DOCSLIB.ORG

Isomers Have Same Molecular Formula, but Different Structures Constitutional Isomers Differ in the Order of Attachment

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Isomers Have Same Molecular Formula, but Different Structures Constitutional Isomers Differ in the Order of Attachment Isomers Have same molecular formula, but different structures Constitutional Isomers Stereoisomers Differ in the order of Atoms are connected in the attachment of atoms same order, but differ (different bond connectivity) in spatial orientation CH3 CH3 H H3C H3C CH3 Enantiomers Diastereomers Image and mirrorimage Not related as image and are not superimposable mirrorimage stereoisomers H H H CH3 F F H3C H3C Br Cl Cl Br CH3 H H H Stereoisomers What is meant by Stereoisomers? They are any structure that have the same constitutional structure (have the same atoms bonded to the same atoms) but are arranged differently in three dimensions Differ in the ORIENTATION of atoms in space Have already seen this with cis and trans alkene structures H CH3 H3C H3C CH3 H H H Trans-2-butene Cis-2-butene The difference in orientation can cause vastly different properties for the molecule Stereoisomers How Do We Tell if Something is Chiral Chiral object has a mirror image that is different from the original object (chiral means “handedness” in Greek) Therefore mirror image is NONSUPERIMPOSABLE Hands are nonsuperimposable, therefore chiral Hammer is superimposable, therefore achiral Importance of Chirality When two chiral objects interact, a unique shape selectivity occurs (we will learn that with chiral organic molecules this leads to an ENERGY difference) Consider a hand and a glove (two chiral objects) Both are chiral, therefore there is a different energetic fit if the left hand goes into a right-handed glove compared to the right hand Would not be able to fit opposite hand into this chiral glove This energetic difference is not present if one of the objects is achiral Importance of Chirality Similar to a chiral hand interacting with a chiral glove leading to different energy interactions, when two chiral molecules interact there will be an energy difference depending upon the chirality Cl-Cl When methyl groups are pointing away and hydroxy or amine groups pointing up, chlorine interacts with chlorine and hydrogen with hydrogen In this enantiomer, however, the chlorine is directed toward the hydrogen and the other chlorine is directed toward other hydrogen Will obtain different energetic interactions when different chirality of compounds interact Chirality of Molecules Whenever a molecule cannot be superimposed on its mirror image it is chiral 2-bromobutane H C CH H C H C 3 Br Br 3 3 Br 3 H H H H H H H H Br H H H H CH3 H3C CH3 CH3 nonsuperimposable therefore chiral 2-bromopropane H C CH H C H C 3 Br Br 3 3 Br 3 Br H H H H H3C CH3 H3C H3C superimposable therefore achiral Definition of Enantiomers Enantiomers: any nonsuperimposable mirror image molecules 2-bromobutane H C CH H C H C 3 Br Br 3 3 Br 3 H H H H H H H H Br H H H H CH3 H3C CH3 CH3 enantiomers Nonsuperimposable mirror images Symmetry Ultimately symmetry distinguishes chiral molecules A chiral molecule can have no internal planes of symmetry Methane Chloromethane Dichloromethane Bromochloro- Bromochloro- (1 of 4 planes) (1 of 3 planes) (1 of 2 planes) methane fluoromethane (achiral) (achiral) (achiral) (achiral) (chiral) Determining Chiral Carbon Atoms A chiral carbon atom must have four different substituents In order to have no internal planes of symmetry all four substituents on a carbon must be different Therefore a double or triple bonded carbon atom is not chiral (2 or 3 substituents, respectively, would be the same) If a compound has only one chiral atom, then the molecule must be chiral Cahn-Ingold-Prelog Naming System for Chiral Carbon Atoms A chiral carbon is classified as being either R or S chirality How to name: 1) Prioritize atoms bonded to chiral carbon atom The higher the molecular weight the higher the priority -if two substituents are the same at the initial substitution point continue (atom-by-atom) until a point of difference *if there is never a difference then the carbon atom is not chiral! Treat double and triple bonded species as multiple bonds to site 1 Br 2 4 CH=CH H 2 CH2CH3 3 Cahn-Ingold-Prelog Naming System for Chiral Carbon Atoms 2) Place lowest priority substituent towards the back and draw an arrow from the highest priority towards the second priority 1 1 Br 2 Br 3 4 CH=CH 4 CH