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Topic: E1cB elimination reaction


  
  Elimination reaction - Wikipedia, the free encyclopedia
An elimination reaction is a type of organic reaction in which two substituents are removed from a molecule in either a one or two-step mechanism.
Either the unsaturation of the molecule increases (as in most organic elimination reactions) or the valence of an atom in the molecule decreases by two, a process known as reductive elimination.
An important class of elimination reactions are reactions involving alkyl halides or alkanes in general with good leaving groups, reacting with a Lewis base to form an alkene in the reverse of an addition reaction.
en.wikipedia.org /wiki/Elimination_reaction   (836 words)

  
 Elimination vs. substitution in the reaction of 3-stannyl esters and 3-stannyl nitriles with sodium tert-butoxide in ...   (Site not responding. Last check: 2007-10-10)
Elimination vs. substitution in the reaction of 3-stannyl esters and 3-stannyl nitriles with sodium tert-butoxide in DMSO
We assumed that with 3-stannyl nitriles as substrates, the substitution reaction is probably preferred over the elimination reaction because the elimination mechanism is slowed by the less acidic character of the hydrogen on C-2.
For example, the reaction of methyl (E)-3-phenyl propenoate (22) with triphenyl- or tributylstannyl lithium in THF and quenching the intermediate carbanions with methyl iodide led to the corresponding adducts 4 and 9 in 85% and 65% yield, respectively.
www.arkat-usa.org /ark/journal/2003/I10_Rossi-Ruveda/RR-887C/887C.asp   (1914 words)

  
 Ingold Eliminations   (Site not responding. Last check: 2007-10-10)
Eliminations can be described as the loss of two groups from two different centres with concomitant formation of an unsaturation in the molecule (double or triple bonds) or the formation of a new ring.
This is an elimination reaction in which all the components are in the same chemical species.
This is taken as the second (and final) step of a reaction in which a species formed by the loss of a proton (and usually negatively charged) eliminates a chemical group from itself, forming a double bond or cyclic compound.
www-mitchell.ch.cam.ac.uk /macie/Annotation/eliminations.html   (407 words)

  
 Journal of the Chemical Society B: Physical Organic articles
The kinetics and mechanism of the reaction of cyclopentadienylidenetriarylphosphoranes with cyano-olefins.
Kinetic data on the base-catalysed elimination of hydrogen cyanide from 2-(1-phenyl-1,2,2-tricyanoethyl)-cyclopentadienylidenetriphenylphosphorane in benzene, benzene—acetonitrile, or acetonitrile solvent are reported.
Activation parameters are reported for the reaction with triethylamine and the kinetic results are interpreted in terms of a E1CB reaction involving fast proton loss to form an ion-pair, followed by rate-determining elimination of cyanide ion from the ion-pair intermediate.
www.rsc.org /Publishing/Journals/J2/article.asp?doi=J29710000220   (183 words)

  
 New Page 1
E1-Elimination reaction occurs when the Leaving group departs first to produce a reasonably stable cation; the proton is then lost to form the alkene.
The E1 and E1cB should be considered the two extremes of a spectrum of elimination mechanism in which E2 is in the center.
E1cB example (ewg makes C-H acidic; anion needed to kick out poor L) Hetero E1cB example: Protons on heteroatom are usually acidic due electronegativity of the heteroatom, so the first step is deprotonation, and can occur with a rather weak base.
www.uiowa.edu /intlinet/unijos/departments/pharmacology/Afolabi/ELIMINATION.htm   (482 words)

  
 Chapter 5 : Elimination   (Site not responding. Last check: 2007-10-10)
Elimination reactions are important as a method for the preparation of alkenes.
The term "elimination" describes the fact that a small molecule is lost during the process.
In many cases the elimination reaction may proceed to alkenes that are constitutional isomers with one formed in excess of the other.
library.tedankara.k12.tr /carey/ch5-4.html   (162 words)

  
 M
An acceptable mechanism of a specified reaction (and there may be a number of such alternative mechanisms not excluded by the evidence) must be consistent with the reaction stoichiometry, the rate law, and with all other available experimental data, such as the stereochemical course of the reaction.
The effect (on reaction rates, ionization equilibria, etc.) attributed to a substituent due to overlap of its p or pi orbitals with the p or pi orbitals of the rest of the molecular entity.
(For reactions in solution this number is always taken to exclude molecular entities that form part of the medium and which are involved solely by virtue of their solvation of solutes.) A reaction with a molecularity of one is called "unimolecular", one with a molecularity of two "bimolecular" and of three "termolecular".
www.chem.qmul.ac.uk /iupac/gtpoc/M.html   (2808 words)

  
 Details of Elimination Mechanisms
As previously mentioned, actual examples of this reaction type are very rare (the example given in the previous page is hypothetical!).
Two known examples are; elimination of HCN from cyanohydrins (shown below) and HF from Cl As was mentioned in the nucleophilic substitution section, species which are basic are often also nucleophilic.
In E2, the RDS involves removal of the proton at the same time as the leaving group departs - therefore both base strength and leaving group ability are important for the rate of E2.
www.everyscience.com /Chemistry/Organic/Eliminations/b.1205.php   (525 words)

  
 Current research in CMM
Reactions that take place in a concerted fashion cannot easily be described by a single reaction coordinate.
Examples are found in enzymatic catalysis and in reactions in solution for which the interaction between solvent and reactions is strong.
The fundamental elimination (E2) reaction is an intrinsically multi-dimensional problem, as it is a priori not possible to predict whether the leaving group leaves first (E1-like mechanism), the attacking group attacks first (E1cb-like mechanism) or the reaction is truly concerted.
www.cmm.upenn.edu /research/current/FE/main.html   (646 words)

  
 esm_bruice_organic_4|Elimination Reactions of Alkyl Halides;|Current Topics
The elimination of alkyl halides to form alkenes in the presence of a strong base can occur by one of two different mechanisms, the E2 and E1cB (regular E1 only happens when a weak base is present).
In the E1cB mechanism the base abstracts the proton to give an intermediate carbanion, which then goes on to eliminate the leaving group.
This is done by examining the rate of reaction with two different isotopes of the same element.
wps.prenhall.com /esm_bruice_organic_4/0,7842,742637-,00.utf8.html   (370 words)

  
 [No title]   (Site not responding. Last check: 2007-10-10)
Calculations on fluorine-substituted carbanions permit the assessment of the role of negative ion hyperconjugation, and studies of elimination reactions of fluorohydrocarbons address the borderline between E1cb and E2 reaction pathways.
Among the superficially simplest chemical reactions are those involving the transfer of a single positively-charged hydrogen atom (a proton) from one place in a molecule to another.
His studies also extend to the analysis of reactions initiated by the loss of a proton, focusing in particular on whether the overall reaction path for such reactions involves simultaneous elimination of an additional group or if such elimination occurs subsequently to the initial proton loss.
www.cs.utexas.edu /users/yguan/NSFAbstracts/Abstracts/MPS/CHE.MPS.a9706242.txt   (279 words)

  
 Curly arrows; elimination reactions
This is elimination, or removal of HX from an alkyl halide.
It can be seen that elimination could well take place under the same reaction conditions as substitution, if there is a hydrogen atom b to the halogen.
In the elimination of 2-bromo-2-methylbutane with a strong, unhindered base such as sodium ethoxide in ethanol, the two alkene products are formed in the ratio 7:3 respectively.
www.abdn.ac.uk /curly-arrows/org/e1.htm   (594 words)

  
 C6 - complement component 6
S configuration, respectively, of the methyl group indicate that (a) the oxidoreductase reaction involves transfer of H-4 to C-6, (b) the transfer is predominantly intramolecular, and (c) the transfer is stereospecific, H-4 replacing the C-6 hydroxyl group with inversion of configuration.
The reaction mechanisms for these types of dehydratases are quite complicated with proton abstraction from the sugar 4'-hydroxyl group and hydride transfer to NAD+, proton abstraction at C-5, and elimination of the hydroxyl group at C-6 of the sugar, and finally return of a proton to C-5 and a hydride from NADH to C-6.
Thirdly, it is reaction-non-selective, forming simultaneously isomaltulose and a second hitherto uncharacterized alpha-(1----1)-linked disaccharide (trehalulose), by hydrolysis of sucrose followed by reaction of glucose with the C-6 and C-1 positions of the fructofuranose respectively.
www.ihop-net.org /UniPub/iHOP/gg/86832.html   (7222 words)

  
 Potential Energy Diagrams for Reactions
This is related to the likelihood that the molecules have the proper orientation for the reaction.
The overall rate of the reaction is approximately determined by the reaction step that has the highest energy TS.
It is possible that the reaction could proceed by the E1 or E1cb mechanisms, but it turns out that the TS for the RDS for those mechanisms with these reactants is higher in energy than the TS for the E2 mechanism.
www.ipfw.edu /chem/261/261Fall03/261nt02q.htm   (1033 words)

  
 Overview of Elimination Mechanisms   (Site not responding. Last check: 2007-10-10)
The most common type of elimination is a so-called 1, 2- elimination where the two species are removed from adjacent carbon atoms, usually causing the formation of a multiple bond.
This is very similar to E1, except this time the C-H bond is broken first rather than the C-X bond (this implies that the RDS is again the loss of X, not the initial loss of H).
In fact this reaction mechanism is the least favourable of the three, and for reactions such as the example above, the energy of activation for E2 is lower than that for E1 This surprisingly stands for Elimination with a bimolecular RDS.
www.everyscience.com /Chemistry/Organic/Eliminations/a.1204.php   (282 words)

  
 Characteristics of E2 Mechanism
In the following reactions, it is possible for an H to be anti to give each of the products without difficulty.
Elimination reactions are often not strictly E1 or E2.
The E1cB mechanism occurs when L is a poor leaving group and the structure of the substrate has an acidic H. It will be acidic because certain structural features are present that stabilize the conjugate base anion.
www.ipfw.edu /chem/261/261Fall03/261nt02p.htm   (685 words)

  
 Bugalicia Database /All Locations
We discovered the existence of three simultaneous reaction paths: spontaneous decomposition of the neutral form of the N-nitrosoguanidine, decomposition of the monoanion, and decomposition through the form of the dianion.
This behavior is consistent with the mechanism E1cB, whereby the stabler the negative charge, the slower the elimination reaction.
When dealing with the case of the elimination through the neutral form we observe that the reaction rate increases together with the capacity of stabilization of the positive charge on the nitrogen atom adjacent to the imino group.
biblio.cesga.es:81 /record=b1049914*gag   (294 words)

  
 Organic II - Exams   (Site not responding. Last check: 2007-10-10)
Each question is worth 15 points for atotal of 105 points.
(ii) The following elimination reaction is occuring by the E
(i) propose a mechanism for the reaction :
www.cs.stedwards.edu /chem/Chemistry/CHEM30/spring/Exam1/Exam1.html   (200 words)

  
 Terminology   (Site not responding. Last check: 2007-10-10)
E1cb: Elimination, unimolecular, conjugate base; elimination occurring from the conjugate base of the substrate
Hoffman elimination: an elimination process in which the major product is the less substituted, and hence less stable alkene; normally requires either a bulky base or a poor leaving group, or both
Leaving group: the group that is displaced in a substitution or elimination reaction
chemistry.umeche.maine.edu /CHY251/Terms6.html   (413 words)

  
 VOH // Questions and Answers for 10D-2
This is the reaction of a carbonyl compound with an alkyl lithium reagent (in the first step, the alkyl lithium reagent is made from the alkyl halide and lithium.
So for reactions of carbonyl compounds in the presence of acid, you should ALWAYS show protonation of the carbonyl oxygen as the first step (this holds true for acetal formation, ketal formation, and attack of a carbonyl by ANY nucleophile, as long as there is acid present).
Can't we consider such a reaction as the combination of two concerted reations (P->I and I->R) and so considering the reaction I->R the activation energey would be from the energey of I to the energy of the transition state (the 2nd trans.
voh.chem.ucla.edu /vohtar/spring98/10D-2/qa.html   (17443 words)

  
 ROA Research Descriptions
His current research interests are in the area of mass spectrometry, including ion/molecule reactions, instrumental development of ICR mass spectrometry and electrochemical assisted FAB mass spectrometry, and in solvation effects on chemical reactivity.
Areas of recent effort include the E2/E1cb elimination reaction, and the effect that hydrogen bonding of a single molecule of solvent to the ion has on its reactivity.
A particular region of the DNA is then amplified using the polymerase chain reaction and then analyzed to identify a genetic disease in the patient, a bacterial contaminant in tomato soup, an organism suitable for bioremediation, or a suspect in a crime.
www.chem.utk.edu /~acsec/resdescrip.htm   (7131 words)

  
 [No title]   (Site not responding. Last check: 2007-10-10)
A reaction that gives the less substituted alkene as the major product is said to follow Hoffman orientation.
A reaction that results with the formation of most stable, highly substituted alkene is said to follow Zaitsev orientation.
Vinyl halides can be prepared by elimination of HX from geminal or vicinal dihalides.
facstaff.uww.edu /kumpatyh/Eliassig.html   (320 words)

  
 Chem 223
the reaction of the tosylate prevents elimination reactions from
Give the electron flow mechanism for this reaction and explain why the stereochemistry is not retained.
2 reaction is very sensitive to the polarity of the solvent.
faculty.maryvillecollege.edu /bunde/C223EXAM.htm   (850 words)

  
 The Structure of Chondroitin B Lyase Complexed with Glycosaminoglycan Oligosaccharides Unravels a Calcium-dependent ...
by the concentration of enzyme in the reaction.
In the E2 reaction the proton abstraction and leaving-group
The proton abstraction is performed by Lys-250, whereas the glycosidic bond is protonated by Arg-271, followed by leaving group elimination.
www.jbc.org /cgi/content/full/279/31/32882   (7482 words)

  
 Chlorination of Sulfides: A theoretical study   (Site not responding. Last check: 2007-10-10)
In the case we are studying (reaction of dimethylsulfide with molecular chlorine, it is clear that dissociation is not an open channel, due to an exceedingly high barrier (more than 30 kcal/mol at the best level of calculation).
The meaning of this transition state structure is that we are in the presence of a typical E1 elimination, not an E2 as advanced by Wilson and Huang [6] and not an E1cb (although it does not rule out these mechanisms when other substituents are present).
This view is also supported by the size of the charge on the Cl atom (0.61 electrons at the best level of calculation) and the More-O'Ferrall-Albery-Jencks diagrams.
bilbo.edu.uy /dequifim/Proyectos/Denis/Sulfuranos/Nov-18.html   (193 words)

  
 Carleton College: Chemistry   (Site not responding. Last check: 2007-10-10)
Steve continues to study the reaction of nitric oxide with iron porphyrins immobilized on carbon electrode surfaces.
Angela Fleischhacker ('02) and Laura Phillips ('03) are studying the stereochemistry and kinetics of the elimination reaction of a stereospecifically-deuterated *-phenoxythioester, necessary to compare the thioester to a ketone substrate in the same solvent mixture.
In an attempt to discover if there is a unifying theme in enolate protonation and E1cB-like elimination reactions, Jane Coughlin ('03) and Min Chong ('04) are comparing the stereochemistry of H/D exchange of -phenoxybutyrate esters, thioesters, and ketones to the stereochemistry of the 1,2-elimination reactions on analogous substrates.
www.acad.carleton.edu /curricular/chem/annualreports_past/annualreport2001_2002/arfaculty.htm   (5684 words)

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