DOCSLIB.ORG

Exploring Student Thinking About Addition Reactions Solaire A

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Exploring Student Thinking About Addition Reactions Solaire A Exploring Student Thinking About Addition Reactions Solaire A. Finkenstaedt-Quinn*, Field M. Watts, Michael N. Petterson, Sabrina R. Archer, Emma P. Snyder-White, and Ginger V. Shultz Department of Chemistry, University of Michigan, Ann Arbor, Michigan 48109, United States 5 ABSTRACT Organic chemistry is a required course sequence for many STEM students, however research indicates that organic chemistry reaction mechanisms are especially challenging for students due to a mixture of underlying conceptual difficulties, the process-oriented thinking inherent to the discipline, and the representations commonly used to depict mechanisms. While student reasoning about many of the 10 reaction types covered in the organic chemistry curriculum have been studied previously, there is minimal research focused specifically on how students think about the mechanisms of addition reactions. This study addresses that gap by probing first semester organic chemistry students’ thinking using think-aloud interviews as they worked through two different addition reactions. To elicit a range of thinking, students worked through the mechanisms using either paper and pencil or 15 an app that dynamically represents the molecules. Overall, students were able to identify the steps of the two addition reactions but did not always successfully apply chemical thinking during the mechanistic steps. Specifically, both groups of students struggled with the concepts related to carbocation stability, frequently misapplying stabilization via substitution and demonstrating difficulty in identifying the potential for resonance stabilization. Our results suggest that instructors should 20 emphasize the conceptual grounding directing mechanistic steps, in particular when determining carbocation stability. GRAPHICAL ABSTRACT KEYWORDS 25 Second-year undergraduate, chemical education research, organic chemistry, computer-based learning, problem solving/decision making, addition reactions INTRODUCTION The organic chemistry sequence is a two-semester prerequisite for numerous fields of study with the aim of teaching students the fundamentals of organic chemistry reactions, developing students’ 30 ability to reason through reaction mechanisms, and strengthening students’ understanding of structure-property relationships.1 Unlike in general chemistry, where reactions are presented simply as reactants and products separated by an arrow, organic chemistry emphasizes reaction mechanisms, which require students to envision electron flow, denoted by the curved-arrow formalism, from reactant through intermediate molecules to the desired product.2, 3 However, reaction 35 mechanisms have been identified as one of the most difficult concepts covered in organic chemistry,1 as students must engage in multivariate thinking, weighing considerations of chemical reactivity and spatial constraints while also interpreting numerous representations.4 Thus, it is important to characterize how students reason through reaction mechanisms in order to better support their learning. 40 Prior research focused on how students approach and think about organic chemistry reaction mechanisms indicates that students use mechanistic steps primarily to move from the reactant to the product.2, 5-8 Additionally, students have a tendency to focus on the explicit features of the molecules involved in the reaction.7-10 However, research focused specifically on student reasoning about addition reactions is minimal.5, 9 Addition reactions are often introduced early in the organic chemistry 45 curriculum and require students to begin applying new content knowledge as they weigh multiple reaction pathways, utilizing reasoning processes that they will engage in throughout the course. As such, investigating student thinking on this topic has the potential to inform our understanding of how students may approach more difficult reactions that similarly require consideration of multiple reaction pathways. This study explores how students reason about addition reactions during think- 50 aloud interviews where students work through reaction mechanisms on paper or through an app. Mechanistic Reasoning An understanding of reaction mechanisms is a key component of the organic chemistry curriculum and requires students to integrate several sources of knowledge including knowledge of chemical reactivity and mechanistic representations.3-4 With its importance and complexity, a number of studies 55 have been conducted that investigate the processes students use when approaching mechanisms.2, 5-15 These studies indicate that students are not successfully applying the conceptual knowledge they have developed throughout general and organic chemistry and focus on explicit structural features, neglecting implicit, electronic features when making mechanistic decisions.2, 7-10, 15 For example, in two studies using card sort tasks, students tended to sort organic reactions based on two main criteria: 60 similarities in the type of reaction, without any discussion of the underlying mechanistic patterns, and by surface features, such as the presence of similar functional groups.9, 15 Additionally, students have been found to rely on memorized mechanisms that lead them to the desired products.2, 5-8, 14-15 These difficulties may be due to underdeveloped knowledge or a poor ability to apply the knowledge of electronics and electron flow