CH H 2 H 2 3 CH2CH3 CH=CH2 3 2 1 1 Br Br H3CH2C CH=CH2 H2C=HC CH2CH3 3 2 2 3 R S If this arrow is clockwise it is labeled R (Latin, rectus, “upright) If this arrow is counterclockwise it is labeled S (Latin, sinister, “left”) Enantiomers have Identical Energies Most typical physical properties are identical (melting point, boiling point, density, etc.) How can we distinguish enantiomers? Usually to characterize different compounds we would record the physical properties of a pure sample and compare, but this does not distinguish enantiomers One important characteristic – their optical activity is different Chiral compounds rotate plane polarized light Optical Rotation Amount (and direction) of Optical Rotation distinguishes enantiomers Enantiomeric compounds rotate the plane of polarized light by exactly the same amount but in OPPOSITE directions A compound will be labeled by its specific rotation ([α]) [α] = α(observed) / (c • l) HO H H OH (R)-2-butanol -13.5˚ (S)-2-butanol +13.5˚ Optical Rotation Diagram for a polarimeter Chiral compounds will rotate plane polarized light Achiral compounds do not rotate plane polarized light Racemic Mixtures Few Chiral Compounds are Obtained in Only One Enantiomeric Form A solution of a chiral molecule might contain a majority of one enantiomer but a small fraction of the opposite enantiomer If there are equal amounts of both enantiomers present it is called a RACEMIC mixture (or RACEMATE) The optical rotation will be zero (since each enantiomer has opposite sign of optical rotation) Racemic Mixtures A racemate can be formed in two ways: 1) Add equal amounts of each pure enantiomer (hard) 2) React an achiral molecule to generate a chiral center using only achiral reagents O H2 HO H H OH achiral Chiral products Enantiomeric Excess (or optical purity) For many cases where there is an abundance of one enantiomer relative to the other the sample is characterized by its enantiomeric excess (e.e.) The enantiomeric purity is defined by this e.e. [(R – S) / (R + S)] (100%) = e.e. Therefore if a given solution has 90% of one enantiomer (say R) and 10% of the other enantiomer (S) then the enantiomeric excess is 80% [(90 – 10) / (90 + 10)](100%) = 80% Fischer Projection Another convenient way to represent stereochemistry is with a Fischer projection To draw a Fischer projection: 1) Draw molecule with extended carbon chain in continuous trans conformation 2) Orient the molecule so the substituents are directed toward the viewer CO H HO 2 H CH3 ** Will need to change the view for each new carbon position along the main chain 3) Draw the molecule as flat with the substituents as crosses off the main chain CO2H HO H CH3 Fischer Projection Important Points - Crosses are always pointing out of the page - Extended chain is directed away from the page CO2H CO2H HO H HO H CH3 CH3 Rotation of Fischer Projections A Fischer projection can be rotated 180˚, but not 90˚ CO2H 180˚ CH3 Convention is to place HO H H OH more oxidized carbon at top, but obtain same CH3 CO2H stereoisomer A 90˚ rotation changes whether substituents are coming out or going into the page It changes the three dimensional orientation of the substituents CO2H 90˚ OH HO H H3C CO2H CH3 H CO2H CO2H H C H H C OH 3 OH 3 H Fischer Projection Fischer projections are extremely helpful with long extended chains with multiple stereocenters Orient view at each chiral center H C CH3 3 Br H Br H H H Cl Cl CH 3 CH3 An enantiomer is easily seen with a Fischer projection CH3 CH3 H Br Br H H Cl Cl H CH3 CH3 Merely consider the “mirror” image of the Fischer projection Number of Stereoisomers With n Stereocenters there are a Maximum Possible 2n Stereoisomers Therefore with two stereocenters there are a possible four stereoisomers enantiomers enantiomers CH3 CH3 CH3 CH3 H Br Br H H Br Br H H Cl Cl H Cl H H Cl CH3 CH3 CH3 CH3 These two stereoisomers are not related by a mirror plane, therefore they are not enantiomers Diastereomers - Any stereoisomer that is not an enantiomer Therefore the two stereoisomers are not mirror images Remember enantiomers: mirror images that are not superimposable Already observed diastereomers with cis and trans alkenes H CH3 H3C H3C CH3 H H H Stereoisomers, but not mirror images therefore diastereomers Meso Compounds Sometimes there are compounds that are achiral but have chiral carbon atoms (called MESO compounds) Maximum number of stereoisomers for a compound is 2n (where n is the number of chiral atoms) Enantiomers Diastereomers Identical (nonsuperimposable (not mirror related) (meso) mirror images) CH3 CH3 CH3 CH3 HO H H OH HO H H OH H OH HO H HO H H OH CH3 CH3 CH3 CH3 This compound has only 3 stereoisomers even though it has 2 chiral atoms Meso Compounds The meso compounds are identical (therefore not stereoisomers) therefore this compound has 3 stereoisomers Meso compounds are generally a result of an internal plane of symmetry bisecting two (or more) symmetrically disposed chiral centers CH3 HO H 2,3-(2R,3S)-butanediol has an internal plane of HO H symmetry as shown CH3 Any compound with an internal plane of symmetry is achiral Number of Stereoisomers Determining number of stereoisomers of HO2CCH(OH)CH(OH)CH(OH)CO2H " using Fischer projections Maximum number is 2n, therefore 23 = 8 1 2 3 2 CO2H CO2H CO2H CO2H HO H HO H HO H HO H HO H HO H H OH H OH HO H H OH HO H H OH CO2H CO2H CO2H
Load more

Recommended publications
  • Influence of Chirality on the Electromagnetic Wave Electrical and Computer Engineering
    Influence of Chirality on the Electromagnetic Wave Propagation: Unbounded Media And Chirowaveguides Priyá Dilipa Gaunço Dessai Dissertation submitted to obtain the Master Degree in Electrical and Computer Engineering Jury President: Professor José Manuel Bioucas Dias Supervisor: Professor Carlos Manuel dos Reis Paiva Co- Supervisor: Professor António Luís da Silva Topa Member Professor Sérgio de Almeida Matos December 2011 Abstract When a chiral medium interacts with the polarization state of an electromag- netic plane wave and couples selectively with either the left or right circularly polarized component, we call this property the optical activity. Since the beginning of the 19th century, the study of complex materials has intensied, and the chiral and bi-isotropic (BI) media have generated one of the most interesting and challenging subjects in the electromagnetic research groups in terms of theoretical problems and potential applications. This dissertation addresses the theoretical interaction between waves and the chiral media. From the study of chiral structures it is possible to observe the eect of the polarization rotation, the propagation modes and the cuto frequencies. The reection and transmission coecients between a simple isotropic media (SIM) and chiral media are also analyzed, as well as the relation between the Brewster angle and the chiral parameter. The BI planar structures are also analyzed for a closed guide, the parallel-plate chirowaveguide, and for a semi-closed guide, the grounded chiroslab. From these structures
    [Show full text]
  • Locally Enhanced and Tunable Optical Chirality in Helical Metamaterials
    hv photonics Article Locally Enhanced and Tunable Optical Chirality in Helical Metamaterials Philipp Gutsche 1,*, Raquel Mäusle 1 and Sven Burger 1,2 1 Zuse Institute Berlin, Takustr. 7, 14195 Berlin, Germany; [email protected] (R.M.); [email protected] (S.B.) 2 JCMwave GmbH, Bolivarallee 22, 14050 Berlin, Germany * Correspondence: [email protected]; Tel.: +49-30-841-85-203 Received: 31 October 2016; Accepted: 18 November 2016; Published: 23 November 2016 Abstract: We report on a numerical study of optical chirality. Intertwined gold helices illuminated with plane waves concentrate right and left circularly polarized electromagnetic field energy to sub-wavelength regions. These spots of enhanced chirality can be smoothly shifted in position and magnitude by varying illumination parameters, allowing for the control of light-matter interactions on a nanometer scale. Keywords: metamaterials; optical chirality; computational nano-optics 1. Introduction Helical metamaterials strongly impact the optical response of incident circularly (CPL) and linearly polarized light. They serve as efficient circular polarizers [1] and are candidates for chiral near-field sources [2]. Both, advancement of fabrication techniques [3–5] and design which employs fundamental physical properties [6] have significantly increased the performance of these complex structures. Most experimental, numerical and theoretical studies focus on the far-field response of helical metamaterials. Nevertheless, near-field interaction of light and chiral matter is expected to be enhanced in chiral near-fields. Recently, the quantity of optical chirality has been introduced quantifying this phenomenon [7]. In the weak-coupling regime, the interplay of electromagnetic fields and chiral molecules, which are not superimposable with their mirror image, is directly proportional to this near-field measure [8].
