Ch.6 Alkenes: Structure and Reactivity Alkene = Olefin
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Proton Nmr Spectroscopy
1H NMR Spectroscopy (#1c) The technique of 1H NMR spectroscopy is central to organic chemistry and other fields involving analysis of organic chemicals, such as forensics and environmental science. It is based on the same principle as magnetic resonance imaging (MRI). This laboratory exercise reviews the principles of interpreting 1H NMR spectra that you should be learning right now in Chemistry 302. There are four questions you should ask when you are trying to interpret an NMR spectrum. Each of these will be discussed in detail. The Four Questions to Ask While Interpreting Spectra 1. How many different environments are there? The number of peaks or resonances (signals) in the spectrum indicates the number of nonequivalent protons in a molecule. Chemically equivalent protons (magnetically equivalent protons) give the same signal in the NMR whereas nonequivalent protons give different signals. 1 For example, the compounds CH3CH3 and BrCH2CH2Br all have one peak in their H NMR spectra because all of the protons in each molecule are equivalent. The compound below, 1,2- dibromo-2-methylpropane, has two peaks: one at 1.87 ppm (the equivalent CH3’s) and the other at 3.86 ppm (the CH2). 1.87 1.87 ppm CH 3 3.86 ppm Br Br CH3 1.87 ppm 3.86 10 9 8 7 6 5 4 3 2 1 0 2. How many 1H are in each environment? The relative intensities of the signals indicate the numbers of protons that are responsible for individual signals. The area under each peak is measured in the form of an integral line. -
Course Material 2.Pdf
Reactive Intermediates Source: https://www.askiitians.com/iit-jee-chemistry/organic-chemistry/iupac- and-goc/reaction-intermediates/ Table of Content • Carbocations • Carbanions • Free Radicals • Carbenes • Arenium Ions • Benzynes Synthetic intermediate are stable products which are prepared, isolated and purified and subsequently used as starting materials in a synthetic sequence. Reactive intermediate, on the other hand, are short lived and their importance lies in the assignment of reaction mechanisms on the pathway from the starting substrate to stable products. These reactive intermediates are not isolated, but are detected by spectroscopic methods, or trapped chemically or their presence is confirmed by indirect evidence. • Carbocations Carbocations are the key intermediates in several reactions and particularly in nucleophilic substitution reactions. Structure of Carbocations : Generally, in the carbocations the positively charged carbon atom is bonded to three other atoms and has no nonbonding electrons. It is sp 2 hybridized with a planar structure and bond angles of about 120°. There is a + vacant unhybridized p orbital which in the case of CH 3 lies perpendicular to the plane of C—H bonds. Stability of Carbocations: There is an increase in carbocation stability with additional alkyl substitution. Thus one finds that addition of HX to three typical olefins decreases in the order (CH 3)2C=CH 2>CH 3—CH = CH 2 > CH 2 = CH 2. This is due to the relative stabilities of the carbocations formed in the rate determining step which in turn follows from the fact that the stability is increased by the electron releasing methyl group (+I), three such groups being more effective than two, and two more effective than one. -
Reactions of Benzene & Its Derivatives
Organic Lecture Series ReactionsReactions ofof BenzeneBenzene && ItsIts DerivativesDerivatives Chapter 22 1 Organic Lecture Series Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon: Halogenation: FeCl3 H + Cl2 Cl + HCl Chlorobenzene Nitration: H2 SO4 HNO+ HNO3 2 + H2 O Nitrobenzene 2 Organic Lecture Series Reactions of Benzene Sulfonation: H 2 SO4 HSO+ SO3 3 H Benzenesulfonic acid Alkylation: AlX3 H + RX R + HX An alkylbenzene Acylation: O O AlX H + RCX 3 CR + HX An acylbenzene 3 Organic Lecture Series Carbon-Carbon Bond Formations: R RCl AlCl3 Arenes Alkylbenzenes 4 Organic Lecture Series Electrophilic Aromatic Substitution • Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile H E + + + E + H • In this section: – several common types of electrophiles – how each is generated – the mechanism by which each replaces hydrogen 5 Organic Lecture Series EAS: General Mechanism • A general mechanism slow, rate + determining H Step 1: H + E+ E El e ctro - Resonance-stabilized phile cation intermediate + H fast Step 2: E + H+ E • Key question: What is the electrophile and how is it generated? 