DOCSLIB.ORG

Reaction Mechanism in Organic Chemistry(Ii)

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text

Load more

Group B REACTION MECHANISM IN ORGANIC CHEMISTRY(II) Dr. Akanksha Upadhyay Assistant Professor Department of Chemistry Women’s College, Samastipur 1 Organic Reactions : Organic reactions are chemical reactions which involve organic compounds. Reaction Mechanism : The steps of an organic reaction showing the breaking and formation of new bonds leading to the formation of product through transitory intermediates. In other words, In organic chemistry terms, a reaction mechanism is a formalized description of how a reaction takes place from reactants to products. Intermediate Reactant or Product Transition State Most of the attacking reagents carry either a positive or a negative charge. 2 Types of Organic Reaction The reactions in organic chemistry are mainly classified into following classes: 1. Substitution Reactions 2. Addition Addition 2. Reactions Organic Reaction Reactions 3. 3. Elimination 4. Rearrangement Reactions 3 1. Substitution Reactions Substitution reactions are defined as reactions in which the functional group of one chemical compound is substituted by another group. or It is a reaction which involves the replacement of one atom or a molecule of a compound with another atom or molecule. Examples: Benzene reacted with Cl2 will produce dichlorobenzene and HCl. This substitution reaction replaces the hydrogen atoms on the original molecule with the Cl atom. 4 Types of Substitution Reaction Substitution reaction may be initiated by a nucleophile, electrophile or free radical. Therefore, Substitution reactions are of three types: 1. Free-Radical Substitution Reaction 2. Nucleophilic Substitution Reaction 3. Electrophilic Substitution Reaction 1. Free-Radical Substitution Reactions A free radical substitution reaction is initiated by free radical. A simple example of substitution is the reaction between alkane and chlorine/bromine in the presence of UV light (or sunlight). Free radicals : Free radicals are atoms or groups of atoms which have a single unpaired electron formed by homolytic fission ( studied in earlier lecture). 5 Mechanism: The mechanism for the chlorination of methane involves the following steps- 1. Initiation Step - A chlorine molecule undergoes homolytic fission in the presence of UV light to give chlorine free radicals. Cl2 2Cl 2. Propagation Step – A chlorine free radical attacks the methane molecule to give methyl free radical and hydrogen chloride. Further, the methyl free radical attacks a chlorine molecule to yield methyl chloride and chlorine radical. CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl These propagation reactions are repeated again and again. 6 3. Termination Steps – These involve the formation of stable molecules by combination of free radicals. Cl + Cl Cl2 CH3 + Cl CH3Cl CH3 + CH3 CH3-CH3 2. Nucleophilic Substitution Reaction When a substitution reaction involves the attack by a nucleophile, the reaction is referred to as SN (S stands for substitution and N for nucleophile) Nucleophilic Substitution Reaction. _ _ Example: R-X + OH R - OH + X The hydrolysis of alkyl halides by aqueos NaOH is an example of nucleophilic substitution reaction. Remember the role of a nucleophile by its Greek roots: Nucleo-(nucleus)-phile-(lover) – it is attracted to the nucleus, which is positively charged! Nucleophiles are therefore negatively charged or strongly δ-. 6 The nucleophilic substitution reactions are divided into two classes: 1. SN2 Reaction 2. SN1 Reaction 1. SN2 Reaction The SN2 reaction is a nucleophilic substitution reaction where a bond is broken and another is formed simultaneously or we can say that where simultaneous attack of the nucleophile and displacement of the leaving grouptake place. The term ‘SN2’ stands for – Substitution Nucleophilic Bimolecular. When the rate of a nucleophilic substitution reaction depends on the concentration of both the substrate and nucleophile,the reaction is second order reaction and termed as SN2 reaction. Rate∞ [Substrate][Nucleophile] Evidently, the rate determining step include the participation of both the substrate and the nucleophile. 7 SN2 Reaction Mechanism: Consider the hydrolysis of methyl chloride by aqueous NaOH. The reaction mechanism is represented here- Fig. Nucleophilic substitution by SN2 Mechanism This reaction proceeds through a backside attack by the nucleophile on the substrate. The nucleophile approaches the given substrate at an angle of 180o to the carbon-leaving group bond. The carbon-nucleophile bond forms and carbon-leaving group bond breaks simultaneously through a transition state. Notice that intermediate is not formed in an SN2 reaction, just a transition state is obtained. In the course of the reaction, the configuration of the carbon is inverted and designated as Walden Inversion. Factors Affecting Rate of SN2 Reaction : 1. Nucleophilicity : Since the nucleophile is involved in the rate-determining step of SN2 reactions, stronger nucleophiles react faster. Stronger nucleophiles are said to have increased nucleophilicity and thus rate of reaction will increase. 