    [Show full text]
  • Chirality in Chemical Molecules
    Chirality in Chemical Molecules. Molecules which are active in human physiology largely function as keys in locks. The active molecule is then called a ligand and the lock a receptor. The structures of both are highly specific to the degree that if one atom is positioned in a different position than that required by the receptor to be activated, then no stimulation of the receptor or only partial activation can take place. Again in a similar fashion if one tries to open the front door with a key that looks almost the same than the proper key for that lock, one will usually fail to get inside. A simple aspect like a lengthwise groove on the key which is on the left side instead of the right side, can mean that you can not open the lock if your key is the “chirally incorrect” one. The second aspect to understand is which molecules display chirality and which do not. The word chiral comes from the Greek which means “hand-like”. Our hands are mirror images of each other and as such are not identical. If they were, then we would not need a right hand and left hand glove. We can prove that they are not identical by trying to lay one hand on top of the other palms up. When we attempt to do this, we observe that the thumbs and fingers do not lie on top of one another. We say that they are non-super imposable upon one another. Since they are not the same and yet are mirror images of each other, they are said to exhibit chirality.
    [Show full text]
  • Chapter 4: Stereochemistry Introduction to Stereochemistry
    Chapter 4: Stereochemistry Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C7H16: CH3 CH3 2-methylhexane 3-methylhexane Me Me Me C Me H Bu Bu Me Me 2-methylhexane H H mirror Me rotate Bu Me H 2-methylhexame is superimposable with its mirror image Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C7H16: CH3 CH3 2-methylhexane 3-methylhexane H C Et Et Me Pr Pr 3-methylhexane Me Me H H mirror Et rotate H Me Pr 2-methylhexame is superimposable with its mirror image Introduction To Stereochemistry Consider two of the compounds we produced while finding all the isomers of C7H16: CH3 CH3 2-methylhexane 3-methylhexane .Compounds that are not superimposable with their mirror image are called chiral (in Greek, chiral means "handed") 3-methylhexane is a chiral molecule. .Compounds that are superimposable with their mirror image are called achiral. 2-methylhexane is an achiral molecule. .An atom (usually carbon) with 4 different substituents is called a stereogenic center or stereocenter. Enantiomers Et Et Pr Pr Me CH3 Me H H 3-methylhexane mirror enantiomers Et Et Pr Pr Me Me Me H H Me H H Two compounds that are non-superimposable mirror images (the two "hands") are called enantiomers. Introduction To Stereochemistry Structural (constitutional) Isomers - Compounds of the same molecular formula with different connectivity (structure, constitution) 2-methylpentane 3-methylpentane Conformational Isomers - Compounds of the same structure that differ in rotation around one or more single bonds Me Me H H H Me H H H H Me H Configurational Isomers or Stereoisomers - Compounds of the same structure that differ in one or more aspects of stereochemistry (how groups are oriented in space - enantiomers or diastereomers) We need a a way to describe the stereochemistry! Me H H Me 3-methylhexane 3-methylhexane The CIP System Revisited 1.
