Alkyl Halides
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Chapter 12 Lecture Notes
Chemical Kinetics “The study of speeds of reactions and the nanoscale pathways or rearrangements by which atoms and molecules are transformed to products” Chapter 13: Chemical Kinetics: Rates of Reactions © 2008 Brooks/Cole 1 © 2008 Brooks/Cole 2 Reaction Rate Reaction Rate Combustion of Fe(s) powder: © 2008 Brooks/Cole 3 © 2008 Brooks/Cole 4 Reaction Rate Reaction Rate + Change in [reactant] or [product] per unit time. Average rate of the Cv reaction can be calculated: Time, t [Cv+] Average rate + Cresol violet (Cv ; a dye) decomposes in NaOH(aq): (s) (mol / L) (mol L-1 s-1) 0.0 5.000 x 10-5 13.2 x 10-7 10.0 3.680 x 10-5 9.70 x 10-7 20.0 2.710 x 10-5 7.20 x 10-7 30.0 1.990 x 10-5 5.30 x 10-7 40.0 1.460 x 10-5 3.82 x 10-7 + - 50.0 1.078 x 10-5 Cv (aq) + OH (aq) → CvOH(aq) 2.85 x 10-7 60.0 0.793 x 10-5 -7 + + -5 1.82 x 10 change in concentration of Cv Δ [Cv ] 80.0 0.429 x 10 rate = = 0.99 x 10-7 elapsed time Δt 100.0 0.232 x 10-5 © 2008 Brooks/Cole 5 © 2008 Brooks/Cole 6 1 Reaction Rates and Stoichiometry Reaction Rates and Stoichiometry Cv+(aq) + OH-(aq) → CvOH(aq) For any general reaction: a A + b B c C + d D Stoichiometry: The overall rate of reaction is: Loss of 1 Cv+ → Gain of 1 CvOH Rate of Cv+ loss = Rate of CvOH gain 1 Δ[A] 1 Δ[B] 1 Δ[C] 1 Δ[D] Rate = − = − = + = + Another example: a Δt b Δt c Δt d Δt 2 N2O5(g) 4 NO2(g) + O2(g) Reactants decrease with time. -
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 . -
Chemistry 0310 - Organic Chemistry 1 Chapter 6
Dr. Peter Wipf Chemistry 0310 - Organic Chemistry 1 Chapter 6. SN2 Reactions Nucleophilic substitution reactions occur when nucleophiles (electron-rich species) displace leaving groups on electrophiles (electron-poor species). The net result is the substitution of one group for another bonded to a carbon atom. Nucleophilicity is increased by a negative charge and an increase in the polarizability. The greater the size and the lower the electronegativity of an atom, the greater its polarizability. Leaving groups are stable species that can be detached with their bonding electrons from a molecule during reaction. Most good leaving groups are the conjugate bases of strong acids. Sulfonates (mesylates, tosylates, triflates, etc.) are popular leaving groups since they can be readily obtained from alcohols. Solvents also influence nucleophilicity. SN2 reactions are second-order reactions whose rates depend o the concentration of both the alkyl halide and the nucleophile. The transition state of the one-step SN2 process involves a trigonal bipyramidal carbon and results in an overall inversion of configuration. The order of reactivity of alkyl halides and alkyl sulfonates in SN2 reactions is CH3>1°>2°>3°. The activation energy, DG‡, determines the rate of a reaction: k = koexp(-DG‡/RT). The overall change in free energy, DG, determines the position of the thermodynamic equilibrium. A positive sign of DG is characteristic for an endergonic reaction, a negative DG is found for an exergonic process. The rate of a reaction can be measured and provides information about its mechanism. The reaction order is the sum of the exponents in the rate equation. Relative Leaving Group Abilities: Leaving Group Common Name krel AcO acetate 1 x 10-10 Cl chloride 0.0001 Br bromide 0.001 I iodide 0.01 O mesylate 1.00 H3C S O O O tosylate 0.70 H3C S O O O brosylate 2.62 Br S O O O nosylate 13 O2N S O O O fluorosulfate 29,000 F S O O O triflate 56,000 CF3 S O O O nonaflate 120,000 C4F9 S O O. -
Stereochemistry, Alkyl Halide Substitution (SN1 & SN2)
