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stereoisomers & chirality

stereoisomers & chirality - CHM—23E Rice information...

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Unformatted text preview: CHM—23E, Rice, information Sheet: Steteoisomets and chiraiity. Page 1 1. Definitions: (1) An enantiomet (has one or more asymmetric centers) is chiral and has a mirror image that is non-superimposahle, thus they are two separate compounds. (2) An achiral compound (has no asymmetric centers) is not chiral and thus has a superimposabie mirror image. In other words the two compounds are identicai. (3) A diastereomer (has two or more asymmetric centers) does not have a superimposable mirror image, or a non— superimposable mirror image, but the other compound is not identical to it either. (4) A mesa compound is an isomer (with two or more asymmetric centers) with a mirror image that is identicai to it and has a plane of symmetry. 2. Examples: (1) Example 1: Aianine, a chiral amino acid with only 1 asymmetric: center. Alanine has two steteoisomers that are enan‘n'omers. These are enantiomers. If you imagine rotating one of the molecules clockwise, you would never be able to get all the groups to overlap. COO“ uumH Try buiiding them with your model kit to prove it. + HEN CH3 , This is a nonwsuperimposahle mirror image. Enanuomers (2) Example 2: Threonine, a chiral amino acid with two asymme‘nfic centers. Threonine has 4 stereoisomers that are comprised of two pairs of enandomers and four pairs of diastereomers. The groups around each asymmetric center are different. (This is the same example I used in ciass). Enantiomers Enanti omers These are two pairs of enantiomers because both asymmetric centers are non—superimposahle. 1 8t 2 and 3 8c 4. I have shown this by the different colors of the group. Try building them with your model kit. CHE/[£31, Rice, Information Sheet: Stereoisomers and chitality. Page 2 (2) Example 2 continued“ .. The following pairs of diastereomers exist: l&3;l&4 and 2&3;2&4. They are diastcreomers because one of the asymmetric centers in the pair is identical (not mirror images) to the other. The other asymmetric Diastereomers centers are different from each other (they are actually mirror images). Diastereomers Look at the bottom asymmetric centers in compound 1 and compound 3. They are colored green because they are identical. Now look at the top asymmetric centers in compound 1 and compound 3. They are colored in different colors because they are non-superimposable mirror images. (4011') RIC the other com Guilds and 011 ‘V‘Jfll SEC 9. similar “altern. You can 3.180 t: ‘ bulldlfl [hé COEEK‘OufidS With 0111’ t’ l.” Y mOClfil kit to COHVlIlCC yourself. (3) Example 3: 2,3vDibromobutane, a chiral compound with two asymmetric centers. 2,3—Dibromobutane has the same groups around both asymmetric centers. This Compound has only 3 stereojsomers that are comprised of 1 pair of enantiomers and two pairs of diastereomers. One of the stereoisomers is a meso compound. I 1 is a meso compound I .._._---..._. ___................G_ Rotate the molecule 180 degrees Dias tereomers These two compounds are identical. The meso compound (compound 3) is shown in the box to the left above. This compoun as an i entica mlrror image and it has a plane of symmetry. If you were to fold the molecule in half (along the labeled mirror plane) all the groups would overlap. Efyou rotate the molecule (that is not numbered) by 180°, all the groups would overlap with compound 1. CHMA231, Rice, Information Sheet: Stereoisomers and chiraljty. Page 3 3. thsical properties of enantiomers and diastereomers: {1} All the physical properties of enantiomers are the same. Reaction with achiral compounés is the same. (2) Differences are with optical activity and reaction with chiral compounds (think receptors!). (3) All the physical properties of diastereomers are different, incluciing reaction with achiral compounds. (4) They react in the same way with a pair of enantiorners. 4. Chemical reactions: When a reactant that does not have an asymmetric center reacts to form a Chiral product then the racemic mixture can form. Look at the example in the notes (pg. 6, Chapter 6). The racemic compound forms because the reactant can attack from either face so it forms an equal amount of both the R and S enantiomer. ...
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