6 Organic Lecture Series + + 7 Organic Lecture Series Chlorination Step 1: formation of a chloronium ion Cl Cl + + - - Cl Cl+ Fe Cl Cl Cl Fe Cl Cl Fe Cl4 Cl Cl Chlorine Ferric chloride A molecular complex An ion pair (a Lewis (a Lewis with a positive charge containing a base) acid) on ch lorine ch loronium ion Step 2: attack of -
Lecture Outline a Closer Look at Carbocation Stabilities and SN1/E1 Reaction Rates We Learned That an Important Factor in the SN
Lecture outline A closer look at carbocation stabilities and SN1/E1 reaction rates We learned that an important factor in the SN1/E1 reaction pathway is the stability of the carbocation intermediate. The more stable the carbocation, the faster the reaction. But how do we know anything about the stabilities of carbocations? After all, they are very unstable, high-energy species that can't be observed directly under ordinary reaction conditions. In protic solvents, carbocations typically survive only for picoseconds (1 ps - 10–12s). Much of what we know (or assume we know) about carbocation stabilities comes from measurements of the rates of reactions that form them, like the ionization of an alkyl halide in a polar solvent. The key to connecting reaction rates (kinetics) with the stability of high-energy intermediates (thermodynamics) is the Hammond postulate. The Hammond postulate says that if two consecutive structures on a reaction coordinate are similar in energy, they are also similar in structure. For example, in the SN1/E1 reactions, we know that the carbocation intermediate is much higher in energy than the alkyl halide reactant, so the transition state for the ionization step must lie closer in energy to the carbocation than the alkyl halide. The Hammond postulate says that the transition state should therefore resemble the carbocation in structure. So factors that stabilize the carbocation also stabilize the transition state leading to it. Here are two examples of rxn coordinate diagrams for the ionization step that are reasonable and unreasonable according to the Hammond postulate. We use the Hammond postulate frequently in making assumptions about the nature of transition states and how reaction rates should be influenced by structural features in the reactants or products. -
Spectroscopic and Photophysical Properties of the Trioxatriangulenium Carbocation and Its Interactions with Supramolecular Systems
View metadata,Downloaded citation and from similar orbit.dtu.dk papers on:at core.ac.uk May 03, 2019 brought to you by CORE provided by Online Research Database In Technology Spectroscopic and Photophysical Properties of the Trioxatriangulenium Carbocation and its Interactions with Supramolecular Systems Reynisson, Johannes Publication date: 2000 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Reynisson, J. (2000). Spectroscopic and Photophysical Properties of the Trioxatriangulenium Carbocation and its Interactions with Supramolecular Systems. Roskilde: Risø National Laboratory. Denmark. Forskningscenter Risoe. Risoe-R, No. 1191(EN) General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Spectroscopic and Photophysical Properties of the Trioxatriangulenium Carbocation and its Interactions with Supramolecular Systems Jóhannes Reynisson OO O Risø National Laboratory, Roskilde, Denmark June 2000 1 Abstract Trioxatriangulenium (TOTA+, 4,8,12-trioxa-4,8,12,12c-tetrahydro- dibenzo[cd,mn]-pyrenylium) is a closed shell carbocation which is stable in its crystalline form and in polar solvents at ambient temperatures. -
Nonclassical Carbocations: from Controversy to Convention