2. Solvent Effect : SN2 reactions are much faster in polar aprotic solvents (e.g. acetonitrile, dimethylsulfoxide, dimethylformamide, etc.) compared with polar protic solvents (e.g. alcohols, water). 3. Steric Hindrance : SN2 reactions are particularly sensitive to steric factors, since they are greatly retarded by steric hindrance (crowding) at the site of reaction. In general, the order of reactivity of alkyl halides in SN2 reactions is: methyl > 1° > 2°. 3° alkyl halides are so crowded that they do not generally react by an SN2 mechanism In an SN2 reaction, the transition state has 5 groups around the central C atom. As a consequence of the steric requirements at this center, less highly substituted systems (i.e. more smaller H groups) will favour an SN2 reaction by making it easier to achieve the transition state. 8 2. SN1 Reaction The SN1 reaction is a unimolecular nucleophilic substitution reaction. When the rate of a nucleophilic substitution reaction depends only on the concentration of the alkyl halide, hence it is first order reaction. This reaction involves the formation of a carbocation intermediate. SN1 Reaction Mechanism: Consider the hydrolysis of tertiary butyl bromide as an example, the mechanism of the SN1 reaction consists of two steps: Step 1. Formation of Carbocation: tert-butyl bromide Carbocation This is the rate determining step. The carbon-bromine bond is a polar covalent bond. The cleavage of this bond allows the removal of the leaving group (bromide ion). When the bromide ion leaves the tertiary butyl bromide, a carbocation intermediate is formed. 9 Step 2. Attack of Nucleophile : The second step is a bond making process where the electron rich nucleophile attack over an electron poor electrophile (carbocation). tert-butyl alcohol Summary Of Nucleophilic Substitution Reaction Factors SN1 Reaction SN2 Reaction Molecularity Unimolecular Bimolecular Kinetics First Order Second Order Steps Two steps One step Intermediates Carbocation No intermediate o o o o o o Alkyl halide 3 > 2 , No 1 or CH3 CH3 > 1 > 2 , No 3 Solvent Polar protic solvent Polar aprotic solvent Nucleophile Weak nucleophile Strong nucleophile 10 Addition Reactions Addition Reactions are those in which atoms or group of atoms are added to a double or triple bond without the elimination of any atom or molecules. Addition reactions are typical of unsaturated organic compounds— i.e., alkenes, which contain a C-C double bond, and alkynes, which have a C-C triple bond—and aldehydes and ketones, which have a C=O double bond. Types of Addition Reactions These reactions may be initiated by electrophiles or nucleophiles: 1. Electrophilic Addition reactions 2. Nucleophilic Addition reactions Electrophilic Addition reactions An electrophilic addition reaction is a reaction in which a substrate is initially attacked by an electrophile, and the overall result is the addition of one or more relatively simple molecules across a multiple bond. The addition of HBr to ethylene is an example of electrophilic addition- Mechanism: + - Br2 gives a Br (electrophile) and Br (nucleophile). Nucleophilic Addition reactions When an addition reaction involves the initial attack by a nucleophile, the reaction is referred to as nucleophilic addition reaction. Aldehydes and ketones which contain carbon-oxygen double bonds undergo such reactions. Reactivity of aldehydes and ketones: Aldehyde and ketones demonstrate polar nature: Since, oxygen is more electronegative than carbon, so electron density is higher on the oxygen side of the bond and lower on the carbon side. Recall that bond polarity can be depicted with a dipole arrow, or by showing the oxygen as holding a partial negative charge and the carbonyl carbon a partial positive charge. Carbon becomes more electrophilic Fig. Structure of Carbonyl group Therefore, C-centre behaves as an electrophilic target for attack by an electron-rich nucleophilic group. Fig. Nucleophilic Addition Reaction Relative Reactivity of Carbonyl Compounds to Nucleophilic Addition Aldehydes are more reactive and readily undergo nucleophilic addition reactions in comparison to ketones. In the case of ketones, two large substituents are present in the structure of ketones which causes steric hindrance when the nucleophile approaches the carbonyl carbon.However, aldehydes contain one substituent and thus the steric hindrance to the approaching nucleophile is less. Moreover, electronically aldehydes demonstrate better reactivity than ketone. This is because ketones contain two
Recommended publications
  • Organic Chemistry

