有機(jī)化學(xué)英文課件chapter3-PPT課件

1、Organic ChemistryWilliam H. BrownChristopher S. FooteBrent L. IversonStereoisomerismand ChiralityChapter 3IsomersIsomers: different compounds with the same molecular formulaConstitutional isomers: isomers with a different connectivityStereoisomers: isomers with the same connectivity but a different

2、orientation of their atoms in spaceChiralityChiral: from the Greek, cheir, handan object that is not superposable on its mirror imageAchiral: an object that lacks chirality; one that lacks handednessan achiral object has at least one element of symmetryplane of symmetry: an imaginary plane passing t

3、hrough an object dividing it so that one half is the mirror image of the other halfcenter of symmetry: a point so situated that identical components are located on opposite sides and equidistant from that point along the axis passing through itElements of SymmetrySymmetry in objectsElements of Symme

4、tryPlane of symmetry (contd)Chiral CenterThe most common (but not the only) cause of chirality in organic molecules is a tetrahedral atom, most commonly carbon, bonded to four different groupsA carbon with four different groups bonded to it is called a chiral centerall chiral centers are stereocente

5、rs, but not all stereocenters are chiral centers (see Figure 3.5)Enantiomers: stereoisomers that are nonsuperposable mirror imagesrefers to the relationship between pairs of objectsEnantiomers2-Butanolhas one chiral centerhere are four different representations for one enantiomerusing (4) as a model

6、, here are two different representations for the enantiomer of (4)EnantiomersThe enantiomers of lactic aciddrawn in two different representationsEnantiomers2-ChlorobutaneEnantiomers3-ChlorocyclohexeneEnantiomersA nitrogen chiral centerR,S ConventionPriority rules1. Each atom bonded to the chiral cen

7、ter is assigned a priority based on atomic number; the higher the atomic number, the higher the priority2. If priority cannot be assigned per the atoms bonded to the chiral center, look to the next set of atoms; priority is assigned at the first point of differenceR,S Convention3. Atoms participatin

8、g in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bondsNaming Chiral Centers1. Locate the chiral center, identify its four substituents, and assign priority from 1 (highest) to 4 (lowest) to each substituent2. Orient the molecule so that the

9、group of lowest priority (4) is directed away from you3. Read the three groups projecting toward you in order from highest (1) to lowest priority (3)4. If the groups are read clockwise, the configuration is R; if they are read counterclockwise, the configuration is S (S)-2-ChlorobutaneNaming Chiral

10、Centers(R)-3-Chlorocyclohexene(R)-Mevalonic acidEnantiomers & DiastereomersFor a molecule with 1 chiral center, 21 = 2 stereoisomers are possibleFor a molecule with 2 chiral centers, a maximum of 22 = 4 stereoisomers are possibleFor a molecule with n chiral centers, a maximum of 2n stereoisomers are

11、 possibleEnantiomers & Diastereomers2,3,4-Trihydroxybutanaltwo chiral centers 22 = 4 stereoisomers exist; two pairs of enantiomersDiastereomers:stereoisomers that are not mirror imagesrefers to the relationship among two or more objectsEnantiomers & Diastereomers2,3-Dihydroxybutanedioic acid (tartar

12、ic acid)two chiral centers; 2n = 4, but only three stereoisomers existMeso compound: an achiral compound possessing two or more chiral centers that also has chiral isomersEnantiomers & Diastereomers2-MethylcyclopentanolEnantiomers & Diastereomers1,2-CyclopentanediolHHOHHOHHOHHOHHOOHHOHHHHOcis-1,2-Cy

13、clopentanediol (a meso compound)trans-1,2-Cyclopentanediol (a pair of enantiomers)diastereomersEnantiomers & Diastereomerscis-3-MethylcyclohexanolEnantiomers & Diastereomerstrans-3-MethylcyclohexanolIsomersProperties of StereoisomersEnantiomers have identical physical and chemical properties in achi

14、ral environmentsDiastereomers are different compounds and have different physical and chemical propertiesmeso tartaric acid, for example, has different physical and chemical properties from its enantiomers (see Table 3.1)Plane-Polarized LightOrdinary light: light vibrating in all planes perpendicula

15、r to its direction of propagationPlane-polarized light: light vibrating only in parallel planesOptically active: refers to a compound that rotates the plane of plane-polarized lightPlane-Polarized Lightplane-polarized light is the vector sum of left and right circularly polarized lightcircularly pol

16、arized light reacts one way with an R chiral center, and the opposite way with its enantiomerthe result of interaction of plane-polarized light with a chiral compound is rotation of the plane of polarizationPlane-Polarized LightPolarimeter: a device for measuring the extent of rotation of plane-pola

17、rized lightOptical Activityobserved rotation: the number of degrees, , through which a compound rotates the plane of polarized lightdextrorotatory (+): refers to a compound that rotates the plane of polarized light to the rightlevorotatory (-): refers to a compound that rotates of the plane of polar

18、ized light to the leftspecific rotation: observed rotation when a pure sample is placed in a tube 1.0 dm in length and concentration in g/mL (density); for a solution, concentration is expressed in g/ 100 mLOptical PurityOptical purity: a way of describing the composition of a mixture of enantiomers

19、Enantiomeric excess: the difference between the percentage of two enantiomers in a mixtureoptical purity is numerically equal to enantiomeric excess, but is experimentally determinedEnantiomeric Excess Example: a commercial synthesis of naproxen, a nonsteroidal anti-inflammatory drug (NSAID), gives

20、the S enantiomer in 97% eeCalculate the percentages of the R and S enantiomers in this mixtureResolutionRacemic mixture: an equimolar mixture of two enantiomersbecause a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific rotation is zeroResolution: the

21、separation of a racemic mixture into its enantiomersResolutionOne means of resolution is to convert the pair of enantiomers into two diastereomers diastereomers are different compounds and have different physical propertiesA common reaction for chemical resolution is salt formation after separation

22、of the diastereomers, the enantiomerically pure acids are recoveredResolutionracemic acids can be resolved using commercially available chiral bases such as 1-phenylethanamineracemic bases can be resolved using chiral acids such asResolutionEnzymes as resolving agentsAmino Acidsthe 20 most common am

23、ino acids have a central carbon, called an a-carbon, bonded to an NH2 group and a COOH groupin 19 of the 20, the a-carbon is a chiral center 18 of the 19 a-carbons have the R configuration, one has the S configurationin the D,L system, all have the L configurationat neutral pH, an amino acid exists

24、as an internal saltin this structural formula, the symbol R = a side chainProteinsproteins are long chains of amino acids covalently bonded by amide bonds formed between the carboxyl group of one amino acid and the amino group of another amino acidChirality in the Biological WorldExcept for inorganic salts and a few low-molecular-weight organic substances, the molecules of living systems are chiralAlthough these molecules can exist as a number of stereoisomers, generally only one is produced and used in a given biological systemIts a chiral world!Chirality in the Biologic