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HĨA LẬP THỂ Stereochemistry Chirality • “Handedness”: găng tay phải khơng chồng khít với găng tay trái • Đối tượng ảnh qua gương khác với đối tượng gốc ban đầu => Chiral Carbons • Carbon tứ diện với nhóm khác chiral • ảnh qua gương hợp chất khác (enantiomer) => Mặt phẳng đối xứng gương • Nếu hai nhóm giống nhau, carbon achiral • Phân tử có mặt phẳng đối xứng khơng chiral.* Lưu ý! Nếu khơng có mặt phẳng đối xứng, phân tử chiral achiral ảnh qua gương có chồng khít lên hay không mirror Cl Cl H H Are these two structures identical? Cl Cl H H • The Biological Importance of Chirality • The binding specificity of a chiral receptor site for a chiral molecule is usually only favorable in one way "How would you like to live in Looking-Glass House, Kitty? I wonder if they'd give you milk in there? Perhaps Lookingglass milk isn't good to drink " Through the Looking Glass by Lewis Carroll Alice looked through the keyhole of this door into the Looking Glass garden (the Cathedral garden) Is looking glass milk safe to drink? ? Different enantiomers of a drug can have different effects H2N H stimulant amphetamine H NH2 adverse cardiovascular effects – Problem: Are A and B identical or enantiomers? • Manipulate B to see if it will become superposable with A • Exchange groups to try to convert B into A – One exchange of groups leads to the enantiomer of B – Two exchanges of groups leads back to B The Cahn Ingold Prelog R-S Notational System Assign (R) or (S) • Assign a priority number to each group attached to the chiral carbon • Atom with highest atomic number assigned the highest priority #1 • Working in 3D, rotate molecule so that lowest priority group is in back • Draw an arrow from highest to lowest priority group • Clockwise = (R), Counterclockwise = (S) If the order of decreasing precedence traces a clockwise path, the absolute configuration is R If the path is anticlockwise, the configuration is S d d b a a b c c Order of decreasing rank: a>b>c >d clockwise R anticlockwise S Enantiomers of 2-butanol H H CH2CH3 CH3CH2 C OH H3C (S)-2-Butanol HO C CH3 (R)-2-Butanol The Cahn-Ingold-Prelog Rules Orient the molecule so that lowest-ranked (lightest) substituent points away from you Trace the path from highest to lowest Clockwise (R), Counter Clockwise (S) Enantiomers of 2-butanol H H CH2CH3 CH3CH2 C OH H3C (S)-2-Butanol HO C CH3 (R)-2-Butanol Stereogenic center in a ring R or S ? H3C H R H H —CH2C=C > —CH2CH2 > —CH3 > —H Groups with double or triple bonds are assigned priorities as if their atoms were duplicated or triplicated Vinyl vs isopropyl – Problem: Are A and B identical or enantiomers? – Name them using R- S- designation (R)-1-bromo-1chloroethane http://www.cem.msu.edu/~reusch/VirtualText/sterism3.htm