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TAMU CHEM 227 - Enantiomers and Diastereomers
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CHEM 227 1st Edition Lecture 7 Outline of Last LectureI. StereochemistryII. Chiralitya. What is Chirality?b. Chirality CentersIII. Optical ActivityIV. Pasteur’s DiscoveryV. Sequence rules for specification of configurationOutline of Current Lecture I. Rules for assigning priority/R and S (CONT.)II. 5.6 DiastereomersIII. 5.7 Meso compoundsIV. 5.8 Racemic Mixtures and the Resolution of Enantiomers (Separation)V. 5.9 Review of IsomerismCurrent Lecture- Continue from Last week’s Rules on assigning priorities- Rule 3:o Multiple-bonded atoms are equivalent to the same number of single bonded atomso When the hydrogen is pointing away from you, the clockwise/counterclockwise orientation should be exactly as it is on the paper, but if the hydrogen is pointing toward you, you have to mentally turn the molecule, and the R/S switches (because you need to look at it from the other side (because when assigning R or S, the hydrogen (or lowest ranking group) should be pointing away from you))o PRACTICE THIS5.6 DiastereomersThese notes represent a detailed interpretation of the professor’s lecture. GradeBuddy is best used as a supplement to your own notes, not as a substitute.- Molecules with more than one chirality center usually have mirror image stereoisomers that are enantiomers- When you can have more than one place in the molecule that can be R or S, you have many different combinations of the molecule that could existo If you have 2 chirality centers, you could have R, R or R, S, or S, S or S, Ro If you have 3 chirality centers you could have R, R, R or R, R, S or R, S, S, etc. There are manyyyyy- In addition they can have stereoisomeric forms that are not mirror images, called diastereomers- MAXIMUM Number of Stereoisomers = 2no Where n = number of chirality centerso Example: 2 chirality centers- Number of stereoisomers = 22 = 4 3 chirality centers- Number of stereoisomers = 23 = 8Here is an example of a molecule with 2 chirality centers (the 4 (22) stereoisomers):- 2R,3R and 2S,3S are enantiomers because they are mirror images of each other- 2R,3S and 2S,3R are enantiomers because they are mirror images of each other- The only difference between the mirror images of each other is their optical rotationo One will be dextrorotatoryo One will be levorotatory- But they are not all mirror images of each othero The molecules that are not mirror images of each other are called Diastereomerso When one chirality center is the same but one is differento Ex: 2R,3R and 2R,3S are diastereomers of each other5.7 Meso compounds- Tartaric acid has two chirality centers and two diastereomeric forms.- One form is chiral and the other is achiral, but both have two chirality centers- An achiral compound with chirality centers is called a meso compound- Consider the figure belowo The two structures on the left are chiral and mirror images of each othero The two structures on the right in the figure are identical so the compound (2R, 3S) is achiral- When the two chirality centers have the same attachmentso When there is a plane of symmetry even though there are chiral centers5.8 Racemic Mixtures and the Resolution of Enantiomers- Diastereomers have different properties from each othero This makes it possible to separate them- But how do we separate mixtures of enantiomers? They have the same properties. The only difference is their optical rotation- A 50:50 mixture of two chiral compounds that are mirror images does not rotate light is called a racemic mixture (named “racemic acid” that is the double salt of (+) and (-) tartaric acid- To have a pure compound is necessary to separate or resolved it from the mixture (called a racemate)- To separate components of a racemate (reversibly) we make a derivative of each with a chiral substance that is free of its enantiomer (resolving agent)- This gives diastereomers that are separated by their different solubility - The resolving agent is then removed- Enantiomers reacting with an achiral amine… NO separationo Because your results are enantiomers, they are pretty much identical and you can not separate them- Enantiomers reacting with a chiral amine there is a resolutiono Because your results are diastereomers and not enantiomers, it is possible to separate them Ex: using chromatography, one diastereomer will run farther down the strip than the other5.9 Review of Isomerism- There are many types of Isomers we have learned so far- Here is a breakdown of them:- Constitutional Isomerso Different connectivity gives different carbon backbones and/or different functional groups- Same connections, different spatial arrangement of atomso Enantiomers (nonsuperimposable mirror images)o Diastereomers (all other stereoisomers)  Includes cis, trans,


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TAMU CHEM 227 - Enantiomers and Diastereomers

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