Stereochemistry Chiral Molecules

Slides:



Advertisements
Similar presentations
chemistry in three dimensions
Advertisements

153 Symmetry Monarch butterfly: bilateral symmetry= mirror symmetry Whenever winds blow butterflies find a new place on the willow tree -Basho (~1644 -
Optical Activity Enantiomers are different compounds:
Chapter 5- Chirality. Chirality A chiral object is an object that possesses the property of handedness A chiral object, such as each of our hands, is.
Stereochemistry Chiral Molecules
Organic Chemistry 4 th Edition Paula Yurkanis Bruice Irene Lee Case Western Reserve University Cleveland, OH ©2004, Prentice Hall Chapter 5 Stereochemistry.
Organic Chemistry Stereochemistry. Isomers compounds with the same molecular formula but not identical structures.
The study of the three dimensional structure of molecules.
Chapter 5 Stereochemistry
Bio Organic Chemistry Stereochemistry. Review of Isomers.
William H. Brown Christopher S. Foote Brent L. Iverson
Chapter 6 Stereochemistry.
© 2013 Pearson Education, Inc. Stereochemistry Stereochemistry refers to the 3-dimensional properties and reactions of molecules. It has its own language.
CHE 240 Unit IV Stereochemistry, Substitution and Elimination Reactions CHAPTER FIVE Terrence P. Sherlock Burlington County College 2004.
Chapter 5 Stereochemistry.
Chapter 5 Stereochemistry
Why Stereochemistry? Stereo isomers Optical Activity – Optical Isomers Optical Rotation Chirality-Chiral atom-Chiral molecules Enantiomers Specifying.
Stereochemistry.
3 3-1 Organic Chemistry William H. Brown & Christopher S. Foote.
Stereochemistry Stereoisomerism.
Chapter 4: Stereochemistry. Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C 7 H 16 : 2-methylhexame.
Stereochemistry & Chiral Molecules. Isomerism Isomers are different compounds with the same molecular formula 1) Constitutional isomers: their atoms are.
Created by Professor William Tam & Dr. Phillis Chang Chapter 5 Stereochemistry Chiral Molecules Copyright © 2014 by John Wiley & Sons, Inc. All rights.
111 Fall 2008Dr. Halligan CHM 234 Stereochemistry Chapter 5.
1 Stereoisomerism Chapter 26 Hein * Best * Pattison * Arena Colleen Kelley Chemistry Department Pima Community College © John Wiley and Sons, Inc. Version.
Fischer Projections Fischer projection: a two- dimensional representation showing the configuration of a stereocenter –horizontal lines represent bonds.
Chapter 5 Stereochemistry Jo Blackburn Richland College, Dallas, TX Dallas County Community College District  2003,  Prentice Hall Organic Chemistry,
Created by Professor William Tam & Dr. Phillis Chang Ch Chapter 5 Stereochemistry Chiral Molecules.
Stereochemistry 1. Stereoisomerism 2. Chirality
Introduction to Organic Chemistry 2 ed William H. Brown
Properties of Chiral Molecules: Optical Activity.
Stereochemistry. 2 Chirality Handedness“ Handedness ”: Right glove doesn’t fit the left hand. Mirror-imageMirror-image object is different from the original.
Stereochemistry Chiral Molecules
Chapter 5 Stereochemistry
Constitutional Isomers
Stereochemistry at Tetrahedral Centers
Chapter 5 Stereochemistry: Chiral Molecules 1.
Chemistry 2100 Chapter 15. Enantiomers Enantiomers: Enantiomers: Nonsuperposable mirror images. –As an example of a molecule that exists as a pair of.
Chapter 5 Stereochemistry: Chiral Molecules
William Brown Thomas Poon Chapter Six Chirality: The Handedness of Molecules.
Chapter 5 Stereochemistry Jo Blackburn Richland College, Dallas, TX Dallas County Community College District  2006,  Prentice Hall Organic Chemistry,
Chiral Molecules Chapter 5.
Stereochemistry 240 Chem Chapter 5 1. Isomerism Isomers are different compounds that have the same molecular formula.
Isomers Are different compounds with the same molecular formula
Stereochemistry & Chiral Molecules
chemistry in three dimensions
Organic Chemistry, 9th Edition L. G. Wade, Jr.
Stereochemistry Stereochemistry refers to the
9. Stereochemistry.
University of California,
Stereoisomerism and Chirality Unit 5.
Stereoisomerism and Chirality Unit 5.
Stereoisomerism.
Chapter 20.3: Stereoisomerism
Stereochemistry Stereochemistry refers to the
Chapter 5 Stereochemistry: Chiral Molecules
Isomers In this chapter, we concentrate on enantiomers and diastereomers.
Chapter 5 Stereochemistry: Chiral Molecules
Stereoisomerism and Chirality Unit 6.
240 Chem Stereochemistry Chapter 5.
Chirality - the Handedness of Molecules
Chapter 4: Stereochemistry
240 Chem Stereochemistry Chapter 5.
TOPIC 4. STEREOCHEMISTRY (Chapter 5)
Symmetry Monarch butterfly: bilateral symmetry= mirror symmetry 153.
TOPIC 4. STEREOCHEMISTRY (Chapter 5)
Stereochemistry Stereochemistry refers to the
240 Chem Stereochemistry Chapter 5.
Presentation transcript:

Stereochemistry Chiral Molecules Chapter 5 Stereochemistry Chiral Molecules

Chirality & Stereochemistry An object is achiral (not chiral) if the object and its mirror image are identical

A chiral object is one that cannot be superposed on its mirror image

1A. The Biological Significance of Chirality Chiral molecules are molecules that cannot be superimposable with their mirror images One enantiomer causes birth defects, the other cures morning sickness

One enantiomer is a bronchodilator, the other inhibits platelet aggregation

66% of all drugs in development are chiral, 51% are being studied as a single enantiomer Of the $475 billion in world-wide sales of formulated pharmaceutical products in 2008, $205 billion was attributable to single enantiomer drugs

2B. Stereoisomers Stereoisomers are NOT constitutional isomers Stereoisomers have their atoms connected in the same sequence but they differ in the arrangement of their atoms in space. The consideration of such spatial aspects of molecular structure is called stereochemistry

2C. Enantiomers & Diastereomers Stereoisomers can be subdivided into two general categories: enantiomers & diasteromers Enantiomers – stereoisomers whose molecules are nonsuperposable mirror images of each other Diastereomers – stereoisomers whose molecules are not mirror images of each other

Geometrical isomers (cis & trans isomers) are: Diastereomers

Enantiomers and Chiral Molecules Enantiomers occur only with compounds whose molecules are chiral A chiral molecule is one that is NOT superposable on its mirror image The relationship between a chiral molecule and its mirror image is one that is enantiomeric. A chiral molecule and its mirror image are said to be enantiomers of each other

(I) and (II) are nonsuperposable mirror images of each other

A Single Chirality Center Causes a Molecule to Be Chiral The most common type of chiral compounds that we encounter are molecules that contain a carbon atom bonded to four different groups. Such a carbon atom is called an asymmetric carbon or a chiral center and is usually designated with an asterisk (*)

(III) and (IV) are nonsuperposable mirror images of each other

(V) and (VI) are superposable ⇒ not enantiomers ⇒ achiral

achiral chiral Plane of symmetry: TEST FOR CHIRALITY plane of symmetry No plane of symmetry chiral

Naming Enantiomers: R,S-System Using only the IUPAC naming that we have learned so far, these two enantiomers will have the same name: 2-Butanol This is undesirable because each compound must have its own distinct name

Example ① ④ ② or ③ ② or ③ ① ④ ③ ② (C, H, H) (H, H, H)

① OH Et Me H ④ ③ ② OH Et Me H Arrows are clockwise (R)-2-Butanol

Other examples ① Counter- clockwise ④ ③ (S) ② Clockwise ② (R) ④ ③ ①

Other examples (R) Rotate C–Cl bond such that H is pointed to the back ② ④ ③ ① Clockwise (R)

Rule 4 For groups containing double or triple bonds, assign priorities as if both atoms were duplicated or triplicated

Example ③ (S) ④ ② ①

Other examples ② (R) ④ ③ ① ③ ④ ② ② (S) ③ ①

Properties of Enantiomers: Optical Activity Mirror images that are not superposable

Enantiomers have identical physical properties (e. g Enantiomers have identical physical properties (e.g. melting point, boiling point, refractive index, solubility etc.) Compound bp (oC) mp (oC) (R)-2-Butanol 99.5 (S)-2-Butanol (+)-(R,R)-Tartaric Acid 168 – 170 (–)-(S,S)-Tartaric Acid (+/–)-Tartaric Acid 210 – 212

Enantiomers Have the same chemical properties (except reaction/interactions with chiral substances) Show different behavior only when they interact with other chiral substances Turn plane-polarized light on opposite direction

Optical activity The property possessed by chiral substances of rotating the plane of polarization of plane-polarized light

8A. Plane-Polarized Light The electric field (like the magnetic field) of light is oscillating in all possible planes When this light passes through a polarizer (Polaroid lens), we get plane-polarized light (oscillating in only one plane) Polaroid lens

8B. The Polarimeter A device for measuring the optical activity of a chiral compound a = observed optical rotation

ℓ 8C. Specific Rotation observed rotation temperature wavelength of light (e.g. D-line of Na lamp, l=589.6 nm) concentration of sample solution in g/mL length of cell in dm (1 dm = 10 cm)