Chem 261 Assignment & Lecture Outline 3: Stereochemistry, Alkyl Halide Substitution (SN1 & SN2) Read Organic Chemistry, Solomons, Fryle & Snyder 12th Edition (Electronic) • Functional Group List – Learn to recognize – Please see Green Handout – also p 76 of text • Periodic Table – Inside Front Cover - know 1st 10 elements (up through Neon) • Relative Strength of Acids and Bases – Inside Front cover (reference only) • Chapter 5 – Stereochemistry • Chapter 6 –Ionic Reactions: Nucleophilic Substitution of Alkyl Halides Problems: Do Not turn in, answers available in "Study Guide Student Solutions Manual " Solomons, Fryle, Snyder • Chapter 5: 5.1 to 5.15; 5.18 to 5.21; 5.26; 5.28; 5.33a-d; 5.46 • Chapter 6: 6.1 to 6.5; 6.7 to 6.10; 6.13; 6.20; 6.26; 6.27 Lecture Outline # 3 I. Comparison of 2 Structures: Same Molecular Formula ? -> If Yes, Possibly Isomers or Identical Same Arrangement (Sequence) of Groups ? If No -> Structural Isomers If Yes -> Superposable? If Yes -> Identical Structures If No -> Stereoisomers Non-Superposable Mirror Images ? If NO -> Diastereomers If Yes -> Enantiomers II. Chirality and Stereoisomers A. The Concept of Chirality 1. Identification of chiral objects a) achiral = not chiral b) planes of symmetry within a molecule 2. Types of stereoisomers – enantiomers and diastereomers B. Location of stereogenic (chiral) centres – 4 different groups on tetrahedral atom 1. Enantiomers & diastereomers 2. Meso compounds - chiral centers with plane of symmetry within molecule 3. Molecules with more than one chiral centre 4. Recognition of chiral centers in complex molecules - cholesterol - 8 chiral centres Drawing the enantiomer of cholesterol and its potential 255 stereoisomers 5. -
Ochem ACS Review 23 Aryl Halides
ACS Review Aryl Halides 1. Which one of the following has the weakest carbon-chlorine bond? A. I B. II C. III D. IV 2. Which compound in each of the following pairs is the most reactive to the conditions indicated? A. I and III B. I and IV C. II and III D. II and IV 3. Which of the following reacts at the fastest rate with potassium methoxide (KOCH 3) in methanol? A. fluorobenzene B. 4-nitrofluorobenzene C. 2,4-dinitrofluorobenzene D. 2,4,6-trinitrofluorobenzene 4. Which of the following reacts at the fastest rate with potassium methoxide (KOCH 3) in methanol? A. fluorobenzene B. p-nitrofluorobenzene C. p-fluorotoluene D. p-bromofluorobenzene 5. Which of the following is the kinetic rate equation for the addition-elimination mechanism of nucleophilic aromatic substitution? A. rate = k[aryl halide] B. rate = k[nucleophile] C. rate = k[aryl halide][nucleophile] D. rate = k[aryl halide][nucleophile] 2 6. Which of the following is not a resonance form of the intermediate in the nucleophilic addition of hydroxide ion to para -fluoronitrobenzene? A. I B. II C. III D. IV 7. Which chlorine is most susceptible to nucleophilic substitution with NaOCH 3 in methanol? A. #1 B. #2 C. #1 and #2 are equally susceptible D. no substitution is possible 8. What is the product of the following reaction? A. N,N-dimethylaniline B. para -chloro-N,N-dimethylaniline C. phenyllithium (C 6H5Li) D. meta -chloro-N,N-dimethylaniline 9. Which one of the reagents readily reacts with bromobenzene without heating? A. -
Amide Activation: an Emerging Tool for Chemoselective Synthesis
Featuring work from the research group of Professor As featured in: Nuno Maulide, University of Vienna, Vienna, Austria Amide activation: an emerging tool for chemoselective synthesis Let them stand out of the crowd – Amide activation enables the chemoselective modification of a large variety of molecules while leaving many other functional groups untouched, making it attractive for the synthesis of sophisticated targets. This issue features a review on this emerging field and its application in total synthesis. See Nuno Maulide et al., Chem. Soc. Rev., 2018, 47, 7899. rsc.li/chem-soc-rev Registered charity number: 207890 Chem Soc Rev View Article Online REVIEW ARTICLE View Journal | View Issue Amide activation: an emerging tool for chemoselective synthesis Cite this: Chem. Soc. Rev., 2018, 47,7899 Daniel Kaiser, Adriano Bauer, Miran Lemmerer and Nuno Maulide * It is textbook knowledge that carboxamides benefit from increased stabilisation of the electrophilic carbonyl carbon when compared to