Nonclassical Carbocations H H C From Controversy to Convention H H H A Stoltz Group Literature Meeting brought to you by Chris Gilmore June 26, 2006 8 PM 147 Noyes Outline 1. Introduction 2. The Nonclassical Carbocation Controversy - Winstein, Brown, and the Great Debate - George Olah and ending the discussion - Important nonclassical carbocations 3. The Nature of the Nonclassical Carbocation - The 3-center, 2-electron bond - Cleaving C-C and C-H σ−bonds - Intermediate or Transition state? Changing the way we think about carbocations 4. Carbocations, nonclassical intermediates, and synthetic chemistry - Biosynthetic Pathways - Steroids, by W.S. Johnson - Corey's foray into carbocationic cascades - Interesting rearrangements - Overman and the Prins-Pinacol Carbocations: An Introduction Traditional carbocation is a low-valent, trisubstituted electron-deficient carbon center: R superacid R R LG R R R R R R "carbenium LGH ion" - 6 valence e- - planar structure - empty p orbital Modes of stabilization: Heteroatomic Assistance π-bond Resonance σ-bond Participation R X H2C X Allylic Lone Pair Anchimeric Homoconjugation Hyperconjugation Non-Classical Resonance Assistance Stabilization Interaction Outline 1. Introduction 2. The Nonclassical Carbocation Controversy - Winstein, Brown, and the Great Debate - George Olah and ending the discussion - Important nonclassical carbocations 3. The Nature of the Nonclassical Carbocation - The 3-center, 2-electron bond - Cleaving C-C and C-H σ−bonds - Intermediate or Transition state? Changing the way we think about carbocations 4. Carbocations, nonclassical intermediates, and synthetic chemistry - Biosynthetic Pathways - Steroids, by W.S. Johnson - Corey's foray into carbocationic cascades - Interesting rearrangements - Overman and the Prins-Pinacol The Nonclassical Problem: Early Curiosities Wagner, 1899: 1,2 shift OH H - H+ Meerwein, 1922: 1,2 shift OH Meerwin, H. -
George A. Olah 151
MY SEARCH FOR CARBOCATIONS AND THEIR ROLE IN CHEMISTRY Nobel Lecture, December 8, 1994 by G EORGE A. O L A H Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089-1661, USA “Every generation of scientific men (i.e. scientists) starts where the previous generation left off; and the most advanced discov- eries of one age constitute elementary axioms of the next. - - - Aldous Huxley INTRODUCTION Hydrocarbons are compounds of the elements carbon and hydrogen. They make up natural gas and oil and thus are essential for our modern life. Burning of hydrocarbons is used to generate energy in our power plants and heat our homes. Derived gasoline and diesel oil propel our cars, trucks, air- planes. Hydrocarbons are also the feed-stock for practically every man-made material from plastics to pharmaceuticals. What nature is giving us needs, however, to be processed and modified. We will eventually also need to make hydrocarbons ourselves, as our natural resources are depleted. Many of the used processes are acid catalyzed involving chemical reactions proceeding through positive ion intermediates. Consequently, the knowledge of these intermediates and their chemistry is of substantial significance both as fun- damental, as well as practical science. Carbocations are the positive ions of carbon compounds. It was in 1901 that Norris la and Kehrman lb independently discovered that colorless triphe- nylmethyl alcohol gave deep yellow solutions in concentrated sulfuric acid. Triphenylmethyl chloride similarly formed orange complexes with alumi- num and tin chlorides. von Baeyer (Nobel Prize, 1905) should be credited for having recognized in 1902 the salt like character of the compounds for- med (equation 1). -
Introduction to Alkenes and Alkynes in an Alkane, All Covalent Bonds