    [Show full text]
  • The Power of Crowding for the Origins of Life
    Orig Life Evol Biosph (2014) 44:307–311 DOI 10.1007/s11084-014-9382-5 ORIGIN OF LIFE The Power of Crowding for the Origins of Life Helen Greenwood Hansma Received: 2 October 2014 /Accepted: 2 October 2014 / Published online: 14 January 2015 # Springer Science+Business Media Dordrecht 2015 Abstract Molecular crowding increases the likelihood that life as we know it would emerge. In confined spaces, diffusion distances are shorter, and chemical reactions produce fewer and more regular products. Crowding will occur in the spaces between Muscovite mica sheets, which has many advantages as a site for life’s origins. Keywords Muscovite mica . Molecular crowding . Origin of life . Mechanochemistry. Abiogenesis . Chemical confinement effects . Chirality. Protocells Cells are crowded. Protein molecules in cells are typically so close to each other that there is room for only one protein molecule between them (Phillips, Kondev et al. 2008). This is nothing like a dilute ‘prebiotic soup.’ Therefore, by analogy with living cells, the origins of life were probably also crowded. Molecular Confinement Effects Many chemical reactions are limited by the time needed for reactants to diffuse to each other. Shorter distances speed up these reactions. Molecular complementarity is another principle of life in which pairs or groups of molecules form specific interactions (Root-Bernstein 2012). Current examples are: enzymes & substrates & cofactors; nucleic acid base pairs; antigens & antibodies; nucleic acid - protein interactions. Molecular complementarity is likely to have been involved at life’s origins and also benefits from crowding. Mineral surfaces are a likely place for life’s origins and for formation of polymeric molecules (Orgel 1998).
    [Show full text]
  • Chiral Metamaterials: Simulations and Experiments
    HOME | SEARCH | PACS & MSC | JOURNALS | ABOUT | CONTACT US Chiral metamaterials: simulations and experiments This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2009 J. Opt. A: Pure Appl. Opt. 11 114003 (http://iopscience.iop.org/1464-4258/11/11/114003) The Table of Contents and more related content is available Download details: IP Address: 139.91.179.8 The article was downloaded on 10/11/2009 at 12:27 Please note that terms and conditions apply. IOP PUBLISHING JOURNAL OF OPTICS A: PURE AND APPLIED OPTICS J. Opt. A: Pure Appl. Opt. 11 (2009) 114003 (10pp) doi:10.1088/1464-4258/11/11/114003 REVIEW ARTICLE Chiral metamaterials: simulations and experiments Bingnan Wang1, Jiangfeng Zhou1, Thomas Koschny1,2,3, Maria Kafesaki2,3 and Costas M Soukoulis1,2,3 1 Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA 2 Institute of Electronic Structure and Laser, FORTH, Heraklion, Crete, 71110, Greece 3 Department of Materials Science and Technology, University of Crete, Heraklion, Crete, 71110, Greece E-mail: [email protected] Received 20 February 2009, accepted for publication 6 May 2009 Published 16 September 2009 Online at stacks.iop.org/JOptA/11/114003 Abstract Electromagnetic metamaterials are composed of periodically arranged artificial structures. They show peculiar properties, such as negative refraction and super-lensing, which are not seen in natural materials. The conventional metamaterials require both negative and negative μ to achieve negative refraction. Chiral metamaterial is a new class of metamaterials offering a simpler route to negative refraction.
    [Show full text]
  • Metamaterials with Magnetism and Chirality
    1 Topical Review 2 Metamaterials with magnetism and chirality 1 2;3 4 3 Satoshi Tomita , Hiroyuki Kurosawa Tetsuya Ueda , Kei 5 4 Sawada 1 5 Graduate School of Materials Science, Nara Institute of Science and Technology, 6 8916-5 Takayama, Ikoma, Nara 630-0192, Japan 2 7 National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, 8 Japan 3 9 Advanced ICT Research Institute, National Institute of Information and 10 Communications Technology, Kobe, Hyogo 651-2492, Japan 4 11 Department of Electrical Engineering and Electronics, Kyoto Institute of 12 Technology, Matsugasaki, Sakyo, Kyoto 606-8585, Japan 5 13 RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan 14 E-mail: [email protected] 15 November 2017 16 Abstract. This review introduces and overviews electromagnetism in structured 17 metamaterials with simultaneous time-reversal and space-inversion symmetry breaking 18 by magnetism and chirality. Direct experimental observation of optical magnetochiral 19 effects by a single metamolecule with magnetism and chirality is demonstrated 20 at microwave frequencies. Numerical simulations based on a finite element 21 method reproduce well the experimental results and predict the emergence of giant 22 magnetochiral effects by combining resonances in the metamolecule. Toward the 23 magnetochiral effects at higher frequencies than microwaves, a metamolecule is 24 miniaturized in the presence of ferromagnetic resonance in a cavity and coplanar 25 waveguide. This work opens the door to the realization of a one-way mirror and 26 synthetic gauge fields for electromagnetic waves. 27 Keywords: metamaterials, symmetry breaking, magnetism, chirality, magneto-optical 28 effects, optical activity, magnetochiral effects, synthetic gauge fields 29 Submitted to: J.
