Good notes on stereochemistry

Good notes on stereochemistry - Isomerism: Constitutional...

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1 Stereochemistry: Chiral Molecules Chapter 5 Constitutional Isomers = same molecular formula, different connectedness Stereoisomers = same molecular formula, same connectivity of atoms but different arrangement of atoms in space Isomerism: Constitutional Isomers and Stereoisomers Examples of Constitutional Isomers formula constitutional isomers C 3 H 8 O CH 3 CH 2 CH 2 OH CH 3 CHCH 3 C 4 H 10 3 2 2 3 3 CHCH 3 OH 3 Constitutional Isomers - Review Same molecular formula – different bond connectivities Always different properties Very different properties if different functional groups 1. Enantiomers: stereoisomers whose molecules are nonsuperposable mirror images 2. Diastereomers: stereoisomers whose molecules are not mirror images of each other L Example: cis and trans double bond isomers L Example: cis and trans cycloalkane isomers Two types of stereoisomers t Chiral molecule - has the property of handedness l Not superposable on its mirror image l Can exist as a pair of enantiomers t Pair of enantiomers l A chiral molecule and its mirror image t Achiral molecule l Superposable on its mirror image Enantiomers and Chiral Molecules Mirror images = handedness Left hand cannot be superimposed on the right hand
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2 Mirror image = converts right hand into left A chiral molecule: 2-butanol I and II are mirror images of each other I and II are not superposable and so are enantiomers C H OH CH 3 CH3 A C CH3 H HO CH3 B rotate 2- propanol is not chiral B is mirror image of A, but is superimposable by 180 o rotation Everything has a mirror image, the question is whether it is superimposable 1. A molecule with a single tetrahedral carbon bonded to four different groupswill always be chiral 2. Switching two groups at the tetrahedral center leads to the enantiomeric molecule 3. A molecule with more than one tetrahedral carbon bonded to four different groups is not always chiral l Stereogenic center (stereo center) L An atom bearing groups of such nature that an interchange of anytwo groups will produce a stereoisomer L Carbons at a tetrahedral stereogenic center are designated with an asterisk (*) l Example: 2-butanol Chiral molecules and stereogenic centers Mirror images not superimposable = enantiomers Tests for achirality 1. Draw mirror image. Is it superimposable? 2. Does the species have a bisecting plane of symmetry?
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3 An imaginary plane that bisects a molecule in such a way that the two halves of the molecule are mirror images of each other A molecule with a plane of symmetry cannot be chiral Cl H Plane of Symmetry = achiral H Cl 2-chloropropane If any two groups on a C are identical, achiral 2 -chlorobutane: no plane of Symmetry H Cl * Compounds with 4 different groups attached to one Carbon must be chiral unless a mesocompound (2 stereocenters) t The binding specificity of a chiral receptor site for a chiral molecule is usually only favorable in one way Many biological processes depend on chirality 1.
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This note was uploaded on 12/10/2009 for the course CHEM 3311 at Colorado.

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Good notes on stereochemistry - Isomerism: Constitutional...

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