有机化学英文chapter3.pptx
Stereoisomerismand ChiralityChapter 3Chapter 3第1页/共43页Isomers Isomers:Isomers:different compounds with the same molecular formula Constitutional isomers:Constitutional isomers:isomers with a different connectivity Stereoisomers:Stereoisomers:isomers with the same connectivity but a different orientation of their atoms in space第2页/共43页Chirality Chiral:Chiral:from the Greek,cheir,handan object that is not superposable on its mirror image Achiral:Achiral:an object that lacks chirality;one that lacks handednessan achiral object has at least one element of symmetry plane of symmetry:plane of symmetry:an imaginary plane passing through an object dividing it so that one half is the mirror image of the other half center of symmetry: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第3页/共43页Elements of Symmetry Symmetry in objectsSymmetry in objects第4页/共43页Elements of SymmetryPlane of symmetry(contd)第5页/共43页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 centerchiral centerall chiral centers are stereocenters,but not all stereocenters are chiral centers(see Figure 3.5)Enantiomers:Enantiomers:stereoisomers that are nonsuperposable mirror imagesrefers to the relationship between pairs of objects第6页/共43页Enantiomers2-Butanolhas one chiral centerhere are four different representations for one enantiomerusing(4)as a model,here are two different representations for the enantiomer of(4)第7页/共43页EnantiomersThe enantiomers of lactic aciddrawn in two different representations第8页/共43页Enantiomers2-Chlorobutane第9页/共43页Enantiomers3-Chlorocyclohexene第10页/共43页EnantiomersA nitrogen chiral center第11页/共43页R,S ConventionPriority rules1.Each atom bonded to the chiral center 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 difference第12页/共43页R,S Convention3.Atoms participating in a double or triple bond are considered to be bonded to an equivalent number of similar atoms by single bonds第13页/共43页Naming 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 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 R;if they are read counterclockwise,the configuration is S S (S)-2-Chlorobutane第14页/共43页Naming Chiral Centers(R)-3-Chlorocyclohexene(R)-Mevalonic acid第15页/共43页Enantiomers&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 possible第16页/共43页Enantiomers&Diastereomers2,3,4-Trihydroxybutanaltwo chiral centers 22=4 stereoisomers exist;two pairs of enantiomers Diastereomers:Diastereomers:stereoisomers that are not mirror imagesrefers to the relationship among two or more objects第17页/共43页Enantiomers&Diastereomers2,3-Dihydroxybutanedioic acid(tartaric acid)two chiral centers;2n=4,but only three stereoisomers exist Meso compound:Meso compound:an achiral compound possessing two or more chiral centers that also has chiral isomers第18页/共43页Enantiomers&Diastereomers2-Methylcyclopentanol第19页/共43页Enantiomers&Diastereomers1,2-CyclopentanediolHHOH HOHHOH HOHHOOHHOHHHHOcis-1,2-Cyclopentanediol (a meso compound)trans-1,2-Cyclopentanediol (a pair of enantiomers)diastereomers第20页/共43页Enantiomers&Diastereomerscis-3-Methylcyclohexanol第21页/共43页Enantiomers&Diastereomerstrans-3-Methylcyclohexanol第22页/共43页Isomers第23页/共43页Properties of StereoisomersEnantiomers have identical physical and chemical properties in achiral 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)第24页/共43页Plane-Polarized Light Ordinary light:Ordinary light:light vibrating in all planes perpendicular to its direction of propagation Plane-polarized light:Plane-polarized light:light vibrating only in parallel planes Optically active:Optically active:refers to a compound that rotates the plane of plane-polarized light第25页/共43页Plane-Polarized Lightplane-polarized light is the vector sum of left and right circularly polarized lightcircularly polarized 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 polarization第26页/共43页Plane-Polarized Light Polarimeter:Polarimeter:a device for measuring the extent of rotation of plane-polarized light第27页/共43页Optical Activity observed rotation:observed rotation:the number of degrees,through which a compound rotates the plane of polarized light dextrorotatory(+):dextrorotatory(+):refers to a compound that rotates the plane of polarized light to the right levorotatory(-):levorotatory(-):refers to a compound that rotates of the plane of polarized light to the left specific rotation:specific 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 mL第28页/共43页Optical Purity Optical purity:Optical purity:a way of describing the composition of a mixture of enantiomers Enantiomeric excess:Enantiomeric excess:the difference between the percentage of two enantiomers in a mixtureoptical purity is numerically equal to enantiomeric excess,but is experimentally determined第29页/共43页Enantiomeric Excess Example:Example:a commercial synthesis of naproxen,a nonsteroidal anti-inflammatory drug(NSAID),gives the S enantiomer in 97%eeCalculate the percentages of the R and S enantiomers in this mixture第30页/共43页Resolution Racemic mixture:Racemic mixture:an equimolar mixture of two enantiomersbecause a racemic mixture contains equal numbers of dextrorotatory and levorotatory molecules,its specific rotation is zero Resolution:Resolution:the separation of a racemic mixture into its enantiomers第31页/共43页ResolutionOne 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 of the diastereomers,the enantiomerically pure acids are recovered第32页/共43页Resolutionracemic acids can be resolved using commercially available chiral bases such as 1-phenylethanamineracemic bases can be resolved using chiral acids such as第33页/共43页第34页/共43页ResolutionEnzymes as resolving agents第35页/共43页Amino Acidsthe 20 most common amino acids have a central carbon,called an-carbon,bonded to an NH2 group and a COOH groupin 19 of the 20,the-carbon is a chiral center 18 of the 19-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 as an internal saltin this structural formula,the symbol R R=a side chain第36页/共43页Proteins proteinsproteins 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第37页/共43页Chirality 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 system Its a chiral world!Its a chiral world!第38页/共43页Chirality in the Biological WorldConsider chymotrypsin,a protein-digesting enzyme in the digestive system of animalschymotrypsin contains 251 chiral centersthe maximum number of stereoisomers possible is 2251there are only 238 stars in our galaxy!第39页/共43页Chirality in the Biological WorldEnzymes are like hands in a handshakethe substrate fits into a binding site on the enzyme surfacea left-handed molecule will only fit into a left-handed binding site anda right-handed molecule will only fit into a right-handed binding siteenantiomers have different physiological properties because of the handedness of their interactions with other chiral molecules in living systems第40页/共43页Chirality in the Biological Worlda schematic diagram of an enzyme surface capable of binding with(R)-glyceraldehyde but not with(S)-glyceraldehyde第41页/共43页Stereoisomerism and ChiralityEnd Chapter 3End Chapter 3第42页/共43页感谢您的观看！第43页/共43页