    [Show full text]
  • Supramolecular Chirality of Hydrogen-Bonded Rosette Assemblies Mercedes Crego-Calama
    Supramolecular chirality of hydrogen-bonded rosette assemblies Mercedes Crego-Calama To cite this version: Mercedes Crego-Calama. Supramolecular chirality of hydrogen-bonded rosette assemblies. Supramolecular Chemistry, Taylor & Francis: STM, Behavioural Science and Public Health Titles, 2007, 19 (01-02), pp.95-106. 10.1080/10610270600981716. hal-00513491 HAL Id: hal-00513491 https://hal.archives-ouvertes.fr/hal-00513491 Submitted on 1 Sep 2010 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Supramolecular Chemistry For Peer Review Only Supramolecular chirality of hydrogen-bonded rosette assemblies Journal: Supramolecular Chemistry Manuscript ID: GSCH-2006-0028 Manuscript Type: Review Date Submitted by the 31-May-2006 Author: Complete List of Authors: crego-calama, mercedes; University of Twente, SMCT Supramolecular chirality, noncovalent synthesis, amplification of Keywords: chirality, diastereomeric synthesis, enantiomeric synthesis Note: The following files were submitted by the author for peer review, but cannot be converted to PDF. You must view these files (e.g. movies) online. tif.sit URL: http:/mc.manuscriptcentral.com/tandf/gsch Email: [email protected] Page 1 of 20 Supramolecular Chemistry 1 2 3 4 5 Supramolecular chirality of hydrogen-bonded 6 7 rosette assemblies 8 9 10 Socorro Vázquez-Campos, Mercedes Crego-Calama*, and David N.
    [Show full text]
  • Chirality: the Handedness of Molecules
    06 Chirality: The Handedness of Molecules Tartaric acid is found in grapes and other fruits, both free and as its salts (see Section 6.4B). Inset: A model of tartaric acid. (© fatihhoca/iStockphoto) KEY QUESTIONS 6.1 What Are Stereoisomers? 6.9 What Is the Significance of Chirality in the 6.2 What Are Enantiomers? Biological World? 6.3 How Do We Designate the Configuration of a 6.10 How Can Enantiomers Be Resolved? Stereocenter? HOW TO 6.4 What Is the 2n Rule? 6.1 How to Draw Enantiomers 6.5 How Do We Describe the Chirality of 6.2 How to Determine the R & S Configuration Cyclic Molecules with Two Stereocenters? without Rotating the Molecule 6.6 How Do We Describe the Chirality 6.3 How to Determine Whether Two Compounds Are of Molecules with Three or More the Same, Enantiomers, or Diastereomers without Stereocenters? the Need to Spatially Manipulate the Molecule 6.7 What Are the Properties of Stereoisomers? 6.8 How Is Chirality Detected in the CHEMICAL CONNECTIONS Laboratory? 6A Chiral Drugs IN THIS CHAPTER, we will explore the relationships between three-dimensional ob- jects and their mirror images. When you look in a mirror, you see a reflection, or mirror Mirror image The reflection image, of yourself. Now, suppose your mirror image becomes a three-dimensional object. of an object in a mirror. 167 168 CHAPTER 6 Chirality: The Handedness of Molecules We could then ask, “What is the relationship between you and your mirror image?” By relationship, we mean “Can your reflection be superposed on the original ‘you’ in such a way that every detail of the reflection corresponds exactly to the original?” The answer is that you and your mirror image are not superposable.
