Stereochemistry PDF

Stereochemistry Determination of Absolute Configuration  Dextrototatory & levorotatory In The Polarimeter :If the analyzer is rotated in a clockwise direction, the rotation, a (measured in degrees), is said to be positive (+). If the rotation is counterclockwise, the rotatio...

Stereochemistry Determination of Absolute Configuration  Dextrototatory & levorotatory In The Polarimeter :If the analyzer is rotated in a clockwise direction, the rotation, a (measured in degrees), is said to be positive (+). If the rotation is counterclockwise, the rotation is said to be negative (-). ❖ A substance that rotates plane-polarized light in the clockwise direction is also said to be dextrorotatory, ❖ and one that rotates plane-polarized light in a counterclockwise direction is said to be levorotatory ❖ (Latin: dexter, right, and laevus, left).  Naming Enantiomers: The R,S-System Because enantiomers are two different compounds, we need to distinguish them by name. If we name these two enantiomers using only the IUPAC system, both enantiomers will have the same name: 2-butanol (or sec-butyl alcohol) Three scientists devised a system of nomenclature, This system, called the R,S-system or the Cahn–Ingold– Prelog system  We will study the configuration for two representative models  How To Assign (R) and (S) Configurations We assign (R ) and (S ) configurations based on the following rules : For Fischer projection Rule 1 : Assign priorities (1, 2, 3, or 4) to the atoms directly bonded to the Stereogenic center in order of decreasing atomic number. The atom of highest atomic number gets the highest priority (1). Rule 2 If two atoms on a Stereogenic center are the same, assign priority based on the atomic number of the atoms bonded to these atoms. One atom of higher atomic number determines a higher priority. Rule 3 To assign a priority to an atom that is part of a multiple bond, treat a multiply bonded atom as an equivalent number of singly bonded atoms. Rule 4 Trace a circle from priority group 1 --- 2 --- 3 If the circle goes in the clockwise direction (to the right ) so the isomer is named R If the circle goes in the counterclockwise direction (to the left ) the isomer is named S Counterclockwise Clockwise Two Enantiomers S R In “Fischer projection” When the 4th group on the vertical line , we assign the direction Clockwise-------- R Counterclockwise---------- S  When the 4th group on the Horizontal line so after we assign the direction, we reverse it to get the right assigning Two Enantiomers counterclockwise clockwise It looks like an S isomer, but we It looks like an R isomer, but we must reverse the answer because must reverse the answer because The group 4 on horizontal line so , S R. The group 4 on horizontal line so , R S. S R For Flying Wedge : the same rules applied with these instructions R R S S R S S S  What about Compounds contains More than one chiral center? We have seen that if the compound contain one chiral center it will have two isomers (enantiomers) but what if the compound contains more than one chiral center, how many isomers it will have? We can calculate the number of isomers for any chiral compounds contain any chiral centers by this equation So if a compound has one chiral center so 21= 2………it will have 2 stereoisomers So if a compound has Two chiral center so 22= 4………it will have 4 stereoisomers So if a compound has Three chiral center so 23= 8………it will have 8 stereoisomers And so on….. Fischer Projections for compound with 2 Chiral Centers Asymmetric center Asymmetric center 3-bromo-2-hydroxybutanoic acid To draw the four stereoisomer in Fischer projection we will 1. draw any arrangement 2. then assign its configuration clockwise It looks like an R isomer, but we must reverse the answer because 3 The group 4 on horizontal line so R S. 1 S 4 2  clockwise It looks like an R isomer, but we must reverse the answer because 2 The group 4 on horizontal line so R S. 4 1 S 3  1) To draw the first isomer (reverse the groups on horizontal line) 2) To draw the Third isomer (reverse just ONE group on horizontal line) 3 )To draw the fourth isomer (reverse the groups on horizontal line) Diastereomers : are non mirror images, non superimposable, stereoisomers  Meso Compounds Whereas 2,3-dibromopentane has two Stereogenic centers and the maximum of four stereoisomers, 2,3-dibromobutane has two Stereogenic centers but fewer than the maximum number of stereoisomers. A MESO compound is an achiral compound that contains tetrahedral stereogenic centers. C is a MESO compound. 27 27 Because one stereoisomer of 2,3-dibromobutane is superimposable on its mirror image, there are only three stereoisomers and not four  Tartaric Acid Contains 2 asymmetric centers So, the maximum numbers of isomers is 4 But there are only three isomers only as two of them are identical And 2 only of them are optically active and the third is optically inactive as it Meso compound Identical  Racemic Mixtures An equal amount of two enantiomers is called a racemic mixture or a racemate. A racemic mixture is optically inactive. Because two enantiomers rotate plane-polarized light to an equal extent but in opposite directions, the rotations cancel, and no rotation is observed. Physical Properties of Enantiomers A and B compared 30  Cis and Trans Isomers (geometrical isomers) In an alkene the double bond, is rigid. holds attached groups in fixed positions. makes cis/trans isomers possible. there is no rotation around the double bond in alkenes. groups attached to the double bond are fixed relative to each other. You can make a “double bond” with your fingers with both thumbs on the same side or opposite from each other. 32 Two isomers are possible when groups are attached to the double bond are different. In a cis isomer, groups are attached on the same side of the double bond. In the trans isomer, the groups are attached on opposite sides. 33  H CH2CH3 H3C H C C C C H H H3C H 1-Butene 2-Methylpropene H3C CH3 H3C H C C C C H H H CH3 cis-2-Butene trans-2-Butene  H CH2CH3 H3C H C C C C H H H3C H 1-Butene 2-Methylpropene H3C CH3 C C Constitutional isomers H H cis-2-Butene  H CH2CH3 H3C H C C C C H H H3C H 1-Butene 2-Methylpropene H3C H Constitutional isomers C C H CH3 trans-2-Butene  Stereoisomers (Geometrical isomers) H3C CH3 H3C H C C C C H H H CH3 cis-2-Butene trans-2-Butene  Stereochemical Notation cis trans (identical or analogous (identical or analogous substituents substitutents on same side) on opposite sides)  The E-Z Notational System E: higher ranked substituents on opposite sides Z: higher ranked substituents on same sides. higher lower higher higher C C C C lower higher lower lower Entgegen = Opposite Zusammen = Together THANKS