The value of a depends on the particular experiment (since there are different concentrations with each run) But specific rotation [a] should be the same regardless of the concentration

If analyzer is rotated CLOCKWISE the rotation is (+) and the molecule is Dextrorotatory If analyzer is rotated COUNTERCLOCKWISE the rotation is (-) and the molecule is Levorotatory

Two enantiomers should have the same value of specific rotation, but the signs are opposite

A racemic mixture causes no net rotation of plane-polarized light 9A. Racemic Forms An equimolar mixture of two enantiomers is called a racemic mixture (or racemate or racemic form) A racemic mixture causes no net rotation of plane-polarized light equal & opposite rotation by the enantiomer rotation

9B. Racemic Forms and Enantiomeric Excess A sample of an optically active substance that consists of a single enantiomer is said to be enantiomerically pure or to have an enantiomeric excess of 100%

Enantiomeric excess (ee) Also known as the optical purity Can be calculated from optical rotations

Example A mixture of the 2-butanol enantiomers showed a specific rotation of +6.76. The enantiomeric excess of the (S)-(+)-2-butanol is 50% The net composition is: 75% S (+), 25% R (-)

Molecules with More than One Chirality Center Diastereomers Stereoisomers that are not enantiomers Unlike enantiomers, diastereomers usually have substantially different chemical and physical properties

* * Note: In compounds with n tetrahedral stereocenters, the maximum number of stereoisomers is 2n.

(I) & (II) are enantiomers to each other (III) & (IV) are enantiomers to each other

Diastereomers to each other: (I) & (III), (I) & (IV), (II) & (III), (II) & (IV)

Note: (III) contains a plane of symmetry, is a meso compound, and is achiral ([a] = 0o).

(I) & (II) are enantiomers to each other and chiral (III) & (IV) are identical and achiral

(I) & (III), (II) & (III) are diastereomers Only 3 stereoisomers: (I) & (II) {enantiomers}, (III) {meso}

12B. How to Name Compounds with More than One Chirality Center 2,3-Dibromobutane Look through C2–Ha bond ④ ① ③ ② C2: (R) configuration

Look through C3–Hb bond Full name: (2R, 3R)-2,3-Dibromobutane ④ ① ③ ② C3: (R) configuration Full name: (2R, 3R)-2,3-Dibromobutane

Fischer Projection Formulas 13A. How To Draw and Use Fischer Projections Fischer Projection

Fischer Projection

enantiomers (I) and (II) are both chiral and they are enantiomers with each other

(III) is achiral (a meso compound) Plane of symmetry (III) is achiral (a meso compound) (III) and (I) are diastereomers to each other

Stereoisomerism of Cyclic Compounds a meso compound achiral Plane of symmetry

14A. Cyclohexane Derivatives 1,4-Dimethylcyclohexane Both cis- & trans-1,4-dimethylcyclo-hexanes are achiral and optically inactive The cis & trans forms are diastereomers Plane of symmetry

1,3-Dimethylcyclohexane Plane of symmetry cis-1,3-Dimethylcyclohexane has a plane of symmetry and is a meso compound

1,3-Dimethylcyclohexane NO plane of symmetry trans-1,3-Dimethylcyclohexane exists as a pair of enantiomers

1,3-Dimethylcyclohexane Has two chirality centers but only three stereoisomers

1,2-Dimethylcyclohexane trans-1,2-Dimethylcyclohexane exists as a pair of enantiomers

Although (I) and (II) are enantiomers to each other, they can interconvert rapidly  (I) and (II) are achiral

Relating Configurations through Reactions in Which No Bonds to the Chirality Center Are Broken If a reaction takes place in a way so that no bonds to the chirality center are broken, the product will of necessity have the same general configuration of groups around the chirality center as the reactant

Same configuration Same configuration

15A. Relative and Absolute Configurations Chirality centers in different molecules have the same relative configuration if they share three groups in common and if these groups with the central carbon can be superposed in a pyramidal arrangement

(R)-2-Butanol (S)-2-Butanol The absolute configuration of a chirality center is its (R) or (S) designation, which can only be specified by knowledge of the actual arrangement of groups in space at the chirality center (R)-2-Butanol (S)-2-Butanol enantiomers

The Synthesis of Chiral Molecules 10A. Racemic Forms

10B. Stereoselective Syntheses Stereoselective reactions are reactions that lead to a preferential formation of one stereoisomer over other stereoisomers that could possibly be formed enantioselective – if a reaction produces preferentially one enantiomer over its mirror image diastereoselective – if a reaction leads preferentially to one diastereomer over others that are possible

Kinetic Resolution One enantiomer reacts “fast” and another enantiomer reacts “slow”

e.g.

 END OF CHAPTER 5 