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Đồng phân là một hiện tượng phổ biến và quan trọng trong hóa học hữu cơ. Hiện tượng một công thức phân tử ứng với hai hay nhiều chất khác nhau được gọi là hiện tượng đồng phân. Các đồng phân lập thể (còn gọi là đồng không gian) phân biệt nhau về cấu hình nghĩa là phân biệt nhau về sự phân bố không gian của các nguyên tử hoặc nhóm nguyên tử trong phân tử. Hai loại đồng phân lập thể chủ yếu là đồng phân quang học và đồng phân hình học. Ở hai loại này các chất đồng phân phân biệt nhau do sự phân bố không gian của các nguyên tử, nhóm nguyên tử xung quanh phần “ không trùng với ảnh” hoặc phần cứng nhắc của phân tử.
Stereochemistry: an introduction Chem 30A Fall 2002 Grazia Piizzi, Steve Hardinger Stereochemistry of Tetrahedral Carbons We need: one Carbon sp 3 -hybridized, at least to represent molecules as 3D objects For example: C Br H H Cl C H Br HCl C H H Br Cl H H Br Cl 2D drawing Not appropriate for Stereochem 3D drawing Appropriate for Stereochem 2 Let’s consider some molecules…… identical (same compound) First pair Second pair B r H Cl H H Cl Br H A B F Cl Br H H Cl Br F C D same molecular formula (CH 2 BrCl) same atom connectivity same molecular formula (CHFBrCl) same atom connectivity stereoisomers (two different compounds) superposable nonsuperposable 3 Thus, we can define……… Stereoisomers Stereoisomers: isomers that have same formula and connectivity but differ in the position of the atoms in space Stereochemistry Stereochemistry: chemistry that studies the properties of stereoisomers 4 Historical perspective Christiaan Huygens (1629-1695). Dutch astronomer, mathematician, and physicist. He discovers plane polarized light: Normal light (nonpolarized) Horizontally polarized light Direction of light Horizontal filter Vertical filter Light completely blocked 5 Historical perspective Carl Wilhelm Scheele (1742-1786) “Oh, how happy I am! No care for eating or drinking or dwelling, no care for my pharmaceutical business, for this is mere play to me. But to watch new phenomena this is all my care, and how glad is the enquirer when discovery rewards his diligence; then his heart rejoices" In 1769, he discovers Tartaric Acid from tartar (the potassium salt of tartaric acid, deposited on barrels and corks during fermentation of grape juice). H O C O 2 H CO 2 HHO Tartaric Acid 6 Historical perspective Jean Baptiste Biot (1774-1862) In 1815, he notes that certain natural organic compounds (liquids or solutions) rotate plane polarized light (Optical Activity). plane polarized light tube containing a liquid organic compound or solution plane polarized light IN OUT molecule m o l e c u l e m o l e c u l e m o l e c u l e m o l e c u l e m o l e c u l e 7 Definitions Optically Active Optically Active: the ability of some compounds to rotate plane polarized light. Dextrorotatory Dextrorotatory (+) (+): an optically active compound that rotates plane polarized light in a clockwise direction. Levorotatory Levorotatory ( ( - - ) ) : : an optically active compound that rotates plane polarized light in a counterclockwise direction. N N H CH 3 N CH 3 H H 3 C H (-)-Nicotine (+)-Methamphetamine 8 In 1819, Racemic Acid was discovered. Later shown to have the same formula as Tartaric Acid. Historical perspective H O C O 2 H CO 2 HHO In 1832, Biot notes that Tartaric Acid from grape juice fermentation rotates plane polarized light in a clockwise direction: H O C O 2 H CO 2 HHO H O C O 2 H CO 2 HHO H O C O 2 H CO 2 HHO H O C O 2 H CO 2 HHO plane polarized light tube containing solution of Tartaric Acid (TA) plane polarized light, rotated clockwise IN OUT TA is dextrorotatory 9 In 1819, Racemic Acid was discovered. Later shown to have the same formula as Tartaric Acid. Historical perspective H O C O 2 H CO 2 HHO In 1838, Biot notes that Racemic Acid does not rotate plane polarized light: plane polarized light tube containing solution of Racemic Acid (RA) plane polarized light, unchanged H O C O 2 H CO 2 HHO H O C O 2 H CO 2 HHO H O C O 2 H CO 2 HHO H O C O 2 H CO 2 HHO IN OUT RA is not optically active 10