Introduction to Alkenes and Alkynes In an alkane, all covalent bonds between carbon were σ (σ bonds are defined as bonds where the electron density is symmetric about the internuclear axis) In an alkene, however, only three σ bonds are formed from the alkene carbon -the carbon thus adopts an sp2 hybridization Ethene (common name ethylene) has a molecular formula of CH2CH2 Each carbon is sp2 hybridized with a σ bond to two hydrogens and the other carbon Hybridized orbital allows stronger bonds due to more overlap H H C C H H Structure of Ethylene In addition to the σ framework of ethylene, each carbon has an atomic p orbital not used in hybridization The two p orbitals (each with one electron) overlap to form a π bond (p bonds are not symmetric about the internuclear axis) π bonds are not as strong as σ bonds (in ethylene, the σ bond is ~90 Kcal/mol and the π bond is ~66 Kcal/mol) Thus while σ bonds are stable and very few reactions occur with C-C bonds, π bonds are much more reactive and many reactions occur with C=C π bonds Nomenclature of Alkenes August Wilhelm Hofmann’s attempt for systematic hydrocarbon nomenclature (1866) Attempted to use a systematic name by naming all possible structures with 4 carbons Quartane a alkane C4H10 Quartyl C4H9 Quartene e alkene C4H8 Quartenyl C4H7 Quartine i alkine → alkyne C4H6 Quartinyl C4H5 Quartone o C4H4 Quartonyl C4H3 Quartune u C4H2 Quartunyl C4H1 Wanted to use Quart from the Latin for 4 – this method was not embraced and BUT has remained Used English order of vowels, however, to name the groups -
Ganic Compounds
6-1 SECTION 6 NOMENCLATURE AND STRUCTURE OF ORGANIC COMPOUNDS Many organic compounds have common names which have arisen historically, or have been given to them when the compound has been isolated from a natural product or first synthesised. As there are so many organic compounds chemists have developed rules for naming a compound systematically, so that it structure can be deduced from its name. This section introduces this systematic nomenclature, and the ways the structure of organic compounds can be depicted more simply than by full Lewis structures. The language is based on Latin, Greek and German in addition to English, so a classical education is beneficial for chemists! Greek and Latin prefixes play an important role in nomenclature: Greek Latin ½ hemi semi 1 mono uni 1½ sesqui 2 di bi 3 tri ter 4 tetra quadri 5 penta quinque 6 hexa sexi 7 hepta septi 8 octa octo 9 ennea nona 10 deca deci Organic compounds: Compounds containing the element carbon [e.g. methane, butanol]. (CO, CO2 and carbonates are classified as inorganic.) See page 1-4. Special characteristics of many organic compounds are chains or rings of carbon atoms bonded together, which provides the basis for naming, and the presence of many carbon- hydrogen bonds. The valency of carbon in organic compounds is 4. Hydrocarbons: Compounds containing only the elements C and H. Straight chain hydrocarbons are named according to the number of carbon atoms: CH4, methane; C2H6 or H3C-CH3, ethane; C3H8 or H3C-CH2-CH3, propane; C4H10 or H3C-CH2- CH2-CH3, butane; C5H12 or CH3CH2CH2CH2CH3, pentane; C6H14 or CH3(CH2)4CH3, hexane; C7H16, heptane; C8H18, octane; C9H20, nonane; C10H22, CH3(CH2)8CH3, decane. -
Reactions of Aromatic Compounds Just Like an Alkene, Benzene Has Clouds of Electrons Above and Below Its Sigma Bond Framework
Reactions of Aromatic Compounds Just like an alkene, benzene has clouds of electrons above and below its sigma bond framework. Although the electrons are in a stable aromatic system, they are still available for reaction with strong electrophiles. This generates a carbocation which is resonance stabilized (but not aromatic). This cation is called a sigma complex because the electrophile is joined to the benzene ring through a new sigma bond. The sigma complex (also called an arenium ion) is not aromatic since it contains an sp3 carbon (which disrupts the required loop of p orbitals). Ch17 Reactions of Aromatic Compounds (landscape).docx Page1 The loss of aromaticity required to form the sigma complex explains the highly endothermic nature of the first step. (That is why we require strong electrophiles for reaction). The sigma complex wishes to regain its aromaticity, and it may do so by either a reversal of the first step (i.e. regenerate the starting material) or by loss of the proton on the sp3 carbon (leading to a substitution product). When a reaction proceeds this way, it is electrophilic aromatic substitution. There are a wide variety of electrophiles that can be introduced into a benzene ring in this way, and so electrophilic aromatic substitution is a very important method for the synthesis of substituted aromatic compounds. Ch17 Reactions of Aromatic Compounds (landscape).docx Page2 Bromination of Benzene Bromination follows the same general mechanism for the electrophilic aromatic substitution (EAS). Bromine itself is not electrophilic enough to react with benzene. But the addition of a strong Lewis acid (electron pair acceptor), such as FeBr3, catalyses the reaction, and leads to the substitution product. -