    Organic Chemistry

    Wisebridge Learning Systems Organic Chemistry Reaction Mechanisms Pocket-Book WLS www.wisebridgelearning.com © 2006 J S Wetzel LEARNING STRATEGIES CONTENTS ● The key to building intuition is to develop the habit ALKANES of asking how each particular mechanism reflects Thermal Cracking - Pyrolysis . 1 general principles. Look for the concepts behind Combustion . 1 the chemistry to make organic chemistry more co- Free Radical Halogenation. 2 herent and rewarding. ALKENES Electrophilic Addition of HX to Alkenes . 3 ● Acid Catalyzed Hydration of Alkenes . 4 Exothermic reactions tend to follow pathways Electrophilic Addition of Halogens to Alkenes . 5 where like charges can separate or where un- Halohydrin Formation . 6 like charges can come together. When reading Free Radical Addition of HX to Alkenes . 7 organic chemistry mechanisms, keep the elec- Catalytic Hydrogenation of Alkenes. 8 tronegativities of the elements and their valence Oxidation of Alkenes to Vicinal Diols. 9 electron configurations always in your mind. Try Oxidative Cleavage of Alkenes . 10 to nterpret electron movement in terms of energy Ozonolysis of Alkenes . 10 Allylic Halogenation . 11 to make the reactions easier to understand and Oxymercuration-Demercuration . 13 remember. Hydroboration of Alkenes . 14 ALKYNES ● For MCAT preparation, pay special attention to Electrophilic Addition of HX to Alkynes . 15 Hydration of Alkynes. 15 reactions where the product hinges on regio- Free Radical Addition of HX to Alkynes . 16 and stereo-selectivity and reactions involving Electrophilic Halogenation of Alkynes. 16 resonant intermediates, which are special favor- Hydroboration of Alkynes . 17 ites of the test-writers. Catalytic Hydrogenation of Alkynes. 17 Reduction of Alkynes with Alkali Metal/Ammonia . 18 Formation and Use of Acetylide Anion Nucleophiles .
  • Recent Advances in the Direct Nucleophilic Substitution of Allylic

    Recent Advances in the Direct Nucleophilic Substitution of Allylic

    SHORT REVIEW ▌25 Recentshort review Advances in the Direct Nucleophilic Substitution of Allylic Alcohols through SN1-Type Reactions AlejandroSN1 Reactions of Allylic Alcohols Baeza,* Carmen Nájera* Departamento de Química Orgánica and Instituto de Síntesis Orgánica, University of Alicante, Apdo.99, 03080 Alicante, Spain Fax +34(965)903549; E-mail: [email protected]; E-mail: [email protected] Received: 03.10.2013; Accepted after revision: 06.11.2013 Abstract: Direct nucleophilic substitution reactions of allylic alco- hols are environmentally friendly, since they generate only water as a byproduct, allowing access to new allylic compounds. This reac- tion has, thus, attracted the interest of the chemical community and several strategies have been developed for its successful accom- plishment. This review gathers the latest advances in this methodol- ogy involving SN1-type reactions. 1 Introduction 2SN1-Type Direct Nucleophilic Substitution Reactions of Allylic Alcohols 2.1 Lewis Acids as Catalysts Alejandro Baeza was born in Alicante (Spain) in 1979. He studied 2.2 Brønsted Acids as Catalysts chemistry at the University of Alicante and he received his M.Sc. (2003) and Ph. D. degrees (2006) from here under the supervision of 2.3 Other Promoters Prof. José Miguel Sansano and Prof. Carmen Nájera. He was a post- 3 Conclusions and Outlook doctoral researcher in Prof. Pfaltz’s research group (2007–2010). In 2010 he returned to Alicante and joined the research group of Prof. Key words: S 1 reaction, allylic substitution, carbocations, allylic N Carmen Nájera. His main research interests focus on the development alcohols, green chemistry of new environmentally friendly methodologies, especially in asym- metric synthesis.
  • Chemical Kinetics HW1 (Kahn, 2010)

