有机化学 英文课件 chapter3.ppt

1、Organic Chemistry,William H. Brown Christopher S. Foote Brent L. Iverson,Stereoisomerism and Chirality,Chapter 3,Isomers,Isomers: different compounds with the same molecular formula Constitutional isomers: isomers with a different connectivity Stereoisomers: isomers with the same connectivity but a

2、different orientation of their atoms in space,Chirality,Chiral: from the Greek, cheir, hand an object that is not superposable on its mirror image Achiral: an object that lacks chirality; one that lacks handedness an achiral object has at least one element of symmetry plane of symmetry: an imaginary

3、 plane passing through an object dividing it so that one half is the mirror image of the other half center of symmetry: a point so situated that identical components are located on opposite sides and equidistant from that point along the axis passing through it,Elements of Symmetry,Symmetry in objec

4、ts,Elements of Symmetry,Plane of symmetry (contd),Chiral Center,The most common (but not the only) cause of chirality in organic molecules is a tetrahedral atom, most commonly carbon, bonded to four different groups A carbon with four different groups bonded to it is called a chiral center all chira

5、l centers are stereocenters, but not all stereocenters are chiral centers (see Figure 3.5) Enantiomers: stereoisomers that are nonsuperposable mirror images refers to the relationship between pairs of objects,Enantiomers,2-Butanol has one chiral center here are four different representations for one

6、 enantiomer using (4) as a model, here are two different representations for the enantiomer of (4),Enantiomers,The enantiomers of lactic acid drawn in two different representations,Enantiomers,2-Chlorobutane,Enantiomers,3-Chlorocyclohexene,Enantiomers,A nitrogen chiral center,R,S Convention,Priority

7、 rules 1. Each atom bonded to the chiral center is assigned a priority based on atomic number; the higher the atomic number, the higher the priority 2. 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

8、difference,R,S Convention,3. Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds,Naming Chiral Centers,1. Locate the chiral center, identify its four substituents, and assign priority from 1 (highest) to 4 (lowest) to ea

9、ch substituent 2. Orient the molecule so that the group of lowest priority (4) is directed away from you 3. 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

10、 configuration is S (S)-2-Chlorobutane,Naming Chiral Centers,(R)-3-Chlorocyclohexene (R)-Mevalonic acid,Enantiomers two pairs of enantiomers Diastereomers: stereoisomers that are not mirror images refers to the relationship among two or more objects,Enantiomers 2n = 4, but only three stereoisomers e

11、xist Meso compound: an achiral compound possessing two or more chiral centers that also has chiral isomers,Enantiomers for a solution, concentration is expressed in g/ 100 mL,Optical Purity,Optical purity: a way of describing the composition of a mixture of enantiomers Enantiomeric excess: the diffe

12、rence between the percentage of two enantiomers in a mixture optical purity is numerically equal to enantiomeric excess, but is experimentally determined,Enantiomeric Excess,Example: a commercial synthesis of naproxen, a nonsteroidal anti-inflammatory drug (NSAID), gives the S enantiomer in 97% ee C

13、alculate the percentages of the R and S enantiomers in this mixture,Resolution,Racemic mixture: an equimolar mixture of two enantiomers because a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules, its specific rotation is zero Resolution: the separation of a racemic

14、 mixture into its enantiomers,Resolution,One means of resolution is to convert the pair of enantiomers into two diastereomers diastereomers are different compounds and have different physical properties A common reaction for chemical resolution is salt formation after separation of the diastereomers

15、, the enantiomerically pure acids are recovered,Resolution,racemic acids can be resolved using commercially available chiral bases such as 1-phenylethanamine racemic bases can be resolved using chiral acids such as,Resolution,Enzymes as resolving agents,Amino Acids,the 20 most common amino acids hav

16、e a central carbon, called an a-carbon, bonded to an NH2 group and a COOH group in 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 configuration in the D,L system, all have the L configuration at neutral pH, an amino acid exists as an inte

17、rnal salt in this structural formula, the symbol R = a side chain,Proteins,proteins 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 acid,Chirality in the Biological World,Except for inorganic sal

18、ts and a few low-molecular-weight organic substances, the molecules of living systems are chiral Although these molecules can exist as a number of stereoisomers, generally only one is produced and used in a given biological system Its a chiral world!,Chirality in the Biological World,Consider chymotrypsin, a protein-digesting enzyme in the digestive system of animals chymotrypsin contains 251 chiral centers the maximum number of stereoisomers possible is 2251 there are only 238 stars in our gal