DOCSLIB.ORG

Topic 6 – Alkyl Halide and Carbonyl Compounds Organic Compounds Containing a Halogen

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Topic 6 – Alkyl Halide and Carbonyl Compounds Organic Compounds Containing a Halogen Topic 6 – Alkyl halide and carbonyl compounds Organic compounds containing a halogen Compounds are named in standard way, eg: CH3 H3C CH3 Cl CH3 1 C23 I 4-chlorotoluene (aryl halide) 2-iodo-2-methylpropane (tertiary alkyl halide) 15 CHCl 3 Br Br trichloromethane 2,4-dibromopentane (chloroform) (two secondary alkyl halides) 15 CCl 4 tetrachloromethane (carbon tetrachloride) Br Br (2R,4R)-2,4-dibromopentane (two secondary alkyl halides) Structure and properties Dense liquids and solids which are insoluble in water . The C‐X bond is polar with a slight positive charge (+) residing on the carbon end of the bond and a slight CX X = F, Cl, Br or I negative charge ( ‐) on the halogen end . The carbon‐halogen bond strength decreases in the dipole moment order C‐F > C‐Cl > C‐Br > C‐I. Alkyl fluorides tend to be less reactive than other alkyl halides, mainly due to the higher strength of the C‐F bond. Bond length (pm) Bond strength (kJ mol‐1) Dipole moment (D) CH3‐F 139 452 1.85 CH3‐Cl 178 351 1.87 CH3‐Br 193 293 1.81 CH3‐I 214 234 1.62 40 1. Nucleophilic substitution Electrophile: A species that seeks an electron pair Nucleophile: A species that supplies an electron pair Nu + H3CX Nu CH3 + X nucleophile electrophile nucleophilic substitution Nu + H3CX Nu CH3 + X Examples: A wide range of charged and uncharged nucleophiles may be employed Nucleophile + H3CI Product + I Product class HO + H3CI H3COH + I alcohol RO + H3CI H3COR + I ether NC + H3CI H3CCN + I nitrile RCC + H3CI H3CCCR+ I alkyne H N 2 + H3CI H3CNH2 + I amine R3N + H3CI H3CNR3 + I tetraalkylammonium salt Question: Draw the organic products in the following CN reactions: Br OH NH2 41 nd Mechanism – SN2 (2 order) CH CH H3CH2C 2 3 CH2CH3 ~1/2 ~1/2 CH3OCCBr H3COC Br CH3O Br H H CH3 H CH3 CH3 leaving nucleophile transition state group electrophile SN2 reaction favoured by a H Me primary alkyl halide as the CX CX starting material. H H Nu H Nu H methyl halide primary alkyl halide relative rate: 2,000,000 (FAST) 40,000 Me Me CX CX Me Me Nu H Nu Me secondary alkyl halide tertiary alkyl halide relative rate: 500 <1 (V. SLOW) st Mechanism – SN1 (1 order) 2 carbocations have sp hybridised carbons (trigonlal planar) with a vacant p-orbital CH3 Attack of nucleophile to either side of the planar C Br CH carbocation intermediate H CC 3 is equally likely! 3 CH H C 3 3 CH 3 vacant p-orbital slow CH 3 CH3 CH3 HO OH C C OH HO C fast fast H C CH 3 3 H3C H3C CH3 CH3 Br planar + carbocation leaving intermediate group 42 Evidence for SN1 mechanism Intermediacy of a planar carbocation intermediate means that any stereochemistry in the starting material is lost on conversion to the product. The nucleophilic substitution of (R)-3-bromo-3-methylhexane with methanol leads to racemic methyl ether product H3C Br CH3 slow + Br (R)-3-bromo-3-methylhexane planar carbocation intermediate CH3OH CH3OH rate determining step fast 50% inversion H C OCH H CO CH 50% retention 3 3 3 3 + (R)-3-methoxy-3-methylhexane (S)-3-methoxy-3-methylhexane SN1 stands for substitution, nucleophilic, unimolecular. Unimolecular refers to the fact that only one molecule, the alkyl halide, is involved in the slow rate determining step. The reaction rate is therefore determined by the concentration of the alkyl halide Reaction Rate [electrophile] Carbocations The intermediate in an SN1 reaction is a carbocation. The C atom has six electrons and a positive charge. This is an unstable, highly H3C H3C H3C H reactive intermediate. C CH3 C H C H C H Not all carbocations have the H3C H3C H H a tertiary alkyl a secondary alkyl a primary alkyl a methyl same relative (un)stability . carbocation carbocation carbocation carbocation most least stable stable increasing stability H H C 3 CX CX S 1 reaction more likely when a N H H tertiary carbocation is the H H intermediate methyl halide primary alkyl halide relative rate: (V. SLOW) <1 1 H3C H3C CX CX H3C H3C H H3C secondary alkyl halide tertiary alkyl halide relative rate: 12 (FAST) 1,200,000 43 Comparison of SN1 and SN2 Summary SN2 reaction: Summary SN1 reaction: Substitution, nucleophilic, second order Substitution, nucleophilic, first order The reaction rate is determined by the The reaction rate is determined by the concentration of both nucleophile and concentration the electrophile only electrophile Involves the two step substitution of an Involves the direct, single step alkyl halide by a nucleophile