during organic chemistry reactions,2, 6, 10 which is indispensable when 65 solving unfamiliar mechanisms. To facilitate thinking about the electronic properties and relationships between atoms and molecules, the ability to draw and manipulate Lewis structures is considered key, yet students are known to have difficulties with this skill.16 Additionally, students are not adept in their use of the curved-arrow formalism,2, 5-6 where they do not always use the formalism when it would be useful to predict reaction products,5 and may follow known reaction steps rather than 70 thinking chemically about the process.6 Additionally, mechanistic arrows have little meaning for students and are not recognized as representing electrons.2, 6 This may be indicative of a profound learning gap—that students do not fully understand the usefulness of mechanistic arrows or what these arrows depict.2, 5-6 Lacking an appreciation for curved arrows may promote reliance on memorization and is especially detrimental as students who use the curved-arrow formalism have 75 been shown to perform better on more difficult problems than those who do not.17 Addition Reactions Pi bond addition reactions are introduced early in the organic chemistry curriculum and start to extend students’ reasoning to incorporate content beyond what they have learned in general chemistry, e.g. decisions about acidity and basicity, yet there is little research focused on how 80 students approach and reason through this type of reaction specificially.5, 9 Grove et al. provided a detailed analysis of students’ drawn responses to an acid-catalyzed addition reaction.5 The percentage of students who provided a mechanism for the reaction, instead of just drawing the product, increased from 56 to 70% over the course of two semesters in organic chemistry. When students did show the mechanism, the diversity of mechanistic pathways depicted, both correct and incorrect, increased over 85 time and only a small percent drew a carbocation intermediate.5 Graulich and Bhattacharyya used variations of a card-sorting task targeting electrophilic addition reactions and found that students focused primarily on surface characteristics.9 When students were directed to consider mechanisms, they created categories based on characteristics of the reactions—e.g. reaction intermediates or steps—but did not always make correct assignments. Throughout the tasks, the most commonly 90 identified mechanistic step was carbocation formation.9 Beyond these two studies, additional research focused on the concepts students use as they reason through addition reactions is merited. Analyzing student reasoning about addition reactions is especially important as the reaction mechanisms begin to be more complex than what students may have seen previously. When reasoning through addition reactions, students must consider multiple reaction pathways—e.g., which carbon of 95 the double bond to protonate—and begin weighing multiple chemical considerations—e.g. substitution versus resonance stabilization. Students are known to struggle with these,6, 8 yet considering multiple factors is especially important in more complex reactions, such as substitution and elimination. This study sought to develop descriptions of students’ thinking about addition reactions by identifying what chemical features and concepts students focused on as they worked through two addition reactions. 100 Student reasoning was elicited through think-aloud interviews that utilized two different modalities, conventional paper-pencil and a touch-screen app. It is important to use multiple modalities to probe how students reason, as there are an increasing number of interfaces with which students can interact with mechanistic representations. Additionally, previous research indicates that multiple modalities can elicit a greater range of reasoning from students.18 This research was guided by the 105 following research questions: 1. How do undergraduate students in organic chemistry think about addition reactions when working through mechanisms on paper or using a touch-screen application? 2. When considering addition reaction mechanisms,
Load more

Recommended publications
  • Chemical Kinetics HW1 (Kahn, 2010)
    Chemical Kinetics HW1 (Kahn, 2010) Question 1. (6 pts) A reaction with stoichiometry A = P + 2Q was studied by monitoring the concentration of the reactant A as a function of time for eighteen minutes. The concentration determination method had a maximum error of 6 M. The following concentration profile was observed: Time (min) Conc (mM) 1 0.9850 2 0.8571 3 0.7482 4 0.6549 5 0.5885 6 0.5183 7 0.4667 8 0.4281 9 0.3864 10 0.3557 11 0.3259 12 0.3037 13 0.2706 14 0.2486 15 0.2355 16 0.2188 17 0.2111 18 0.1930 Determine the reaction order and calculate the rate constant for decomposition of A. What can be said about the mechanism or molecularity of this reaction? Question 2. (4 pts) Solve problem 2 on pg 31 in your textbook (House) using both linear and non-linear regression. Provide standard errors for the rate constant and half-life based on linear and non-linear fits. Below is the data set for your convenience: dataA = {{0, 0.5}, {10, 0.443}, {20,0.395}, {30,0.348}, {40,0.310}, {50,0.274}, {60,0.24}, {70,0.212}, {80,0.190}, {90,0.171}, {100,0.164}} Question 3. (3 pts) Solve problem 3 on pg 32 in your textbook (House). Question 4. (7 pts) The authors of the paper “Microsecond Folding of the Cold Shock Protein Measured by a Pressure-Jump Technique” suggest that the activated state of folding of CspB follows Hammond- type behavior.
