Water electrophilic addition

We can extend the general principles of electrophilic addition to acid catalyzed hydration In the first step of the mechanism shown m Figure 6 9 proton transfer to 2 methylpropene forms tert butyl cation This is followed m step 2 by reaction of the car bocation with a molecule of water acting as a nucleophile The aUcyloxomum ion formed m this step is simply the conjugate acid of tert butyl alcohol Deprotonation of the alkyl oxonium ion m step 3 yields the alcohol and regenerates the acid catalyst... 

Electrophilic Addition. Electrophilic reagents attack the electron-deficient bond of maleic anhydride (25). Typical addition reagents include halogens, hydrohaHc acids, and water. 

Yet another example of an electrophilic addition is the reaction of alkenes with the hypohalous acids HO—Cl or HO-Br to yield 1,2-halo alcohols, called halohydrins. Halohydrin formation doesn t take place by direct reaction of an alkene with HOBr or HOC1, however. Rather, the addition is done indirectly by reaction of the alkene with either Br2 or Cl2 in the presence of water. 

Acid-catalyzed hydration of isolated double bonds is also uncommon in biological pathways. More frequently, biological hydrations require that the double bond be adjacent to a carbonyl group for reaction to proceed. Fumarate, for instance, is hydrated to give malate as one step in the citric acid cycle of food metabolism. Note that the requirement for an adjacent carbonyl group in the addition of water is the same as that we saw in Section 7.1 for the elimination of water. We ll see the reason for the requirement in Section 19.13, but might note for now that the reaction is not an electrophilic addition but instead occurs... 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Bromine readily adds across an alkenic double bond by electrophilic addition (Figure 8.4). The brominated compound is usually colourless, but bromine in solution ( bromine water ) has a red colour. Addition of bromine water to an aikene is accompanied by a loss of the red colour as reaction proceeds. The stoichiometry of reaction is almost always 1 1, with one molecule of bromine reacting per double bond. 

Ionic reactions of neutral substrates can show large solvent dependence, due to the differential solvent stabilization of the ionic intermediates and their associated dipolar transition states (Reichardt, 1988). This is the case for the electrophilic addition of bromine to alkenes (Ruasse, 1990, 1992 Ruasse et al., 1991) and the bromination of phenol (Tee and Bennett, 1988a), both of which have Grunwald-Winstein m values approximately equal to 1 so that the reactions are very much slower in media less polar than water. Such processes, therefore, would be expected to be retarded or even inhibited by CDs for two reasons (a) the formation of complexes with the CD lowers the free concentrations of the reactants and (b) slower reaction within the microenvironment of the less polar CD cavity (if it were sterically possible). 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

It is important to be able to look at a molecular structure and deduce the possible reactions it can undergo. Take an alkene, for example. It has a 7t bond that makes it electron-rich and able to attack electrophiles such as water, halogens and hydrogen halides in electrophilic addition reactions. Haloalkanes, on the other hand, contain polar carbon-halogen bonds because the halogen is more electronegative than carbon. This makes them susceptible to attack by nucleophiles, such as hydroxide, cyanide and alkoxide ions, in nucleophilic substitution reactions. 

Subramaniam, 1988]. Hydrochlorination, usually carried out at about 10°C, proceeds by electrophilic addition to give the Markownikoff product with chlorine on the tertiary carbon (Eq. 9-33) [Golub and Heller, 1964 Tran and Prud homme, 1977]. Some cyclization of the intermediate carbocation (XXVI) also takes place (Sec. 9-7). The product, referred to as rubber hydrochloride, has low permeability to water vapor and is resistant to many aqueous solutions (hut not bases or oxidizing acids). Applications include packaging film laminates with metal foils, paper, and cellulose films, although it has been largely replaced by cheaper packaging materials such as polyethylene. 

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

Electrophilic addition of the halogens and related X—Y reagents to alkenes and alkynes has been a standard procedure since the beginning of modem organic chemistry.1 Anti electrophilic bromination of such simple compounds as cyclohexene and ( )- and (Z)-2-butene, and variants of this reaction when water or methanol are solvents (formation of halohydrin or their methyl ethers, respectively), are frequently employed as prototype examples of stereospecific reactions in elementary courses in organic chemistry. A simple test for unsaturation involves addition of a dilute solution of bromine in CCU to the... 

This provides evidence that these 0-vinyloximes are highly prone to electrophilic addition of water across the C=N bond. 

B-6. Which of the species shown is the most stable form of the intermediate in the electrophilic addition of Cl2 in water to cyclohexene to form a halohydrin Electron pairs have been omitted for convenience, and their absence should not be considered as part of the problem. 

Alkenes get converted to alcohols by treatment with aqueous acid (sulphuric or phosphoric acid (Following fig.)). This electrophilic addition reaction involves the addition of water across the double bond. The hydrogen adds to one carbon while a hydroxyl group adds to the other carbon. 

Sometimes the reaction conditions used in this reaction are too harsh since heating is involved and rearrangement reactions can occur. A milder method that gives better results is to treat the alkene with mercuric acetate [Hg(OAc)2] then sodium borohydride. The reaction involves electrophilic addition of the mercury reagent to form an intermediate mercuronium ion. This reacts with water to give an organomercury intermediate. Reduction with sodium... 

Electrophilic addition of mercuric ion gives a vinyl cation, which reacts with water and loses a proton to give an organomercurial alcohol. 

Step 2 of the iodination of benzene shows water acting as a base and removing a proton from the sigma complex. We did not consider the possibility of water acting as a nucleophile and attacking the carbocation, as in an electrophilic addition to an alkene. Draw the reaction that would occur if water reacted as a nucleophile and added to the carbocation. Explain why this type of addition is rarely observed. 

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