via a substitution of an alkyl halide by a carbocation intermediate nucleophile with no intermediates The SN1 reaction of chiral compounds The SN2 reaction proceeds with proceeds with racemisation inversion The rate of the SN1 reaction of alkyl The rate of the SN2 reaction of alkyl halides decreases in the order 3° >> 2° > 1° > methyl halides decreases in the order methyl > 1° > 2° >> 3° Usually only 3° alkyl halides react by this Usually only methyl, 1°and 2° alkyl pathway halides react by this pathway - less crowded Question: Fill in the missing reagents. N(CH3)3 I OH I OCH2CH3 + NaI + NaI + NaI C C CH3 44 2. Elimination of HX Dehydrohalogenation – require H and X on adjacent carbons. Generally concentrated reagents and elevated temperatures are used. Concentrated OH– is used (often KOH in ethanol solvent). c. OH H3C c. OH H3C heat H3C heat HC CH2 HC CH2 HC CH2 H Br Br H + H2O + Br 1-bromopropane 2-bromopropane Zaitsev’s Rule applies – where there is a choice, the most substituted alkene forms. + H O + Br c. OH H3C HC CH CH3 2 H heat H3CCHC CH2 major product H Cl H H3C H2C HC CH2 + H2O + Br R R CC H H R H R R R R H H CC CC CC CC H H H H R H R H R R CC R H unsubstituted monosubstituted disubstituted trisubstituted tetrasubstituted ethylene alkene alkene alkene alkene Br hot, conc K OH + H2O + K Br Examples of eliminations ethanol H3C H3C hot, conc H2SO4 COH CCH2 + H2O H3C H3C H3C hot, conc K OH + H2O ethanol + K Cl Cl major minor trisubstituted C=C bond disubstituted C=C bond 3 carbon substituents 2 carbon substituents 45 Question: Fill in the reagents in the following sequence of reactions. OH OH Br Br 3. Formation of Grignard reagent This is the reaction of an alkyl‐ or aryl‐halide and magnesium. X = Cl, Br, I R X + Mg R Mg X R = alkyl, aromatic eg CH3Br + Mg CH3MgBr + Mg I Mg I Cl MgCl + Mg 46 Carbonyl Compounds – aldehydes and ketones The double bond is composed of one σ and one π bond. H The electron density of a π bond is above and below the plane of the double bond. CO The C=O double bond is polarised. Aldehydes have at least one H attached to the carbonyl group, ketones have two carbon groups attached to the carbonyl group. H Carbon of the carbonyl group is sp2 hybridised. overlap of electrons in p-orbitals give rise to a -bond H oxygen has two lone H C O pairs of electrons CO H H overlap of electrons in sp2-hybridised orbitals gives a -bond in formaldehyde, overlap of electrons in an sp2-hybridised orbital from C and a 1s orbital from H gives a -bond Nomenclature Aldehydes The longest chain containing the CHO group gives the stem; ending –al. If substituents are present, start the numbering from the aldehyde group ‐ C1. Ketones The longest chain containing the carbonyl group gives the stem; ending –one. If substituents are present number from the end of the chain so the carbonyl group has the lowest possible number. There are non‐systematic names for the common aldehydes and ketone, for example: H CH CH CH CH3CH2CH2 3 3 2 CO CO CO CO H H H H formaldehyde acetaldehyde propionaldehyde butyraldehyde (methanal) (ethanal) (propanal) (butanal) O O CH 3 C C H CH CO 3 CH3 acetone benzaldehyde acetophenone (propanone) 47 Question: Name the following compounds. O O O CH2O OO Reactions of carbonyl group – nucleophilic addition Dominated by weak and accessible π‐bond Polarity of C=O directs addition reaction Reaction conducted in two stages . Nucleophile (eg hydride or carbanion) adds to C . Then O– adds to H+ H H H H H Nu CO CO CO stage 1 Nu Nu stage 2 H H H 1. Hydride addition (reduction) – + Use H (LiAlH4 or NaBH4) followed by H Aldehydes give primary alcohols, ketones give secondary alcohols Examples: R R R H H H CO H CO H CO H H H primary alcohol R R R H H H CO H CO H CO R R R secondary alcohol 48 2. Grignard reaction C nucleophile This is a very versatile reaction. use Grignard reagent: RMgX Can use most organohalide compounds to generate Grignard R Mg X reagent. Grignard reagent reacts with aldehydes, ketones and CO2. Must employ anhydrous conditions. R Mg X Examples: H H H H H CO H3C H3C CO H3C CO STEP 1 STEP 2 H H H MgBr nucleophilic MgBr acid-base addition reaction methyl formaldehyde an alkoxide a primary magnesium anion alcohol bromide R R R H H CO H3C H3C CO H3C CO H H H MgBr MgBr methyl other aldehydes secondary magnesium alcohol bromide R R R H H H3C CO H C CO H C CO 3 3 R R R MgBr MgBr methyl all ketones tertiary magnesium alcohol bromide O OO H H3C C H3C C H3C C O O O H MgBr MgBr methyl carbon dioxide carboxylic magnesium acid bromide 49 Question: Draw the products of the following sequence of reactions.
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]