    [Show full text]
  • NEW TANDEM REACTIONS INVOLVING NUCLEOPHILIC AROMATIC SUBSTITUTION by JAMES ERVIN SCHAMMERHORN III Associates of Science Murray
    NEW TANDEM REACTIONS INVOLVING NUCLEOPHILIC AROMATIC SUBSTITUTION By JAMES ERVIN SCHAMMERHORN III Associates of Science Murray State College Tishomingo, Oklahoma 2003 Bachelor of Science in Chemistry Oklahoma State University Stillwater, Oklahoma 2006 Submitted to the Faculty of the Graduate College of the Oklahoma State University in partial fulfillment of the requirements for the Degree of DOCTOR OF PHILOSOPHY May, 2011 1 NEW TANDEM REACTIONS INVOLVING NUCLEOPHILIC ARMOMATIC SUBSTITUTION Thesis Approved: Dr. Richard Bunce Thesis Adviser Dr. K. Darrell Berlin Dr. Ziad El-Rassi Dr. Nicholas Materer Dr. Andrew Mort Dr. Mark E. Payton Dean of the Graduate College ii ACKNOWLEDGMENTS I would like to express my sincere gratitude to Dr. R. A. Bunce, my research advisor, my graduate committee chairman and my friend. I have learned many valuable lessons working alongside him in his research laboratory. His ability to teach both organic theory and laboratory techniques have been instrumental in my research at Oklahoma State University. I am immensely thankful for his support, guidance and patience to me during the course of this research. His unique sense of humor always makes me laugh and makes it a joy to come to lab. I would also like to thank my committee members, Drs. K. D. Berlin, Z. El Rassi, N. F. Materer, and A. J. Mort their acceptance to be on my committee. Their insights into research and graduate life have been invaluable during my graduate career. I am especially grateful to Dr. Berlin for sharing his wealth of organic chemistry knowledge with me during the course of my research. Also, to Dr.
    [Show full text]
  • 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.
    [Show full text]
  • Reactions of Alkenes and Alkynes
    05 Reactions of Alkenes and Alkynes Polyethylene is the most widely used plastic, making up items such as packing foam, plastic bottles, and plastic utensils (top: © Jon Larson/iStockphoto; middle: GNL Media/Digital Vision/Getty Images, Inc.; bottom: © Lakhesis/iStockphoto). Inset: A model of ethylene. KEY QUESTIONS 5.1 What Are the Characteristic Reactions of Alkenes? 5.8 How Can Alkynes Be Reduced to Alkenes and 5.2 What Is a Reaction Mechanism? Alkanes? 5.3 What Are the Mechanisms of Electrophilic Additions HOW TO to Alkenes? 5.1 How to Draw Mechanisms 5.4 What Are Carbocation Rearrangements? 5.5 What Is Hydroboration–Oxidation of an Alkene? CHEMICAL CONNECTIONS 5.6 How Can an Alkene Be Reduced to an Alkane? 5A Catalytic Cracking and the Importance of Alkenes 5.7 How Can an Acetylide Anion Be Used to Create a New Carbon–Carbon Bond? IN THIS CHAPTER, we begin our systematic study of organic reactions and their mecha- nisms. Reaction mechanisms are step-by-step descriptions of how reactions proceed and are one of the most important unifying concepts in organic chemistry. We use the reactions of alkenes as the vehicle to introduce this concept. 129 130 CHAPTER 5 Reactions of Alkenes and Alkynes 5.1 What Are the Characteristic Reactions of Alkenes? The most characteristic reaction of alkenes is addition to the carbon–carbon double bond in such a way that the pi bond is broken and, in its place, sigma bonds are formed to two new atoms or groups of atoms. Several examples of reactions at the carbon–carbon double bond are shown in Table 5.1, along with the descriptive name(s) associated with each.
