DOCSLIB.ORG

Nucleophilic Substitution Reactions of Organic Halides

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Nucleophilic Substitution Reactions of Organic Halides Experiment 9: Nucleophilic Substitution Reactions of Organic Halides Introduction: Substitution Reactions Substitution reactions are reactions where the two species involved exchange parts: Na C + Br C C + NaBr C There are two types of substitution reactions that commonly occur in organic chemical reactions, the SN1 and the SN2 reactions. They have different mechanisms which means their ability to occur is determined by factors like sterics around the alkyl halide, the choice of nucleophile used or even the solvent for the reaction. 1. SN1 (Substitution, Nucleophilic, Unimolecular) Pertinent SN1 Facts The SN1 reaction is called “unimolecular” because the rate of this type of reaction is dependent upon only one species in the rate-determining step, the alkyl halide. r.d.s. Nuc C X C + X C Nuc (slow) (fast) The rate of reaction is ONLY dependent on how quickly a carbocation might form by “spontaneous dissociation”. Remember that the nucleophile is NOT involved in this step. Factors that affect the rate of an SN1 Reaction: 1. The Alkyl Halide must form a stable carbocation, and the more stable the carbocation, the faster it will form, causing the overall reaction to occur quicker (more stable intermediate = lower activation barrier). Recall that alkyl groups stabilize carbocations through both inductive effect and hyperconjugation. The order of carbocation stability: 3º > 2º >>>>>> 1º or methyl Resonance Stabilization adds a tremendous amount of stabilization to a carbocation and can also allow an SN1 reaction to occur faster since resonance also stabilizes carbocations – the more the charge can be spread out over multiple atoms, the more stable the charge will be (“delocalization”). An allylic or Benzylic 1º carbocation is easily as stable as a normal 2º carbocation. 2. Leaving Group: Same order of reactivity seen for SN2 reaction ⊖ ⊖ ⊖ ⊖ Halides: I , Br > Cl , H2O >>> F (best) (worst) 3. The Nucleophile does NOT affect the rate of the reaction because it is not part of the rate-determining step. The nucleophiles in this type of reaction are often weak bases, usually neutral molecules like H2O or ROH. Today we are using ethanol, CH3CH2OH, as your nucleophile. 4. The choice of solvent affects rate by affecting a reagent’s energy levels. Solvent molecules do what is called “solvation”, meaning they surround certain species in a reaction. The solvent can affect the rate of SN1 reaction by stabilizing the carbocation intermediate as it forms, thus increasing the rate of reaction. Any POLAR solvent can be used to do this. The solvent today is ethanol, CH3CH2OH, and it will act as both the nucleophile AND the solvent (a “solvolysis” reaction). H CH2CH3 O H H O CH CH H3CH2C O 2 3 O CH CH H3CH2C O 2 3 H O H H CH CH 2 3 SN1 Reaction Conditions: Each compound will be treated with a solution of silver nitrate in ethanol. Ethanol is a polar, protic solvent that favors ionization of the alkyl halide to form stable carbocations. This is facilitated by the silver ion coordinating to the halide through a lone pair on the halogen atom, causing a weakening of the carbon-halogen bond. Silver halide salts are very insoluble and will precipitate from the solution, indicating if a reaction has occurred. The nitrate ion and ethanol solvent are both poor nucleophiles and so SN2 reactions do not occur. Ag+ R X R X Ag R + AgX(s) HOCH2CH3 R-OCH2CH3 2. The SN2 Reaction (Substitution, Nucleophilic, Bimolecular) The SN2 type reaction is a one-step, concerted substitution process (make new bonds, break old bonds simultaneously). Both the alkyl halide and the nucleophile are involved (“bimolecular”) to determine the rate of reaction: 2 Nuc X + X Nuc Factors that affect the SN2 Reaction: 1. Steric Congestion of Alkyl Halide -The more congested and sterically hindered the alkyl halide, the slower the reaction. Remember that alkyl halides have sp3 hybridized carbons, to which the halide is bonded. Fastest Slower Never CH3-X > RCH2-X > R2CH-X >>>>>>>>>>>>>>> R3C-X 2. The Leaving Group is what the halide is referred to in a substitution reaction and it is the group that must leave the alkyl halide. It is almost always expelled with a full negative charge in an SN2 reaction. The best leaving groups will be those that can best stabilize the anion (i.e. the weakest bases). Halides: I⊖ , Br⊖ > Cl⊖ >>> F⊖ (best) (worst) 3. The choice of solvent affects rate by affecting a reagent’s energy levels. Today you will be using acetone, which is a relatively polar and aprotic solvent. Polar, aprotic