有机化学课件Chapter 5 Stereochemistry

1、有机化学课件有机化学课件Chapter 5 StereochemistryChapter 5 Stereochemistry Teaching Objectives and Requirements Understand: Configuration of Molecules, Chirality, Optical Activity, Grasp: Enantiomers, Stereogenic Center, RS Notational System, Fischer Projections, Molecules with Two Stereogenic Centers. Key poin

2、ts and Areas of Difficulty RS Notational System, Fischer Projections.5.1 Configuration of MoleculesStereoisomers have the same connectivity-the same sequence of bonded atoms-but different arrangements of the atoms in space.Stereogenic center is a tetrahedral carbon atom bearing four different groups

3、 or atoms.5.2 Mirror Images and Chirality When an object and its mirror image exactly match, we say that they are superimposable.An object that is not superimposable (non-superimposable) on its mirror is said to be chiral (Greek,Chiron,hand) ,contrarily said to be achiral.Molecular Chirality A molec

4、ule is chiral: contains a stereogenic center, which is a tetrahydral carbon atom attached to four different atoms or groups.5.2 Mirror Images and Chirality5.2 Mirror Images and Chirality5.2 Mirror Images and ChiralityIf a molecule contains a plane of symmetry, it can be superimposable on its mirror

5、image and is therefore achiral.Enantiomers Are Mirror Image IsomersStereoisomers that are related as nonsuper-imposable mirror images are called enantiomers (Greek, enantios, opposite+ meros, part).5.2 Mirror Images and ChiralityA molecule containing a stereogenic center can exist as a pair of enant

6、iomers.Physical Properties of EnantiomersEnantiomers have many identical physical properties, such as density, melting point, and boiling point.In a chiral environment they have different chemical properties and can be distinguished.6.3 Optical ActivityWhen a beam of “ordinary light passes through a

7、 polarizing filter, it is converted to a beam of plane-polarized light that oscillates in a single plane.When a beam of polarized light pass through a solution of chiral compound, its oscillating direction can be changed to the right or left.Because chiral molecules rotate plane-polarized light, the

8、y are called optical active.6.3 Optical Activity6.3 Optical ActivitySpecific RotationThe optical activity of a substance of degrees of rotation of a solution at a concentration, symbolized by D.The specific rotation is the number of degree of rotation of a solution at a concentration of 1g/ml in tub

9、e 1dm long. The standard conditions selected for determination of specific rotation are 25 and yellow light (D line,589)6.3 Optical ActivityIf a chiral substance rotates plane-polarized light to the right, in positive(+), clockwise direction, the substance is dextrorotatory.If a chiral substance rot

10、ates plane-polarized light to the left, in negative(-), clockwise direction, the substance is levorotatory.The dextrorotatory and levorotatory isomers (enantiomers or optical isomers) of a given substance rotate polarized light the same number of degrees but in opposite direction.6.3 Optical Activit

11、yThe (+) isomer is sometimes called the d form, where d refers to dextrorotatory.The (-) isomer is called l form, where l refers to levorotatory.5.4 Fischer Projection FormulasIn a Fischer projection formula, the vertical line is to show the groups behind the paper plane and usually contains most ca

12、rbon containing groups. In a Fischer projection formula, the horizontal line is to show the groups in front of the paper plane.H COHCHOCH2OHHO CHCH2OHCHO5.4 Fischer Projection FormulasIf one projection formula is lifted out of the plane and flipped over, its formula of enantiomer is obtained.5.4 Fis

13、cher Projection Formulas Because cant see the structure in the old era, Fischer arbitrarily assigned the dextrorotatory enantiomer of glyceraldehydes, which rotates plane polarized light in a clockwise direction (+13.5), to the Fischer projection with the hydroxyl group on the right side and called

14、D-glyceraldehyde.5.4 Fischer Projection FormulasThe mirror image compound,(-)-glycer-aldehyde, was assigned a structure in which the hydroxyl group is on the left and was called L-glyceraldehyde.It rotates the plane polarized light in a counterclockwise direction.The D and L refer only to configurat

15、ion at the stereogenic center, while the d and l refer only to the direction of rotation of plane polarized light.5.4 Fischer Projection FormulasThe configurations of other chiral compounds are determined by chemically relating them to either D- or L-glyceraldehyde.Such as both (+)-glyceraldehyde an

16、d (-)-lactic acid are designated with a D configuration, because both have the OH group on the right in the Fischer projection formula.5.4 Fischer Projection FormulasAlthough Fischer projections of glceraldehyde enantiomers is relative configuration, they happened to be correct by X-ray crystallgrap

17、h detection.D-(+)-glyceraldehyde D-(-)-lactic acidHCOHCHOCH2OHseveral stepHCOHCOOHCH35.5 Absolute ConfigurationR,S Configuration In the R,S system,the four groups bonded to each stereogenic carbon atom are arranged from highest to lowest priority(from 1 to 4).Viewing from the position oppsite to the

18、 lowest priority group(number 4)If the other three goups are arranged in clockwise direction from highest to lowest priority groups(from 1 to 3), the configuration of molecule is designated R (latin,rectus,right). 5.5 Absolute Configuration5.5 Absolute ConfigurationIf the other three goups arrange i

19、n counterclockwise direction from highest to lowest priority groups(from 1 to 3),the configuration of molecule is designated L(latin,sinister,left).5.5 Absolute ConfigurationIf the lowest priority group is on the vertical line in its Fischer projection, the other three goups are arranged in clockwis

20、e direction from highest to lowest priority groups (from 1 to 3) just on the paper plane, the configuration of molecule is designated R (latin,rectus,right) If the other three goups arrange in counterclock-wise direction from highest to lowest priority groups (from 1 to 3) just on the paper plane,th

21、e configuration of molecule is designated L (latin, sinister, left).5.5 Absolute ConfigurationIf the lowest priority group is on the horizontal line in its Fischer projection,the actual R,S are converse to the R,S juged by priority sequence just on the paper plane.Priority RulesAtoms.We rank the fou

22、r atoms bonds to a stereogenic carbon atom in order of decreasing atomic number;the higher the atomic number,the higher the priority:IBrClFONC2H1H(from highest to lowest priority)5.5 Absolute ConfigurationGroups of atoms.If a decision cannot be made using rule 1 because two or more of the directly a

23、ttached atoms are the same, move along the chain until a point of first difference is encountered.Then apply rule 1.Multiple bonds.If a group contains a double bond, both atoms are doubled. That is, a double bond is counted as two single bonds to each of the atoms of the double bond.5.5 Absolute Con

24、figuration The same principle is used for a triple bond.5.6 Multiple Stereogenic CentersStereoisomers that are not enantiomers are called diastereomers.Such as:diastereoisomers(and or respectively, and or respectively)CHO CHOCH2OHCH2OHOHHHHOHOHHOHCHOCH2OHHOHHOHCHOCH2OHHOHHOH5.6 Multiple Stereogenic Centers enantiomers enantiomers Melting point liquid liquid 130 130 Solubility in very very slightly slightly Ethanol soluble soluble soluble soluble5.6 Multiple Stereogenic CentersIn contrast to the enantiomers,which have the same chemical and physical properties, diastereoisomers have diff