    [Show full text]
  • UNIT-III: Mechanistic and Sterochemical Aspects of Addition Reaction Involving Electrophiles & Nucleophiles
    MCH-204:ORGANIC CHEMISTRY UNIT-III: Mechanistic and sterochemical aspects of addition reaction Involving electrophiles & nucleophiles Prof.Anand Halve S.O.S in Chemistry Jiwaji University Gwalior ELECTROPHILIC ADDITION REACTIONS markovnikov’ additions markovnikov’s rule Addition of hydrogen to an unsymmetrical olefin occurs at those carbon atoms with maximum number of hydrogen atoms. (i.e., the carbon with least substitution). Electronegative group goes to more substituted carbon atom. Such an addition leads to a stabler carbocation. Such a reaction may lead to constitutional isomers but actually one of the products is formed as major product. X X Formed H HX X=F, Cl. X Olefin Not formed origin … X HX X X=F, Cl . carbocation is more stablised in T.S. Stereo specific product H X X carbocation is not enough stablilised in transition state Olefin Consider two possible sites for hydrogen addition (i) terminal or (ii) internal (substituted carbon). The addition of hydrogen at the terminal carbon leads to better stabilization of carbocation, the chances of stabilization increases with increase in conjugation with olefin. The terminal carbocation require higher activation energy which is not a favorable condition, leading to slower reaction rate. However, the generation of non terminal carbocation is assisted by hyperconjugative stabilization leading to a lower activation energy. Alkenes-some facts Due to trigonal planar geometry of olefin carbon atoms the addition can occur on the same side (syn periplanar) or on opposite sides (anti periplanar). Alkenes are generally nucleophilic. The C=C double bond provides a higher energy HOMO (highest occupied molecular orbitals). Electron donating groups increase the rate for electrophilic attack as they assist in carbocation and positive charge stabilization in the TS.
    [Show full text]
  • 20 More About Oxidation–Reduction Reactions
    More About 20 Oxidation–Reduction Reactions OOC n important group of organic reactions consists of those that O A involve the transfer of electrons C from one molecule to another. Organic chemists H OH use these reactions—called oxidation–reduction reactions or redox reactions—to synthesize a large O variety of compounds. Redox reactions are also important C in biological systems because many of these reactions produce HH energy. You have seen a number of oxidation and reduction reactions in other chapters, but discussing them as a group will give you the opportunity to CH3OH compare them. In an oxidation–reduction reaction, one compound loses electrons and one com- pound gains electrons. The compound that loses electrons is oxidized, and the one that gains electrons is reduced. One way to remember the difference between oxidation and reduction is with the phrase “LEO the lion says GER”: Loss of Electrons is Oxi- dation; Gain of Electrons is Reduction. The following is an example of an oxidation–reduction reaction involving inorganic reagents: Cu+ + Fe3+ ¡ Cu2+ + Fe2+ In this reaction,Cu+ loses an electron, so Cu+ is oxidized. Fe3+ gains an electron, so Fe3+ is reduced. The reaction demonstrates two important points about oxidation– reduction reactions. First, oxidation is always coupled with reduction. In other words, a compound cannot gain electrons (be reduced) unless another compound in the reaction simultaneously loses electrons (is oxidized). Second, the compound that is oxidized (Cu+) is called the reducing agent because it loses the electrons that are used to reduce the other compound (Fe3+). Similarly, the compound that is reduced (Fe3+) is called the oxidizing agent because it gains the electrons given up by the other compound (Cu+) when it is oxidized.
    [Show full text]
  • Aromatic Substitution Reactions: an Overview
    Review Article Published: 03 Feb, 2020 SF Journal of Pharmaceutical and Analytical Chemistry Aromatic Substitution Reactions: An Overview Kapoor Y1 and Kumar K1,2* 1School of Pharmaceutical Sciences, Apeejay Stya University, Sohna-Palwal Road, Sohna, Gurgaon, Haryana, India 2School of Pharmacy and Technology Management, SVKM’s NMIMS University, Hyderabad, Telangana, India Abstract The introduction or replacement of substituent’s on aromatic rings by substitution reactions is one of the most fundamental transformations in organic chemistry. On the basis of the reaction mechanism, these substitution reactions can be divided into electrophilic, nucleophilic, radical, and transition metal catalyzed. This article also focuses on electrophilic and nucleophilic substitution mechanisms. Introduction The replacement of an atom, generally hydrogen, or a group attached to the carbon from the benzene ring by another group is known as aromatic substitution. The regioselectivity of these reactions depends upon the nature of the existing substituent and can be ortho, Meta or Para selective. Electrophilic Aromatic Substitution (EAS) reactions are important for synthetic purposes and are also among the most thoroughly studied classes of organic reactions from a mechanistic point of view. A wide variety of electrophiles can effect aromatic substitution. Usually, it is a substitution of some other group for hydrogen that is of interest, but this is not always the case. For example, both silicon and mercury substituent’s can be replaced by electrophiles. The reactivity of a particular electrophile determines which aromatic compounds can be successfully substituted. Despite the wide range of electrophilic species and aromatic ring systems that can undergo substitution, a single broad mechanistic picture encompasses most EAS reactions.
