ] (enantiomers) (diastereomers, separable) [(-)-baseH+ ,(+)-A- ] + H+  (+)-HA + (-)-baseH+ [(-)-baseH+ ,(-)-A- ] + H+  (-)-HA + (-)-baseH+ A racemic modification is converted by optically active reagents into a mixture of diastereomers which can then be separated. (resolved)
• 34.
  (e) a reactionof a chiral compound in which a bond to a chiral center is broken… In a reaction of a chiral compound in which a bond to a chiral center is broken, the stereochemistry depends on the mechanism of the reaction. CH3CH2CHCH2-Cl + Cl2, hv CH3 CH3CH2CCH2-Cl + isomers CH3 Cl * * (S)-(+)-1-chloro-2-methylbutane racemic-1,2-dichloro-2-methylbutane optically inactive mixture
• 35.
  CH3 H C2H5 CH2-Cl Cl-Cl 2Cl. + Cl. CH3 C2H5 CH2-Cl . + HCl CH3 C2H5CH2-Cl Cl-Cl sp2 hybridized flat free radical C2H5 CH2-Cl CH3 Cl C2H5 CH2-Cl Cl CH3 +
• 36.
  In a reactionof a chiral compound in which a bond to a chiral center is broken, the stereochemistry depends on the mechanism of the reaction. This means that we can use the stereochemistry of such a reaction to give us information about the mechanism for that reaction.