solvent molecules solvate the metal counterion of the nucleophile (in today’s lab: Na+ of the I-). This will expose the iodide anion so it will be destabilized and more reactive. O O O I Na O O exposed!! O 4. Nucleophile – the species that causes a substitution reaction to occur. For atoms in the same family (column) on the periodic table, as they become larger (as you move DOWN a column on the periodic table), they are better nucleophiles. Larger atoms will ⊖ be more polarizable, thus making I an excellent choice for SN2 reactions. SN2 Reaction Conditions: Each compound will be treated with a solution of sodium iodide (excellent nucleophile) in acetone (polar, aprotic solvent). Also, sodium iodide, NaI, is soluble in acetone whereas sodium bromide and sodium chloride are not. The formation of a precipitate of either NaBr or NaCl will therefore indicate that a reaction has occurred. 3 R Cl + Na+I- acetone R I + NaCl(s) + - R Br + Na I R I + NaBr(s) acetone Introduction: Experiment In this experiment the reactivity of seven different alkyl halides towards nucleophilic substitution reactions will be examined. There are several factors that may influence the reactivity, including the structure of the substrate, the leaving group and the reaction conditions (both solvent and temperature). Here, we will perform two sets of experiments to test the reactivity of each, under conditions that favor SN2 and under conditions that favor SN1. The structures of the substrates are given below. Br CH CH CH CH Cl CH -CH-CH CH 3 2 2 2 3 2 3 CH3-CH-CH2CH3 Cl Br 1-chlorobutane 2-chlorobutane 2-bromobutane bromobenzene CH3 CH3-CH-CH2Cl H3C C Cl CH3CH=CHCH2Cl CH3 CH3 1-chloro-2-methylpropane 2-chloro-2-methylpropane 1-chloro-2-butene (cis/trans mixture) FOR YOUR SAFETY Wear Gloves - Organic halides are irritants and in certain cases corrosive. Silver salts will stain skin black and this is very hard to remove. Wear gloves and dispense with the syringes that are provided. Procedure A. Reactivity with AgNO3 in ethanol (SN1) 1. Obtain a thermometer. Add approximately 1 inch depth of hot tap water to a large beaker and place on your hotplate at a setting of 2 to set up a water bath and stabilize the water temperature at 45°C. Clamp a thermometer in the water so that you can monitor the temperature. Make sure the thermometer is not touching the bottom of the beaker (or you’ll be recording the temperature of the glass beaker, not the water!) 2. To your clean test tubes labeled #1-7 (washed from part A), and using the color- coded 1.0 mL syringes provided, add 0.1 mL of an organic halide (in this order, to avoid confusion): (1) 1-chlorobutane (2) 2-chlorobutane (3) 2-bromobutane 4 (4) bromobenzene (5) 1-chloro-2-methylpropane (6) 2-chloro-2-methylpropane (7) 1-chloro-2-butene Place a cork stopper in each of the tubes and stand the tubes in a test tube rack. Again, to avoid confusion during Steps 3-6, do not attempt to do all 7 test tubes in a single trial. Perform Steps 3-6 on the first three compounds listed, then repeat Steps 3-6 on the final four compounds. 3. Rapidly transfer 1 mL of a 1% AgNO3 solution in ethanol to each tube and mix thoroughly by swirling. Replace the cork stopper in each tube and start timing. Note 1: the designated timekeeper should start a running timer (Stopwatch on cell phone!) and make note of the START TIME for each addition to each test tube (i.e. 0:02 min, 0:25 min, 0:50 min, etc). 4. Watch each test tube carefully and note the time at which a precipitate appears (END TIME) and its color. The timekeeper must record the end time from the running stopwatch (1:03 min, 4:15 min, etc). Note 2: The time for the reaction is the difference between the START and the END (0:02 to 1:03 = 1 min, 1 sec and 0:25 to 4:15 = 3 min, 50 sec. Remember we are using min/sec and there are 60 seconds per min! 5. If after 5 minutes no precipitate is visible, move any test tubes that have no precipitate into the 45°C water bath and continue watching them for the formation of precipitates. 6. Heat for no more than 6 more minutes (11 minutes total), noting the END TIME for any precipitates that form in the hot bath, then remove the tubes from the hot bath and cool to room temperature. 7. When your observations are completed, pour all the solutions into the silver salts waste container, rinse the tubes with ethanol and place those washings in the same container. A 10% thiosulfate solution can be used to remove any residual silver halide precipitates (thiosulfate ion dissolves silver halides) – place these washings in the silver waste container also. Then wash with water and rinse with acetone (dispose of acetone rinses in acetone waste).