    [Show full text]
  • Mechanism, Regioselectivity, and the Kinetics of Phosphine-Catalyzed [3+2] Cycloaddition Reactions of Allenoates and Electron-Deficient Alkenes
    FULL PAPER DOI: 10.1002/chem.200701725 Mechanism, Regioselectivity, and the Kinetics of Phosphine-Catalyzed [3+2] Cycloaddition Reactions of Allenoates and Electron-Deficient Alkenes Yong Liang,[a] Song Liu,[a] Yuanzhi Xia,[a, b] Yahong Li,[b, c] and Zhi-Xiang Yu*[a] Abstract: With the aid of computations alyzed [3+ 2] cycloaddition reaction of theories. Isotopic labeling experiments and experiments, the detailed mecha- allenoates and electron-deficient al- combined with DFT calculations nism of the phosphine-catalyzed [3+ 2] kenes. DFT calculations and FMO showed that the commonly accepted cycloaddition reactions of allenoates analysis revealed that an electron-with- intramolecular [1,2]-proton shift should and electron-deficient alkenes has been drawing group is required in the allene be corrected to a water-catalyzed [1,2]- investigated. It was found that this re- to ensure the generation of the 1,3- proton shift. Additional isotopic label- action includes four consecutive pro- dipole kinetically and thermodynami- ing experiments of the hetero-[3+2] cesses: 1) In situ generation of a 1,3- cally. Atoms-in-molecules (AIM) cycloaddition of allenoates and elec- dipole from allenoate and phosphine, theory was used to analyze the stability tron-deficient imines further support 2) stepwise [3+2] cycloaddition, 3) a of the 1,3-dipole. The regioselectivity this finding. This investigation has also water-catalyzed [1,2]-hydrogen shift, of the [3+2] cycloaddition can be ra- been extended to the study of the and 4) elimination of the phosphine tionalized very well by FMO and AIM phosphine-catalyzed [3+2] cycloaddi- catalyst.
    [Show full text]
  • Regioselectivity in the [2 + 2] Cyclo-Addition Reaction of Triplet Carbonyl Compounds to Substituted Alkenes
    J. Chem. Sci., Vol. 117, No. 5, September 2005, pp. 561–571. © Indian Academy of Sciences. Regioselectivity in the [2 + 2] cyclo-addition reaction of triplet carbonyl compounds to substituted alkenes (Paterno–Büchi reaction): A spin-polarized conceptual DFT approach B PINTÉR,1 F DE PROFT,2 T VESZPRÉMI1 and P GEERLINGS2,* 1Inorganic Chemistry Department, Budapest University of Technology and Economics (BUTE), Szent Gellért tér 4, H-1521, Budapest, Hungary 2Eenheid Algemene Chemie (ALGC), Vrije Universiteit Brussel (VUB), Faculteit, Wetenschappen, Pleinlaan 2, 1050 Brussels, Belgium e-mail: [email protected] Abstract. Regioselectivity of the photochemical [2 + 2] cyclo-addition of triplet carbonyl compounds with a series of ground state electron-rich and electron-poor alkenes, the Paterno–Büchi reaction, is studied. Activation barriers for the first step of the triplet reaction are computed in the case of the O-attack. Next, the observed regioselectivity is explained using a series of DFT-based reactivity indices. In the first step, we use the local softness and the local HSAB principle within a softness matching approach, and explain the relative activation barriers of the addition step. In the final step, the regioselectivity is assessed within the framework of spin-polarized conceptual density functional theory, considering response functions of the system’s external potential v, number of electrons N and spin number Ns, being the difference between the number of a and b electrons in the spin-polarized system. Although the concept of local spin philicity, in- troduced recently within this theory, appears less suited to predict the regioselectivity in this reaction, the correct regioselectivity emerges from considering an interaction between the largest values of the generalized Fukui functions fss on both interacting molecules.