other carbonyl and carboxyl derivatives. This results in a considerably reduced reactivity towards nucleophiles. Accordingly, a perception has been developed of amides as significantly less useful functional handles than their ester and acid chloride counterparts. Received 27th April 2018 However, a significant body of research on the selective activation of amides to achieve powerful DOI: 10.1039/c8cs00335a transformations under mild conditions has emerged over the past decades. This review article aims at placing electrophilic amide activation in both a historical context and in that of natural product rsc.li/chem-soc-rev synthesis, highlighting the synthetic applications and the potential of this approach. Creative Commons Attribution 3.0 Unported Licence. -
Leaving Group of Substrate Important
1 Recap... • Last two lectures we looked at substitution reactions • We found that there we two (extreme) mechanisms • SN1 H H H H H PhS R H R Br R SPh Br • And SN2 H H H H PhS + Br R Br PhS R • We looked at many of the factors that influenced which ..mechanism was operating... • Now we will turn our attention to elimination reactions 2 Elimination reactions NaOH NaOH or low Br H2O OH concentration • You have seen SN1 substitution • Reaction not sped up by altering nucleophile - it is not in RDS • In fact if we increase the amount / concentration of nucleophile we get the following... high O H + + + Br Br OH concentration H H • We observe an elimination reaction • Overall HBr is lost from the molecule • Isn't chemistry fun - these are the same conditions as substitution! • Next two lectures will look at the mechanisms for elimination • And how we control the nature of the reaction we observe... 3 Elimination, bimolecular E2 O H Br HO Br H H • E2 - Elimination bimolecular or 2nd order reaction • 2 molecules in the RDS or transition state H H H H H C H C H H C Br Br H O H C H O H C C H H H H C H C H H H • Two molecules in the rate determining step • So both the base and the substrate control the reaction • Both carbon skeleton & leaving group of substrate important In substitutions nucleophile acts as a nucleophile & attacks carbon In eliminations nucleophile acts as a base & attacks proton 4 Elimination, bimolecular E2: MO • Little confusing as need bonding & anti-bonding in same diagram! • Orbitals need to be parallel for maximum overlap -
Carbonyl Chemistry I: Additions to Carbonyl Groups
Paul Bracher Chem 30 – Section 4 Carbonyl Chemistry I: Additions to Carbonyl Groups Section Agenda 1) Lessons from Exam I, Course Feedbeck Mechanism on Web Site 2) Carbonyl Groups: Structure, Bonding, Molecular Orbitals, Geometry, Reactivity 3) Handout: In-Section Problem Set 4) Handout: In-Section Solution Set 5) Weekly Office Hours: Thursday, 3-4PM and 8-9PM, in Bauer Lobby Structure and Bonding · Carbonyl carbons are roughly sp2 hybridized Molecular Orbital Picture and planar with bond angles near 120 degrees bonding p orbital antibonding p* orbital · The C=O p bond is formed by the overlap of two unhybridized p orbitals. The bond is polarized due to the different electronegativities of carbon and oxygen. 105 o Resonance Model O O Bürgi-Dunitz angle · Another way to picture carbonyl groups is I II through an MO picture. Since the coefficient of · Resonance form II suggests that the carbon pO’s contribution is greater in pC=O, then atom is surrounded by less negative charge conservation of blending atomic orbitals into density than oxygen, rendering the carbon more molecular orbitals tells you that pC must be a electrophilic. larger contributor to the p*C=O orbital. · When reagents add to carbonyl double bonds, · The larger coefficient on oxygen in the filled the nucleophile attacks the carbon atom. orbital explains why the oxygen atom behaves as a Lewis base (it commonly complexes with · Due to the increased negative charge density Lewis acids). The larger coefficient on carbon on oxygen, Lewis acids (such as protons) in the empty antibonding orbital explains why it commonly complex with oxygen. -