Load more

Recommended publications
  • 3 + 3HC1 Effect of Water and Ammonia, Respectively, Upon
    VOL. 23, 1937 CHEMISTRY: SLOB UTSKY, A UDRIETHAND CAMPBELL 611 ACID CATALYSIS IN LIQUID AMMONIA. I. AMMONOL YSIS OF DIETHYLMALONA TE BY CHARLES SLOBUTSEY, L. F. AUDRIETH AND R. W. CAMPBELL DEPARTMENT OF CHEMISTRY, UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS Communicated November 3, 1937 Numerous investigations have shown that liquid ammonia possesses unusual properties as a solvent' for many inorganic and organic compounds. Like water it is the parent substance of a system of acids, bases and salts.2 Liquid ammonia also reacts directly with many substances. Solvolysis takes place and in the case of ammonia such reactions are termed ammono- lytic reactions. Just as phosphorus trichloride is hydrolyzed by water (1), so it may be ammonolyzed3 by ammonia to give the nitrogen analogs of phosphorous and hydrochloric acids (2). PCI + 3HOH -- P(OH)3 + 3HC1 (1) PCI3 + 5HNH2 -+ P(NH)(NH2) + 3NH4C1 (2) Many hydrolytic reactions are catalyzed in aqueous solutions by acids or bases. Thus, the inversion of cane sugar is catalyzed by acids and the velocity of the reaction is a function of the concentration of the hydrogen ion. Esters also undergo hydrolysis and such reactions are markedly catalyzed by the hydrogen ion, or in terms of the modern Bronsted concept of acidity, by the onium4 ion, or solvated proton. It was, therefore, to have been expected that the rate of ammonolysis of esters in liquid ammonia would be accelerated by the presence of ammonium salts, which have been shown to possess acid character in liquid ammonia. It has already been demonstrated qualitatively that ammonium salts do exert a catalytic effect in other ammonolytic reactions.
    [Show full text]
  • 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.
    [Show full text]
  • 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.
    [Show full text]
  • 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.
    [Show full text]
  • Nucleophilic Substitution and Elimination Reactions
    8 NUCLEOPHILIC SUBSTITUTION AND ELIMINATION REACTIONS substitution reactions involve the replacement of one atom or group (X) by another (Y): We already have described one very important type of substitution reaction, the halogenation of alkanes (Section 4-4), in which a hydrogen atom is re- placed by a halogen atom (X = H, Y = halogen). The chlorination of 2,2- dimethylpropane is an example: CH3 CH3 I I CH3-C-CH3 + C12 light > CH3-C-CH2Cl + HCI I I Reactions of this type proceed by radical-chain mechanisms in which the bonds are broken and formed by atoms or radicals as reactive intermediates. This 8-1 Classification of Reagents as Electrophiles and Nucleophiles. Acids and Bases mode of bond-breaking, in which one electron goes with R and the other with X, is called homolytic bond cleavage: R 'i: X + Y. - X . + R : Y a homolytic substitution reaction There are a large number of reactions, usually occurring in solution, that do not involve atoms or radicals but rather involve ions. They occur by heterolytic cleavage as opposed to homolytic cleavage of el~ctron-pairbonds. In heterolytic bond cleavage, the electron pair can be considered to go with one or the other of the groups R and X when the bond is broken. As one ex- ample, Y is a group such that it has an unshared electron pair and also is a negative ion. A heterolytic substitution reaction in which the R:X bonding pair goes with X would lead to RY and :X? R~:X+ :YO --' :x@+ R :Y a heterolytic substitution reaction A specific substitution reaction of this type is that of chloromethane with hydroxide ion to form methanol: In this chapter, we shall discuss substitution reactions that proceed by ionic or polar mechanisms' in which the bonds cleave heterolytically.