    [Show full text]
  • The Origin of Biological Homochirality
    Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press The Origin of Biological Homochirality Donna G. Blackmond Department of Chemistry, The Scripps Research Institute, La Jolla, California 92037 Correspondence: [email protected] SUMMARY The fact that sugars, amino acids, and the biological polymers they construct exist exclusively in one of two possible mirror-image forms has fascinated scientists and laymen alike for more than a century. Yet, it was only in the late 20th century that experimental studies began to probe how biological homochirality, a signature of life, arose from a prebiotic world that presumably contained equal amounts of both mirror-image forms of these molecules. This review discusses experimental studies aimed at understanding how chemical reactions, physical processes, or a combination of both may provide prebiotically relevant mechanisms for the enrichment of one form of a chiral molecule over the other to allow for the emergence of biological homochirality. Outline 1 Introduction 5 Combining physical and chemical approaches 2 Models for the origin of homochirality 6 Concluding remarks 3 Chemical models References 4 Physical models Editors: Thomas R. Cech, Joan A. Steitz, and John F. Atkins Additional Perspectives on RNA Worlds available at www.cshperspectives.org Copyright # 2019 Cold Spring Harbor Laboratory Press; all rights reserved; doi: 10.1101/cshperspect.a032540 Cite this article as Cold Spring Harb Perspect Biol 2019;11:a032540 1 Downloaded from http://cshperspectives.cshlp.org/ on October 4, 2021 - Published by Cold Spring Harbor Laboratory Press D.G. Blackmond 1 INTRODUCTION (Roozeboom 1899) a half century before the macroscopic helicity of biopolymers was characterized (Pauling et al.
    [Show full text]
  • Chapter 5: Stereoisomerism- Three-Dimensional Arrangement of Atoms (Groups) in Space 5.1 Overview of Isomerism
    Chapter 5: Stereoisomerism- three-dimensional arrangement of atoms (groups) in space 5.1 Overview of Isomerism Isomers: different chemical compounds with the same formula Constitutional isomers: same formula, but different connectivity of atoms (or groups) C5H12 C4H10O C4H11N NH O 2 OH NH2 butanol diethyl ether different carbon skeleton different functional group different position of FG Stereoisomers: molecules with the same connectivity but different spatial arrangement of atoms (groups) H C CH 3 3 H3C H H H H CH3 cis-1,2-dimethylcyclopropane trans-1,2-dimethylcyclopropane H H H CH3 H C CH H C H 3 3 3 93 cis-2-butene trans-2-butene 5.2 Introduction to Stereoisomerism Enantiomers: non-superimposable mirror image isomers. Enantiomers are related to each other much like a right hand is related to a left hand Enantiomers have identical physical properties, e.g., bp, mp, etc. Chirality (derived from the Greek word for hand). Enantiomers are said to be chiral. 94 47 Chirality Center: Chiral molecules most commonly contain a carbon with four different groups; the carbon is referred to as a chiral center, or asymmetric center, or stereogenic center, or stereocenter. H H HO OH CO2H HO2C H3C CH3 H H H CO2H HO HO CO2H CO2H H3C H3C H3C HO CO2H H HO H3C H H H OH HO HO CO2H CO2H H3C H3C HO H CH3 CO2H HO2C H3C 95 5.3 Designating Configuration Using the Cahn-Ingold-Prelog System – Assigning the Absolute Configuration 1a. Look at the four atoms directly attached to the chiral carbon atom and rank them according to decreasing atomic number.
    [Show full text]
  • 7.3 Designating Configuration of Enantiomers
    Hornback_Ch07_219-256 12/14/04 7:00 PM Page 224 224 CHAPTER 7 I STEREOCHEMISTRY II PROBLEM 7.3 Indicate whether each of these objects or molecules has a plane of symmetry: a) Idealized human face b) Pencil c) Ear Cl CH3 O d) e) f) H3CCH3 O Cl± Br± CH g) C 3 h) C CH3 ± H H Cl Cl ± 7.3 Designating Configuration of Enantiomers Suppose we are working with one enantiomer of a chiral compound. How can we indi- cate which enantiomer is being used when writing about it? Or how can we look up the properties of this enantiomer in a reference book? We need a method to designate the configuration of the enantiomer, to denote the three-dimensional arrangement of the four groups around the chirality center, other than drawing the structure. In the case of many everyday chiral objects, the terms right and left are used, as in a left shoe or right- handed golf clubs. In Section 6.2 we learned how to designate the configuration of geo- metrical isomers using the Cahn-Ingold-Prelog sequence rules and the labels Z and E. The method to designate the configuration of enantiomers uses these same rules. The following steps are used to assign the configuration of a chiral compound: STEP 1 Assign priorities from 1 through 4 to the four groups bonded to the chirality center using the Cahn-Ingold-Prelog sequence rules presented in Section 6.2.The group with the highest priority receives number 1, and the lowest-priority group receives number 4.
    [Show full text]