Surface Chemistry Changes of Weathered HDPE/Wood-Flour
Polymer Degradation and Stability 86 (2004) 1–9 www.elsevier.com/locate/polydegstab Surface chemistry changes of weathered HDPE/wood-flour composites studied by XPS and FTIR spectroscopy* Nicole M. Starka,), Laurent M. Matuanab aU.S. Department of Agriculture, Forest Service, Forest Products Laboratory, One Gifford Pinchot Drive, Madison, WI 53726-2398, United States bDepartment of Forestry, Michigan State University, East Lansing, MI 48824-1222, United States Received 7 August 2003; received in revised form 3 November 2003; accepted 4 November 2003 Abstract The use of wood-derived fillers by the thermoplastic industry has been growing, fueled in part by the use of wood-fiber– thermoplastic composites by the construction industry. As a result, the durability of wood-fiber–thermoplastic composites after ultraviolet exposure has become a concern. Samples of 100% high-density polyethylene (HDPE) and HDPE filled with 50% wood- flour (WF) were weathered in a xenon arc-type accelerated weathering apparatus for 2000 h. Changes in surface chemistry were studied using spectroscopic techniques. X-ray photoelectron spectroscopy (XPS) was used to verify the occurrence of surface oxidation. Fourier transform infrared (FTIR) spectroscopy was used to monitor the development of degradation products, such as carbonyl groups and vinyl groups, and to determine changes in HDPE crystallinity. The results indicate that surface oxidation occurred immediately after exposure for both the neat HDPE and WF/HDPE composites; the surface of the WF/HDPE composites was oxidized to a greater extent than that of the neat HDPE. This suggests that the addition of WF to the HDPE matrix results in more weather-related damage. -
Novel Poly(Vinyl Alcohol)-Based Column Coating for Capillary Electrophoresis of Proteins
Biochemical Engineering Journal 53 (2010) 137–142 Contents lists available at ScienceDirect Biochemical Engineering Journal journal homepage: www.elsevier.com/locate/bej Novel poly(vinyl alcohol)-based column coating for capillary electrophoresis of proteins Liang Xu, Xiao-Yan Dong, Yan Sun ∗ Department of Biochemical Engineering and Key Laboratory of Systems Bioengineering of the Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China article info abstract Article history: A novel and simple method for the preparation of chemically bonded poly(vinyl alcohol) (PVA) coat- Received 30 June 2010 ing to silica capillary inner wall was developed, and the PVA-coated capillary columns were employed Received in revised form for capillary electrophoresis (CE). The coating procedure included pretreatment of the capillary inner 30 September 2010 wall, silanization, aldehyde group functionalization and PVA immobilization. Electroosmotic flow of the Accepted 6 October 2010 coated capillary was almost suppressed over a wide pH range (pH 3–10). High-efficiency separations of cationic proteins (including cytochrome c, lysozyme, ␣-chymotrypsinogen A) at pH 3.0–5.0 and of anionic proteins (including myoglobin and trypsin inhibitor) at pH 10.0 were achieved with the PVA-coated cap- Keywords: Protein illary. Moreover, a “dual-opposite-injection” approach was adopted for simultaneous separations of both Separation cationic and anionic proteins at neutral pH with the prepared column. In this CE mode, positively charged Bioprocess Monitoring proteins migrated from one end of the column to the detector while negatively charged proteins from Adsorption the other end to the detection window. Good run-to-run repeatability was obtained in all of the protein Capillary electrophoresis CE separations performed in this work.