    [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]
  • Aromatic Nucleophilic Substitution Reaction
    Aromatic Nucleophilic Substitution Reaction DR. RAJENDRA R TAYADE ASSISTANT PROFESSOR DEPARTMENT OF CHEMISTRY INSTITUTE OF SCIENCE, NAGPUR Principles There are four principal mechanisms for aromatic nucleophilic substitution which are similar to that of aliphatic nucleophilic substitution. (SN1, SN2, SNi, SET Mechanism) 1. SNAr Mechanism- addition / elimination CF3, CN, CHO, COR, COOH, Br, Cl, I Common Activating Groups for NAS Step [1] Addition of the nucleophile (:Nu–) to form a carbanion Addition of the nucleophile (:Nu–) forms a resonance-stabilized carbanion with a new C – Nu bond— three resonance structures can be drawn. • Step is rate-determining • Aromaticity of the benzene ring is lost Step [2] loss of the leaving group re-forms the aromatic ring. • This step is fast because the aromaticity of the benzene ring is restored. ? Explain why a methoxy group (CH3O) increases the rate of electrophilic aromatic substitution, but decreases the rate of nucleophilic aromatic substitution. 2.ArSN1 Mechanism- elimination /addition • This mechanism operates in the reaction of diazonium salts with nucleophiles. •The driving force resides in the strength of the bonding in the nitrogen molecule that makes it a particularly good leaving group. 3.Benzyne Mechanism- elimination /addition Step [1] Elimination of HX to form benzyne Elimination of H and X from two adjacent carbons forms a reactive benzyne intermediate Step [2] Nucleophilic addition to form the substitution product Addition of the nucleophile (–OH in this case) and protonation form the substitution product Evidence for the Benzyne Mechanism Trapping in Diels/Alder Reaction O O B E N Z Y N E C C O O O D i e l s / A l d e r O N H 3 N N Dienophile Diene A d d u c t Substrate Modification – absence of a hydrogens LG Substituent Substituent No Reaction Base Isotopic Labeling LG Nu H Nu Structure of Benzyne • The σ bond is formed by overlap of two sp2 hybrid orbitals.
    [Show full text]
  • Elimination Reactions Are Described
    Introduction In this module, different types of elimination reactions are described. From a practical standpoint, elimination reactions widely used for the generation of double and triple bonds in compounds from a saturated precursor molecule. The presence of a good leaving group is a prerequisite in most elimination reactions. Traditional classification of elimination reactions, in terms of the molecularity of the reaction is employed. How the changes in the nature of the substrate as well as reaction conditions affect the mechanism of elimination are subsequently discussed. The stereochemical requirements for elimination in a given substrate and its consequence in the product stereochemistry is emphasized. ELIMINATION REACTIONS Objective and Outline beta-eliminations E1, E2 and E1cB mechanisms Stereochemical considerations of these reactions Examples of E1, E2 and E1cB reactions Alpha eliminations and generation of carbene I. Basics Elimination reactions involve the loss of fragments or groups from a molecule to generate multiple bonds. A generalized equation is shown below for 1,2-elimination wherein the X and Y from two adjacent carbon atoms are removed, elimination C C C C -XY X Y Three major types of elimination reactions are: α-elimination: two atoms or groups are removed from the same atom. It is also known as 1,1-elimination. H R R C X C + HX R Both H and X are removed from carbon atom here R Carbene β-elimination: loss of atoms or groups on adjacent atoms. It is also H H known as 1,2- elimination. R C C R R HC CH R X H γ-elimination: loss of atoms or groups from the 1st and 3rd positions as shown below.
    [Show full text]
  • Concerted Nucleophilic Aromatic Substitution Reactions Simon Rohrbach+, Andrew J
    Angewandte Reviews Chemie International Edition: DOI: 10.1002/anie.201902216 Nucleophilic Aromatic Substitution German Edition: DOI: 10.1002/ange.201902216 Concerted Nucleophilic Aromatic Substitution Reactions Simon Rohrbach+, Andrew J. Smith+, Jia Hao Pang+, Darren L. Poole, Tell Tuttle,* Shunsuke Chiba,* and John A. Murphy* Keywords: Dedicated to Professor Koichi concerted reactions Narasaka on the occasion of ·cSNAr mechanism · his 75th birthday Meisenheimer complex · nucleophilicaromatic substitution Angewandte Chemie &&&& 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2019, 58,2–23 Ü Ü These are not the final page numbers! Angewandte Reviews Chemie Recent developments in experimental and computational From the Contents chemistry have identified a rapidly growing class of nucleophilic 1. Aromatic Substitution Reactions 3 aromatic substitutions that proceed by concerted (cSNAr) rather than classical, two-step, SNAr mechanisms. Whereas traditional 2. Some Contributions by SNAr reactions require substantial activation of the aromatic ring Computational Studies 6 by electron-withdrawing substituents, such activating groups are not mandatory in the concerted pathways. 3. Fluorodeoxygenation of Phenols and Derivatives 9 4. Aminodeoxygenation of Phenol 1. Aromatic Substitution Reactions Derivatives 10 Substitution reactions on aromatic rings are central to 5. Hydrides as Nucleophiles 11 organic chemistry. Besides the commonly encountered elec- trophilic aromatic substitution,[1] other mechanisms include 6. P, N, Si, C Nucleophiles 13 [2,3] SNAr nucleophilic aromatic substitutions and the distinct [4] but related SNArH and vicarious nucleophilic substitutions, 7. Organic Rearrangements via Spiro substitutions brought about through benzyne intermedi- Species: Intermediates or Transition ates,[5,6] radical mechanisms including electron transfer- States? 14 [7] based SRN1 reactions and base-promoted homolytic aro- matic substitution (BHAS) couplings,[8] sigmatropic rear- 8.
    [Show full text]
  • S 1 Reaction Remember That a 3˚ Alkyl Halide Will Not Undergo a S 2 Reaction the Steric Hindrance in the Transition State
    SN1 Reaction Remember that a 3˚ alkyl halide will not undergo a SN2 reaction The steric hindrance in the transition state is too high If t-Butyl iodide is reacted with methanol, however, a substitution product is obtained This product does NOT proceed through a SN2 reaction First proof is that the rate for the reaction does not depend on methanol concentration Occurs through a SN1 reaction Substitution – Nucleophilic – Unimolecular (1) SN1 - In a SN1 reaction the leaving group departs BEFORE a nucleophile attacks For t-butyl iodide this generates a planar 3˚ carbocation The carbocation can then react with solvent (or nucleophile) to generate the product If solvent reacts (like the methanol as shown) the reaction is called a “solvolysis” The Energy Diagram for a SN1 Reaction therefore has an Intermediate - And is a two step reaction CH3 Potential H3C energy CH3 I OCH CH3 3 H3C CH3 CH3 H3C CH3 Reaction Coordinate Why does a SN1 Mechanism Occur? - Whenever the potential energy of a reaction is disfavored for a SN2 reaction the SN1 mechanism becomes a possibility SN2 SN1 Potential energy I OCH3 CH3 H3C CH3 CH3 H3C CH3 Reaction Coordinate Rate Characteristics The rate for a SN1 reaction is a first order reaction Rate = k [substrate] The first step is the rate determining step The nucleophile is NOT involved in the rate determining step Therefore the rate of a SN1 reaction is independent of nucleophile concentration (or nucleophile characteristics, e.g. strength) What is Important for a SN1 Reaction? The primary factor concerning the rate of
    [Show full text]
  • Solvolysis Reaction Kinetics, Rates and Mechanism for Phenyl N-Phenyl Phosphoramidochloridate
    Studies of Phenyl N-Phenyl Phosphoramidochloridate Bull. Korean Chem. Soc. 2014, Vol. 35, No. 8 2465 http://dx.doi.org/10.5012/bkcs.2014.35.8.2465 Solvolysis Reaction Kinetics, Rates and Mechanism for Phenyl N-Phenyl Phosphoramidochloridate Hojune Choi, Kiyull Yang, Han Joong Koh,†,* and In Sun Koo* Department of Chemistry Education and Research Instituted of Natural Science, Gyeongsang National University, Jinju 660-701, Korea. *E-mail: [email protected] †Department of Science Education, Chonju National University of Education, Chonju, Korea. *E-mail: [email protected] Received March 29, 2014, Accepted April 29, 2014 The rate constants of solvolysis of phenyl N-phenyl phosphoramidochloridate (PhNHPO(Cl)OPh, Target Compound-TC1) have been determined by a conductivity method. The solvolysis rate constants of TC1 are well correlated with the extended Grunwald-Winstein equation, using the NT solvent nucleophilicity scale and YCl solvent ionizing scale, and sensitivity values of 0.85 ± 0.14 and 0.53 ± 0.04 for l and m, respectively. These l and m values were similar to those obtained previously for the complex chemical substances dimethyl thiophosphorochloridate; N,N,N',N'-tetramethyldiamidophosphorochloridate; 2-phenyl-2-ketoethyl tosylate; diphenyl thiophosphinyl chloride; and 9-fluorenyl chloroformate. As with the five previously studied solvolyses, an SN2 pathway is proposed for the solvolyses of TC1. For four representative solvents, the rate constants were measured at several temperatures, and activation parameters (∆H≠ and ∆S≠) were estimated. These activation parameters are also in line with the values expected for an SN2 reaction. Key Words : Phenyl N-phenyl phosphoramidochloridate, Extended Grunwald-Winstein equation, SN2 mech- anism, Activation parameters Introduction mediate or is the result of a single transition state (TS).2 The reaction mechanism of phosphoryl compounds, such Phosphoryl transfer is an important aspect of biological as that of ‘phenyl N-phenyl phosphoramidochloridate’ chemistry and organic synthesis.
    [Show full text]
  • FULL PAPER a Self-Assembled Cage with Endohedral Acid Groups Both
    FULL PAPER A Self-Assembled Cage with Endohedral Acid Groups both Catalyzes Substitution Reactions and Controls their Molecularity Paul M. Bogie, Lauren R. Holloway, Courtney Ngai, Tabitha F. Miller, Divine K. Grewal, and Richard J. Hooley[a]* [16],[17] Abstract: A self-assembled Fe4L6 cage complex internally decorated many possibilities in controlled biomimetic catalysis, above with acid functions is capable of accelerating the thioetherification of and beyond simply increasing the effective concentration of activated alcohols, ethers and amines by up to 1000-fold. No product bound substrate. The incorporation of active functions in an inhibition is seen, and effective supramolecular catalysis can occur enclosed space enables reagent-controlled reactions to take with as little as 5 % cage. The substrates are bound in the host with place in enclosed cavities, as opposed to cycloadditions[18]-[20] or up to micromolar affinities, whereas the products show binding that is unimolecular rearrangements, [21],[22] which are still the most an order of magnitude weaker. Most importantly, the cage host alters common reactions studied in synthetic hosts. By internalizing the molecularity of the reaction: whereas the reaction catalyzed by reactive functional groups in a cage, the effect of substrate simple acids is a unimolecular, SN1-type substitution process, the rate binding on nucleophilic substitution reactions can be investigated. of the host-mediated process is dependent on the concentration of nucleophile. The molecularity of the cage-catalyzed reaction is substrate-dependent, and can be up to bimolecular. In addition, the catalysis can be prevented by a large excess of nucleophile, where substrate inhibition dominates, and the use of tritylated anilines as substrates causes a negative feedback loop, whereby the liberated product destroys the catalyst and stops the reaction.
    [Show full text]