Enzymatic Kinetic Isotope Effects from First-Principles Path Sampling
Article pubs.acs.org/JCTC Enzymatic Kinetic Isotope Effects from First-Principles Path Sampling Calculations Matthew J. Varga and Steven D. Schwartz* Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States *S Supporting Information ABSTRACT: In this study, we develop and test a method to determine the rate of particle transfer and kinetic isotope effects in enzymatic reactions, specifically yeast alcohol dehydrogenase (YADH), from first-principles. Transition path sampling (TPS) and normal mode centroid dynamics (CMD) are used to simulate these enzymatic reactions without knowledge of their reaction coordinates and with the inclusion of quantum effects, such as zero-point energy and tunneling, on the transferring particle. Though previous studies have used TPS to calculate reaction rate constants in various model and real systems, it has not been applied to a system as large as YADH. The calculated primary H/D kinetic isotope effect agrees with previously reported experimental results, within experimental error. The kinetic isotope effects calculated with this method correspond to the kinetic isotope effect of the transfer event itself. The results reported here show that the kinetic isotope effects calculated from first-principles, purely for barrier passage, can be used to predict experimental kinetic isotope effects in enzymatic systems. ■ INTRODUCTION staging method from the Chandler group.10 Another method that uses an energy gap formalism like QCP is a hybrid Particle transfer plays a crucial role in a breadth of enzymatic ff reactions, including hydride, hydrogen atom, electron, and quantum-classical approach of Hammes-Schi er and co-work- 1 ers, which models the transferring particle nucleus as a proton-coupled electron transfer reactions. -
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
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. -
S.T.E.T.Women's College, Mannargudi Semester Iii Ii M
S.T.E.T.WOMEN’S COLLEGE, MANNARGUDI SEMESTER III II M.Sc., CHEMISTRY ORGANIC CHEMISTRY - II – P16CH31 UNIT I Aliphatic nucleophilic substitution – mechanisms – SN1, SN2, SNi – ion-pair in SN1 mechanisms – neighbouring group participation, non-classical carbocations – substitutions at allylic and vinylic carbons. Reactivity – effect of structure, nucleophile, leaving group and stereochemical factors – correlation of structure with reactivity – solvent effects – rearrangements involving carbocations – Wagner-Meerwein and dienone-phenol rearrangements. Aromatic nucleophilic substitutions – SN1, SNAr, Benzyne mechanism – reactivity orientation – Ullmann, Sandmeyer and Chichibabin reaction – rearrangements involving nucleophilic substitution – Stevens – Sommelet Hauser and von-Richter rearrangements. NUCLEOPHILIC SUBSTITUTION Mechanism of Aliphatic Nucleophilic Substitution. Aliphatic nucleophilic substitution clearly involves the donation of a lone pair from the nucleophile to the tetrahedral, electrophilic carbon bonded to a halogen. For that reason, it attracts to nucleophile In organic chemistry and inorganic chemistry, nucleophilic substitution is a fundamental class of reactions in which a leaving group(nucleophile) is replaced by an electron rich compound(nucleophile). The whole molecular entity of which the electrophile and the leaving group are part is usually called the substrate. The nucleophile essentially attempts to replace the leaving group as the primary substituent in the reaction itself, as a part of another molecule. The most general form of the reaction may be given as the following: Nuc: + R-LG → R-Nuc + LG: The electron pair (:) from the nucleophile(Nuc) attacks the substrate (R-LG) forming a new 1 bond, while the leaving group (LG) departs with an electron pair. The principal product in this case is R-Nuc. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged.