assume a carbocation to have sp sup 3 sup hybridization with an empty sp sup 3 sup orbital giving positive charge. However, the reactivity of a carbocation more closely resembles Orbital hybridisation sp sup 2 sup hybridization with a trigonal planar molecular geometry. Example Methyl carbocation CH sub 3 sub sup sup Definitions A carbocation was previously often called a carbonium ion but questions ... carbonium ion http www.iupac.org goldbook C00839.pdf PDF ref In present day chemistry a carbocation ... . The chemical establishment was reluctant to accept the notion of a carbocation and for a long time ... spectrum of a stable carbocation in solution was published by Doering et al. ref The 1,1,2,3,4,5,6 heptamethylbenzenonium ... Andrew Olah Olah directly observed the tert butyl carbocation by nuclear magnetic resonance as a stable ... III , secondary II , and primary I alkyl carbocations In organic chemistry , a carbocation is often ... carbon. Stability of the carbocation increases with the number of alkyl groups bonded to the charge ... carbocation would be formed. An exception to this occurs when there is a carbon carbon double ... compounds. For example, when 3 pentanol is heated with aqueous HCl, the initially formed 3 pentyl carbocation ... form of the 2 butyl carbocation is essentially 2 butene with a proton directly above the centre ... fr Carbocation it Carbocatione he nl Carbokation ja pl Karbokation pt Carboc tion ru simple Carbocation sr Karbokatjon fi Karbokationi sv Karbokatjon uk ... more details
File Methyl cation.svg thumb 120px Methyl cation A carbenium ion is a carbocation of the trivalent and classical type R sub 3 sub C sup sup . It is one of two types of carbocation, the other being a carbonium ion . In older literature a carbocation of the type R sub 3 sub C sup sup may still be referred to as a carbonium ion, a term that is used now for five coordinate carbon atoms. ref IUPAC Gold Book http goldbook.iupac.org C00839.html carbonium ion ref The current definitions were proposed by the chemist George Andrew Olah in 1972. ref Stable carbocations. CXVIII. General concept and structure of carbocations based on differentiation of trivalent classical carbenium ions from three center bound penta of tetracoordinated nonclassical carbonium ions. Role of carbocations in electrophilic reactions George Andrew Olah J. Am. Chem. Soc. 1972 94 3 808 820. ref and are now widely accepted References reflist Category cations Category Reactive intermediates de Carbeniumion fr Carb nium ion nl Carbeniumion sr Karbenijum jon ... more details
steps Formation of a tert butyl carbocation by separation of a leaving group a bromide anion from ... SN1 mechanism dissociation to carbocation nucleophile Nucleophilic attack the carbocation reacts with the nucleophile. If the nucleophile is a neutral molecule ... carbocation nucleophile.svg Recombination of carbocation with nucleophile Deprotonation Removal of a proton ... carbon increase the rate of carbocation formation because of the relief of steric strain that occurs. The resultant carbocation is also stabilized by both Inductive effect inductive stabilization and hyperconjugation ... will increase the rate of carbocation formation. The S sub N sub 1 mechanism therefore dominates in reactions ... The carbocation intermediate formed in the reaction s rate limiting step is an sp sup 2 sup ... stereoisomer can be observed, as the leaving group can remain in proximity to the carbocation intermediate ... Two common side reactions are elimination reaction s and Rearrangement reaction carbocation rearrangement .... Finally, if the carbocation intermediate can rearrange to a more stable carbocation, it will give a product derived from the more stable carbocation rather than the simple substitution product. Solvent effects Since the S sub N sub 1 reaction involves formation of an unstable carbocation intermediate ... more details
Migratory aptitude is the relative ability of a migrating group to migrate in a rearrangement reaction . This can be affected by the leaving group whichever gives a more stable carbocation and the group that stays whichever can stabilize the positive charge best .It also depends upon the electron density of the migrating group i.e. Hydride tert alkyl sec alkyl phenyl primary alkyl methyl H organic chem stub Category Organic chemistry ... more details
The Criegee rearrangement is a rearrangement reaction named after Rudolf Criegee . In this organic reaction a tertiary alcohol is cleaved in an organic oxidation by a peroxyacid to a ketone . The acid used is often p nitroperoxybenzoic acid because the p nitrobenzoic acid anion is a good leaving group. Image Criegee rearrangement.gif center The reaction mechanism has similarities with the Baeyer Villiger oxidation where the intermediate hydroxyperacid is called a Criegee intermediate . The per acid forms a per ester with the alcohol group. One alkyl substituent migrates from carbon to the adjacent oxygen atom, replacing the carboxylic acid leaving behind a carbocation . A hydrolysis step forms the ketone together with the alcohol. The order of migrationary aptitude is a tert butyl as the best substituent followed by isopropyl then ethyl group ethyl and then the methyl group. From this it is inferred that the migrating group carries a partial positive charge in the transition state leading to the carbocation. The consecutive Criegee Rearrangement is carried out in an acidic environment and an ester forms from the carbocation. This opens the way to multiple O insertion reactions, eventually leading to the ortho ester . In the Criegee reaction a Vicinal chemistry vicinal diol is cleaved by lead tetraacetate to the corresponding ketones and acetic acid . References Criegee, R., Chem. Ber. 1944 , 77, 722 Criegee, R. Kaspar, R., Ann. Chem. 1948 , 560, 127 Trifuoroperacetic acid in consecutive Criegee rearrangement and carboxonium ions generation Pavel A. Krasutsky and Igor V. Kolomitsyn Arkivoc 2005 NZ 1517J pp 151 171 http arkat usa.org ark journal 2005 I04 Zefirov 1517 1517.asp Article open access publishing open access publication Category Rearrangement reactions Category Name reactions fr R arrangement de Criegee id Penataan ulang Criegee ru zh Criegee ... more details
oxonium intermediate 4 and a Prins reaction follows to 5 whereby the transient carbocation is captured ... carbocation intermediate. Secondary carbonations are inherently unstable, however the silicon effect from the silicon atom stabilizes the carbocation. Silicon is able to donate into an empty p orbital ... more details
charge on the other carbon, forming a carbocation intermediate. The more substituted the carbocation ... stable carbocation will still be formed to some degree, and will proceed to form the minor product ... Mechanisms that avoid the carbocation intermediate may react through other mechanisms that are regioselective ... carbocation A , which rearranges to the seemingly less attractive secondary carbocation B ... more details
A Wagner Meerwein rearrangement is a class of carbocation 1,2 rearrangement rearrangement reaction reaction s in which a hydrogen , alkyl or aryl group migrates from one carbon to a neighboring carbon. ref Wagner, G. J. Russ. Phys. Chem. Soc. 1899 , 31 , 690. ref ref cite journal author Hans Meerwein title ber den Reaktionsmechanismus der Umwandlung von Borneol in Camphen Dritte Mitteilung ber Pinakolinumlagerungen. journal Justus Liebig s Annalen der Chemie year 1914 volume 405 pages 129 175 doi 10.1002 jlac.19144050202 ref Several reviews have been published. ref Popp, F. D. McEwen, W. E. Chem. Rev. 1958 , 58 , 375. Review ref ref Cargill, R. L. et al. Accts. Chem. Res. 1974 , 7 , 106 113. Review ref ref George Olah Olah, G. A. Accts. Chem. Res. 1976 , 9 , 41. Review ref ref Hogeveen, H. Van Krutchten, E. M. G. A. Top. Curr. Chem. 1979 , 80 , 89 124. Review ref ref Hanson, J. R. Comp. Org. Syn. 1991 , 3 , 705 719. Review ref The rearrangement was first discovered in bicyclic terpene s for example the conversion of isoborneol to camphene ref JerryMarch ref Image Isoborneol2CampheneConversion.svg Isoborneol Camphene Conversion The story of the rearrangement reveals that many scientists were puzzled with this and related reactions and its close relationship to the discovery of carbocations as intermediates. ref Birladeanu, L. J. Chem. Ed. 2000 , 77 , 858 863. ref In a simple demonstration reaction of 1,4 dimethoxybenzene with either 2 methyl 2 butanol or 3 methyl 2 butanol in sulfuric acid and acetic acid yields the same disubstituted product, ref Carbocation Rearrangement in an Electrophilic Aromatic Substitution Discovery Laboratory Victoria Polito, Christian S. Hamann and Ian J. Rhile J. Chem. Educ., 2010, 87 9 , pp 969 970 DOI 10.1021 ed9000238 ref the latter via a hydride shift of the cationic intermediate File Carbocation rearrangement polito 2010.svg Carbocation rearrangement Polito 2010 Currently, there are works relating to the use of skeletal rearrangement ... more details
In chemistry the reactivity selectivity principle or RSP states that a more reactive chemical compound or reactive intermediate is less selective in chemical reactions. In this context selectivity represents the ratio of reaction rate s. This principle was generally accepted until the 1970s when too many exceptions started to appear. The principle is now considered obsolete ref Minireview The Reactivity Selectivity Principle An Imperishable Myth in Organic Chemistry Herbert Mayr, Armin R. Ofial Angewandte Chemie International Edition Volume 45, Issue 12 , Pages 1844 1854 http dx.doi.org 10.1002 anie.200503273 Abstract ref . A classic example of perceived RSP found in older organic textbooks concerns the free radical halogenation of simple alkane s. Whereas the relatively unreactive bromine reacts with 2 methylbutane predominantly to 2 bromo 2 methylbutane, the reaction with much more reactive chlorine results in a mixture of all four regioisomer s. Another example of RSP can be found in the selectivity of the reaction of certain carbocation s with azide s and water . The very stable triphenylmethyl carbocation derived from solvolysis of the corresponding triphenylmethyl chloride reacts 100 times faster with the azide anion than with water. When the carbocation is the very reactive tertiary adamantane carbocation as judged from diminished reaction rate rate of solvolysis this difference is only a factor of 10. Constant or inverse relationships are just as frequent. For example a group of 3 and 4 substituted pyridine s in their reactivity quantified by their pKa show the same selectivity in their reactions with a group of alkylating reagents. The reason for the early success of RSP was that the experiments involved very reactive intermediates with reactivities close to kinetic diffusion control and as a result the more reactive intermediate appeared to react slower with the faster substrate. General relationships between reactivity and selectivity in chemical reaction ... more details
Summary State 1 The Zn ion is in its tri coordinated resting state. State 2 The third histidine moves out of coordination from the Zn ion 47 , and the quasipenta coordinated Zn ion influences the O of the glutamate side chain to act as an aggressive base, which transfers an electron to the carbocation on the scissile bond polarized by the Zn ion , creating an oxyanion transition state. State 3 The tetrahedral intermediate, the electron transfer leads the electron to the amide nitrogen given the high energy state of the negatively charged oxygen, breaking the scissile bond. State 4 The anhydride bond breaks, given the inclination of the second glutamate oxygen to the Zn ion, and a water molecule protonates the leaving group and hydroxylates the new C terminus. State 5 The Zn ion goes back in coordination with the third histidine, and the cleaved substrate detaches. Licensing cc by 3.0 Category Unclassified Chemical Structures Orphan image ... more details
In chemistry the intimate ion pair concept introduced by Saul Winstein describes the interactions between a cation , anion and surrounding solvent molecules . ref cite journal last Winstein first S. coauthors Clippinger, E. Fainberg, A. H. Heck, R. RobinsonG. C. year 1956 title Salt Effects and Ion Pairs in Solvolysis and Related Reactions. III.1 Common Ion Rate Depression and Exchange of Anions during Acetolysis volume 78 issue 2 pages 328 335 issn DOI 10.1021 ja01583a022 doi 10.1021 ja01583a022 journal Journal of the American Chemical Society ref In ordinary aqueous solutions of inorganic salts an ion is completely solvated and shielded from the counterion. In less Chemical polarity polar solvents two ions can still be connected to some extent. In a tight or intimate or contact ion pair there are no solvent molecules between the two ions. When solvation increases, ionic bonding decreases and a loose or solvent shared ion pair results. The ion pair concept explains stereochemistry in solvolysis . The concept of intimate ion pairs is used to explain the slight tendency for inversion of stereochemistry during an SN1 reaction . It is proposed that solvent or other ions in solution may assist in the removal of a leaving group to form a carbocation which reacts in an SN1 fashion similarly, the leaving group may associate loosely with the cationic intermediate. The association of solvent or an ion with the leaving group effectively blocks one side of the incipient carbocation, while allowing the backside to be attacked by a nucleophile. This leads to a slight excess of the product with inverted stereochemistry, whereas a purely SN1 reaction should lead to a racemic product. See also Ion association References reflist Category Chemical bonding ... more details
The semipinacol rearrangement is a rearrangement reaction in organic chemistry involving a heterosubstituted alcohol of the type R sub 1 sub R sub 2 sub HO C C X R sub 3 sub R sub 4 sub . The hetero substituent can be a halogen Cl, Br, I , a tosylate , a mesylate or a thioxy group. This reaction proceeds by removal of the leaving group X forming a carbocation as electron deficient center. One of the adjacent alkyl groups then migrates to the positive carbon in a 1,2 shift . Simultaneously with the shift, a pi bond forms from the oxygen to carbon, assisting in driving the migrating group off its position. The result is a ketone or aldehyde . ref Strategic applications of named reactions in organic synthesis background and detailed mechanisms L szl K rti, Barbara Czak 2005 ref . In another definition all semipinacol rearrangements share a common reactive species in which an electrophilic carbon center, including but not limited to carbocations, is vicinal to an oxygen containing carbon and can drive the 1,2 migration of a C C or C H bond to terminate the process, generating a carbonyl group ref Semipinacol Rearrangement in Natural Product Synthesis Zhen Lei Song, Chun An Fan, Yong Qiang Tu Chemical Reviews DOI 10.1021 cr200055g ref . The rearrangement reaction can be classified into 4 types. Type 1 concerns all 2 heterosubstitted alcohols. Substrates in type 2 rearrangements are allyl alcohol s. The carbocation is formed by electrophilic addition to the alkene group with electrophiles such as halonium ion s, Br nsted acid s and Lewis acid s. In type 3 the substrates are epoxide s, notably 2,3 epoxy alcohols and type 4 concerns the reactions of acyloin alpha hydroxyketones and alpha hydroxy imines. Reactions of type 4 are also called acyloin rearrangement s. While similar to the pinacol rearrangement , the semipinacol rearrangement differs from the pinacol rearrangement in that the cation is not formed from a vicinal 1,2 diol. With diazoalcohols the reaction is known ... more details
The pinacol rearrangement or pinacol pinacolone rearrangement is a method for converting a diol 1,2 diol to a carbonyl compound in organic chemistry . This 1,2 rearrangement takes place under acidic conditions. The name of the reaction comes from the rearrangement of pinacol to pinacolone . Image 800px Pinacol rearragement.png 500px Pinacol rearrangement This reaction was first described by Wilhelm Rudolph Fittig in 1860. ref name Fittig1860 cite journal title ber einige Derivate des Acetons author Wilhelm Rudolph Fittig journal Annalen der Chemie und Pharmacie volume 114 issue 1 pages 54 63 year 1860 url doi 10.1002 jlac.18601140107 ref An overview of mechanism discussion In the course of this organic reaction , protonation of one of the OH groups occurs and a carbocation is formed. If both the OH groups are not alike, then the one which yields a more stable carbocation participates in the reaction. Subsequently, an alkyl group from the adjacent carbon migrates to the carbocation center. The driving force for this rearrangement step is believed to be the relative stability of the resultant oxonium ion, which has complete octet configuration at all centers as opposed to the preceding carbocation . The migration of alkyl groups in this reaction occurs in accordance with their usual migratory aptitude , i.e. hydride Ph tertiary secondary methyl . Stereochemistry of the rearrangement In cyclic systems, the reaction presents more features of interest. In these reactions, the stereochemistry of the diol plays a crucial role in deciding the major product. An alkyl group which is situated trans to the leaving OH group alone may migrate. If otherwise, ring expansion occurs, i.e. the ring carbon itself migrates to the carbocation centre. This reveals another interesting feature of the reaction, viz. that it is largely concerted. There appears to be a connection between the migration origin and migration terminus throughout the reaction. Moreover, if the migrating alkyl grou ... more details
is enhanced due to the formation of a stable carbocation upon breaking of the C O bond. In a highly ... to form the highly stable triphenylmethylium carbocation. Synthesis The preparation of triphenylmethanol ... more details
Frank Clifford Whitmore 1887 1947 , nicknamed Rocky , was a prominent chemist who submitted significant evidence for the existence of carbocation mechanisms in organic chemistry. He was born in 1887 in the town of North Attleborough , Massachusetts . Academic career Whitmore earned both his bachelor s degree 1911 and Ph.D. 1914 from Harvard University , where his Ph.D. advisor was Charles Loring Jackson . Several prominent contemporaries of Whitmore at Harvard were E.K. Bolton , Farrington Daniels , Roger Adams , James B. Sumner and James Bryant Conant . After graduating from Harvard he became a professor and taught at the University of Minnesota , Northwestern University , and The Pennsylvania State University . At Penn State, Whitmore served as the Dean of the School of Chemistry and Physics from 1929 1947, succeeding his former Harvard colleague Gerald Wendt in the position. He hired several prominent scientists as faculty members, including Russell Marker and Merrell Fenske. Research and publications While at the Pennsylvania State University Whitmore did his research on carbocation s. The field of organic chemistry was struggling to explain how a compound with a double bonded carbon, an alkene , reacts with a halide compound. Whitmore worked on the findings of others and generalized the concept of molecules with a positively charged carbon atom, a carbocation , as an intermediate step in the addition of a halogen element. Whitmore would go on to publish his findings in a paper titled The Common Basis of Intramolecular Rearrangements. ref Journal of the American Chemical Society , 1932, Volume 54 p. 3274 3283 ref They were controversial at the time because many chemists, notably well known chemist Roger Adams , a critic of Whitmore s, believed that a molecule like a carbocation would never be stable enough to exist. Nevertheless, Whitmore published these findings which today are accepted as the most logical explanation for the reactions in question. In 1937, Whi ... more details
Crystal violet lactone CVL is a leuco dye , a lactone derivate of crystal violet 10B. In pure state it is a slightly yellowish crystalline powder, soluble in nonpolar or slightly polar organic solvent s. The central carbon in the leuco dye leuco form is in a tetraedric configuration, forming four covalent bond s. In acidic environment the lactone ring is broken, the central carbon loses one valence and becomes a resonance stabilized carbocation although it might be better to draw the resonance structure with the cation on nitrogen , this planar carbon interconnecting the systems of the aromatic ring s and the amino functional group s to form one large conjugated system acting as a chromophore with strong absorption in visible spectrum , giving this compound its distinctive color. Image Crystal violet lactone reaction.png thumb 300px Transformation between leuco and colored form of crystal violet lactone, halochromism It was the first dye used in carbonless copy paper s, and it is still widely used in this application. It is also the leuco dye component in some thermochromic dyes, e.g. in the Hypercolor line of clothing. One of its novel uses is a fuel dyes security marker for fuels . It may cause allergic contact dermatitis in people handling the carbonless copy paper. References references Unreferenced date October 2007 Category Triarylmethane dyes Category Anilines ... more details
in the following alkyl benzenesulfonate the alkene is able to delocalization delocalise the carbocation ... An aromatic ring can assist in the formation of a carbocation ic intermediate called a phenonium ... more details
for polar effect in genetics polar effect genetics The Polar effect or electronic effect in chemistry is the effect exerted by a substituent on modifying electrostatic force s operating on a nearby chemical reaction reaction center. The main contributors to the polar effect are the inductive effect , mesomeric effect and the through space electronic substituent field effect field effect . An electron withdrawing group or EWG draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion with the presence of the substituent that has a stabilizing effect. Examples of electron withdrawing groups are halogen s F, Cl nitrile s CN carbonyl s CO Nitro compound nitro groups NO2. An electron releasing group or ERG otherwise called electron donating groups or EDG releases electrons into a reaction center and as such stabilizes electron deficient carbocation s. Examples of electron releasing groups are alkyl groups alcohol groups amine amino groups. The total substituent effect is the combination of the polar effect and the combined steric effect s. In electrophilic aromatic substitution and nucleophilic aromatic substitution substituents are divided into activating group s and deactivating group s where the direction of activation or deactivation is also taken into account. External links Polar effect definition by the IUPAC Gold Book http www.iupac.org goldbook P04709.pdf Link Category Physical organic chemistry nl Polarisatie effect ja pt Efeito polar ... more details
, giving rise to a carbocation which eventually becomes a tertiary alcohol after a water molecule ... carbocation . center center The chloride ion attacks the carbocation, forming em t em BuCl. center ... butanol is a tertiary alcohol, the relative stability of the tert butyl carbocation in the Step 2 ... more details
engages in an electrophilic addition with the alkene to the carbocation ic intermediate 4 . Exactly ... are in blue capture of the carbocation by water or any suitable nucleophile through 5 to the 1,3 ... capture of the carbocation by additional carbonyl reactant. In this mode the positive charge is dispersed ... reactions and when the carbocation is very stable the reaction takes a shortcut to the oxetane ... recombining with the carbocation. The cyclization of certain allyl pulegones in scheme ... of a trichlorotitanium alkoxide making possible an easy delivery of chlorine to the carbocation ... more details
acid. With organic acetates such as cumyl acetate the initiating species is the carbocation R ... is again a proton and the carbocation is stabilized by the triodide ion. Polymerizations with Diethylaluminium ... to form the tert butyl carbocation as the electrophile. Efficient initiators that resemble the monomer ... are believed to stabilize the carbocation. The addition of salt for example a ammonium salt tetraalkylammonium ... polymerization Joseph Jagur Grodzinski, ed. 2005 ref The propagating species is not a carbocation ... more details
Sharpless Oxidation With a stereogenic center next to the carbocation the substitution can ... furan can approach the carbocation formed from the least shielded side away from the bulky t butyl ... Stereoselective reaction with carbocation Bach 2005 See also Stereospecific Dynamic stereochemistry ... more details
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