    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.
  • Bsc Chemistry

    Bsc Chemistry

    Subject Chemistry Paper No and Title 05, ORGANIC CHEMISTRY-II (REACTION MECHANISM-I) Module No and Title 15, The Neighbouring Mechanism, Neighbouring Group Participation by π and σ Bonds Module Tag CHE_P5_M15 CHEMISTRY PAPER :5, ORGANIC CHEMISTRY-II (REACTION MECHANISM-I) MODULE: 15 , The Neighbouring Mechanism, Neighbouring Group Participation by π and σ Bonds TABLE OF CONTENTS 1. Learning Outcomes 2. Introduction 3. NGP Participation 3.1 NGP by Heteroatom Lone Pairs 3.2 NGP by alkene 3.3 NGP by Cyclopropane, Cyclobutane or a Homoallyl group 3.4 NGP by an Aromatic Ring 4. Neighbouring Group Participation on SN2 Reactions 5. Neighbouring Group Participation on SN1 Reactions 6. Neighbouring Group and Rearrangement 7. Examples 8. Summary CHEMISTRY PAPER :5, ORGANIC CHEMISTRY-II (REACTION MECHANISM-I) MODULE: 15 , The Neighbouring Mechanism, Neighbouring Group Participation by π and σ Bonds 1. Learning Outcomes After studying this module, you shall be able to Know about NGP reaction Learn reaction mechanism of NGP reaction Identify stereochemistry of NGP reaction Evaluate the factors affecting the NGP reaction Analyse Phenonium ion, NGP by alkene, and NGP by heteroatom. 2. Introduction The reaction centre (carbenium centre) has direct interaction with a lone pair of electrons of an atom or with the electrons of s- or p-bond present within the parent molecule but these are not in conjugation with the reaction centre. A distinction is sometimes made between n, s and p- participation. An increase in rate due to Neighbouring Group Participation (NGP) is known as "anchimeric assistance". "Synartetic acceleration" happens to be the special case of anchimeric assistance and applies to participation by electrons binding a substituent to a carbon atom in a β- position relative to the leaving group attached to the α-carbon atom.
  • Reaction Kinetics in Organic Reactions

    Reaction Kinetics in Organic Reactions

    Autumn 2004 Reaction Kinetics in Organic Reactions Why are kinetic analyses important? • Consider two classic examples in asymmetric catalysis: geraniol epoxidation 5-10% Ti(O-i-C3H7)4 O DET OH * * OH + TBHP CH2Cl2 3A mol sieve OH COOH5C2 L-(+)-DET = OH COOH5C2 * OH geraniol hydrogenation OH 0.1% Ru(II)-BINAP + H2 CH3OH P(C6H5)2 (S)-BINAP = P(C6 H5)2 • In both cases, high enantioselectivities may be achieved. However, there are fundamental differences between these two reactions which kinetics can inform us about. 1 Autumn 2004 Kinetics of Asymmetric Catalytic Reactions geraniol epoxidation: • enantioselectivity is controlled primarily by the preferred mode of initial binding of the prochiral substrate and, therefore, the relative stability of intermediate species. The transition state resembles the intermediate species. Finn and Sharpless in Asymmetric Synthesis, Morrison, J.D., ed., Academic Press: New York, 1986, v. 5, p. 247. geraniol hydrogenation: • enantioselectivity may be dictated by the relative reactivity rather than the stability of the intermediate species. The transition state may not resemble the intermediate species. for example, hydrogenation of enamides using Rh+(dipamp) studied by Landis and Halpern (JACS, 1987, 109,1746) 2 Autumn 2004 Kinetics of Asymmetric Catalytic Reactions “Asymmetric catalysis is four-dimensional chemistry. Simple stereochemical scrutiny of the substrate or reagent is not enough. The high efficiency that these reactions provide can only be achieved through a combination of both an ideal three-dimensional structure (x,y,z) and suitable kinetics (t).” R. Noyori, Asymmetric Catalysis in Organic Synthesis,Wiley-Interscience: New York, 1994, p.3. “Studying the photograph of a racehorse cannot tell you how fast it can run.” J.
  • 2 Reactions Observed with Alkanes Do Not Occur with Aromatic Compounds 2 (SN2 Reactions Never Occur on Sp Hybridized Carbons!)

    2 Reactions Observed with Alkanes Do Not Occur with Aromatic Compounds 2 (SN2 Reactions Never Occur on Sp Hybridized Carbons!)

    Reactions of Aromatic Compounds Aromatic compounds are stabilized by this “aromatic stabilization” energy Due to this stabilization, normal SN2 reactions observed with alkanes do not occur with aromatic compounds 2 (SN2 reactions never occur on sp hybridized carbons!) In addition, the double bonds of the aromatic group do not behave similar to alkene reactions Aromatic Substitution While aromatic compounds do not react through addition reactions seen earlier Br Br Br2 Br2 FeBr3 Br With an appropriate catalyst, benzene will react with bromine The product is a substitution, not an addition (the bromine has substituted for a hydrogen) The product is still aromatic Electrophilic Aromatic Substitution Aromatic compounds react through a unique substitution type reaction Initially an electrophile reacts with the aromatic compound to generate an arenium ion (also called sigma complex) The arenium ion has lost aromatic stabilization (one of the carbons of the ring no longer has a conjugated p orbital) Electrophilic Aromatic Substitution In a second step, the arenium ion loses a proton to regenerate the aromatic stabilization The product is thus a substitution (the electrophile has substituted for a hydrogen) and is called an Electrophilic Aromatic Substitution Energy Profile Transition states Transition states Intermediate Potential E energy H Starting material Products E Reaction Coordinate The rate-limiting step is therefore the formation of the arenium ion The properties of this arenium ion therefore control electrophilic aromatic substitutions (just like any reaction consider the stability of the intermediate formed in the rate limiting step) 1) The rate will be faster for anything that stabilizes the arenium ion 2) The regiochemistry will be controlled by the stability of the arenium ion The properties of the arenium ion will predict the outcome of electrophilic aromatic substitution chemistry Bromination To brominate an aromatic ring need to generate an electrophilic source of bromine In practice typically add a Lewis acid (e.g.
  • 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

    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.
  • Chapter 23: Substituted Hydrocarbons and Their Reactions

    Chapter 23: Substituted Hydrocarbons and Their Reactions

    736-773_Ch23-866418 5/9/06 3:37 PM Page 736 CHAPTER 23 Substituted Hydrocarbons and Their Reactions Chemistry 2.b, 2.d, 2.h, 3.a, 3.g, 8.c, 10.a, 10.b, 10.e I&E 1.b, 1.c, 1.j What You’ll Learn ▲ You will recognize the names and structures of several important organic functional groups. ▲ You will classify reactions of organic substances as sub- stitution, addition, elimina- tion, oxidation-reduction, or condensation and predict products of these reactions. ▲ You will relate the struc- tures of synthetic polymers to their properties. Why It’s Important Whether you are removing a sandwich from plastic wrap, taking an aspirin, or shooting baskets, you’re using organic materials made of substituted hydrocarbons. These com- pounds are in turn made of molecules whose atoms include carbon, hydrogen, and other elements. Visit the Chemistry Web site at chemistrymc.com to find links about substituted hydrocarbons and their reactions. The spooled threads shown in the photo are made from large organ- ic molecules called polymers. 736 Chapter 23 736-773_Ch23-866418 5/9/06 3:37 PM Page 737 DISCOVERY LAB Making Slime Chemistry 10.b n addition to carbon and hydrogen, most organic substances con- Itain other elements that give the substances unique properties. In this lab, you will work with an organic substance consisting of long carbon chains to which many ϪOH groups are bonded. How will the properties of this substance change when these groups react to form bonds called crosslinks between the chains? Safety Precautions Do not allow solutions or product to contact eyes or exposed skin.
  • 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

    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.
  • AROMATIC NUCLEOPHILIC SUBSTITUTION-PART -2 Electrophilic Substitution

    AROMATIC NUCLEOPHILIC SUBSTITUTION-PART -2 Electrophilic Substitution

    Dr. Tripti Gangwar AROMATIC NUCLEOPHILIC SUBSTITUTION-PART -2 Electrophilic substitution ◦ The aromatic ring acts as a nucleophile, and attacks an added electrophile E+ ◦ An electron-deficient carbocation intermediate is formed (the rate- determining step) which is then deprotonated to restore aromaticity ◦ electron-donating groups on the aromatic ring (such as -OH, -OCH3, and alkyl) make the reaction faster, since they help to stabilize the electron-poor carbocation intermediate ◦ Lewis acids can make electrophiles even more electron-poor (reactive), increasing the reaction rate. For example FeBr3 / Br2 allows bromination to occur at a useful rate on benzene, whereas Br2 by itself is slow). In fact, a substitution reaction does occur! (But, as you may suspect, this isn’t an electrophilic aromatic substitution reaction.) In this substitution reaction the C-Cl bond breaks, and a C-O bond forms on the same carbon. The species that attacks the ring is a nucleophile, not an electrophile The aromatic ring is electron-poor (electrophilic), not electron rich (nucleophilic) The “leaving group” is chlorine, not H+ The position where the nucleophile attacks is determined by where the leaving group is, not by electronic and steric factors (i.e. no mix of ortho– and para- products as with electrophilic aromatic substitution). In short, the roles of the aromatic ring and attacking species are reversed! The attacking species (CH3O–) is the nucleophile, and the ring is the electrophile. Since the nucleophile is the attacking species, this type of reaction has come to be known as nucleophilic aromatic substitution. n nucleophilic aromatic substitution (NAS), all the trends you learned in electrophilic aromatic substitution operate, but in reverse.
  • Reaction Mechanism Lecture 3 : Nucleophilic And

    Reaction Mechanism Lecture 3 : Nucleophilic And

    Module 10 : Reaction mechanism Lecture 3 : Nucleophilic and Electrophilic Addition and Abstraction Objectives In this lecture you will learn the following Ligand activation by metal that leads to a direct external attack at the ligand. Nucleophilic addition and nucleophilic abstraction reactions. Electrophilic addition and electrophilic abstraction reactions. The nucleophilic and electrophilic substitution and abstraction reactions can be viewed as ways of activation of substrates to allow an external reagent to directly attack the metal activated ligand without requiring prior binding of the external reagent to the metal. The attacking reagent may be a nucleophile or an electrophile. The nucleophilic attack of the external reagent is favored if the LnM fragment is a poor π−base and a good σ−acid i.e., when the complex is cationic and/or when the other metal bound ligands are electron withdrawing such that the ligand getting activated gets depleted of electron density and can undergo an external attack by a nucleophile Nu−, like LiMe or OH−. The attack of the nucleophiles may result in the formation of a bond between the nucleophiles and the activated unsaturated substrate, in which case it is called nucleophilic addition, or may result in an abstraction of a part or the whole of the activated ligand, in which case it is called the nucleophilic abstraction. The nucleophilic addition and the abstraction reactions are discussed below. Nucleophilic addition An example of a nucleophilic addition reaction is shown below. Carbon monoxide (CO) as a ligand can undergo nucleophilic attack when bound to a metal center of poor π−basicity, as the carbon center of the CO ligand is electron deficient owing to the ligand to metal σ−donation not being fully compensated by the metal to ligand π−back donation.
  • Alkenes Electrophilic Addition

    Alkenes Electrophilic Addition

    Alkenes Electrophilic Addition 1 Alkene Structures chemistry of C C double bond σ C–C BDE ~ 80 kcal/mol π C=C BDE ~ 65 kcal/mol • The p-bond is weaker than the sigma-bond • The, electrons in the p-bond are higher in energy than those in the s-bond • The electrons in the p-bond are more chemically reactive than those in the s-bond Energy σ∗ M.O. π∗ M.O. p p ~ 65 kcal/mol 2 π bond alkene uses the electrons in sp2 sp THIS orbital π M.O. atomic atomic when acting as a orbitals orbitals ~ 81 kcal/mol LEWIS BASE carbon 1 carbon 2 σ bond σ M.O. molecular orbitals • The alkene uses the electrons in the p-bond when it reacts as a Lewis Base/nucleophile How do you break a p-bond? H H H H H D C C rotate C C rotate C C D D D D H 90° D cis-isomer trans-isomer zero overlap of p orbitals: π bond broken! Energy H H H D D D D H ~ 63 kcal/mol 0° 90° 180° • Rotation around a sigma-bond hardly changes the energy of the electrons in the bond because rotation does not significantly change the overlap of the atomic orbitals that make the bonding M.O. • Rotation around a p-bond, however, changes the overlap of the p AOs that are used to make the bonding M.O., at 90° there is no overlap of the p A.O.s, the p-bond is broken Alkenes : page 1 Distinguishing isomers trans- cis- • By now we are very familiar with cis- and trans-stereoisomers (diastereomers) • But what about, the following two structures, they can NOT be assigned as cis- or trans-, yet they are definitely stereoisomers (diastereomers), the directions in which their atoms point in space are different cis- / trans- Br Br We Need a different system to distinguish stereoisomers for C=C double bonds: Use Z/E notation.