    [Show full text]
  • Reactions of Alkenes Since  Bonds Are Stronger Than  Bonds, Double Bonds Tend to React to Convert the Double Bond Into  Bonds
    Reactions of Alkenes Since bonds are stronger than bonds, double bonds tend to react to convert the double bond into bonds This is an addition reaction. (Other types of reaction have been substitution and elimination). Addition reactions are typically exothermic. Electrophilic Addition The bond is localized above and below the C-C bond. The electrons are relatively far away from the nuclei and are therefore loosely bound. An electrophile will attract those electrons, and can pull them away to form a new bond. This leaves one carbon with only 3 bonds and a +ve charge (carbocation). The double bond acts as a nucleophile (attacks the electrophile). Ch08 Reacns of Alkenes (landscape) Page 1 In most cases, the cation produced will react with another nucleophile to produce the final overall electrophilic addition product. Electrophilic addition is probably the most common reaction of alkenes. Consider the electrophilic addition of H-Br to but-2-ene: The alkene abstracts a proton from the HBr, and a carbocation and bromide ion are generated. The bromide ion quickly attacks the cationic center and yields the final product. In the final product, H-Br has been added across the double bond. Ch08 Reacns of Alkenes (landscape) Page 2 Orientation of Addition Consider the addition of H-Br to 2-methylbut-2-ene: There are two possible products arising from the two different ways of adding H-Br across the double bond. But only one is observed. The observed product is the one resulting from the more stable carbocation intermediate. Tertiary carbocations are more stable than secondary.
    [Show full text]
  • 1 Msc(Chem) 1St Sem Dr Sanjeev Kumar Jha M.L.T. College, Saharsa Reaction Mechanism: Structure and Reactivity Types Of
    MSc(Chem) 1st Sem Dr Sanjeev Kumar Jha M.L.T. College, Saharsa Reaction Mechanism: Structure and reactivity Types of reactions:-- 1.Addition reaction An addition reaction , is simply termed as a reaction where two or more molecules combine to form a larger one (the adduct). Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon–carbon double bonds (alkenes), or with triple bonds (alkynes), and compounds that have rings, which are also considered points of unsaturation. Molecules containing carbon— hetero double bonds like carbonyl (C=O) groups, or imine (C=N) groups, can undergo addition, as they too have double-bond character. An addition reaction is the reverse of an elimination reaction. For instance, the hydration of an alkene to an alcohol is reversed by dehydration. There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition. Two non-polar addition reactions exist as well, called free-radical addition and cycloadditions. Addition reactions are also encountered in polymerizations and called addition polymerization. 1 General overview of addition reactions. Top to bottom: electrophilic addition to alkene, nucleophilic addition of nucleophile to carbonyl and free-radical addition of halide to alkene. 2.Elimination reaction An elimination reaction is a type of reaction in which two substituents are removed from a molecule in either a one or two-step mechanism. The one- step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 reaction. The numbers refer not to the number of steps in the mechanism, but rather to the kinetics of the reaction: E2 is bimolecular (second-order) while E1 is unimolecular (first-order).
    [Show full text]
  • Elimination Reaction
    Elimination reaction An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either one or two-step mechanism.[2] The one-step mechanism is known as the E2 reaction, and the two-step mechanism is known as the E1 reaction. The numbers refer not to the number of steps in the mechanism, but rather to the kinetics of the reaction: E2 is Elimination reaction of cyclohexanol to bimolecular (second-order) while E1 is unimolecular (first-order). cyclohexene with sulfuric acid and In cases where the molecule is able to stabilize an anion but heat[1] possesses a poor leaving group, a third type of reaction, E1CB, exists. Finally, the pyrolysis of xanthate and acetate esters proceed through an "internal" elimination mechanism, the Ei mechanism. Contents Loss of hydron (H+) E2 mechanism E1 mechanism Competition among mechanisms Elimination reactions other than β-elimination See also References External links Loss of hydron (H+) In most organic elimination reactions, at least one hydron (H+) is lost to form the double bond: the unsaturation of the molecule increases. It is also possible that a molecule undergoes reductive elimination, by which the valence of an atom in the molecule decreases by two, though this is more common in inorganic chemistry. An important class of elimination reactions is those involving alkyl halides, with good leaving groups, reacting with a Lewis base to form an alkene. Elimination may be considered the reverse of an addition reaction. When the substrate is asymmetric, regioselectivity is determined by Zaitsev's rule or through Hofmann elimination if the carbon with the most substituted hydrogen is inaccessible.
    [Show full text]