Electrophilic attachment

The solvomercuration reaction is thought to be a two-step process. In the first step (equation 147), electrophilic attachment of mercury ion to the alkene produces a positively charged intermediate. In the second step (equation 148), a nucleophile (generally a solvent molecule) reacts with the intermediate leading to the organomercury compound. 

Diels-alder adducts at 0°C. This cation radical-vinylcyclobutane rearrangement is non-stereospecific, thus accounting for the formation of a cis-trans mixture of Diels-Alder adducts. Kinetic studies revealed (Scheme 8) that the ionization of these ethers involves an inner-sphere electron-transfer mechanism involving strong covalent (electrophilic) attachment to the substrate via oxygen (oxonium ion) or carbon (carbocation). 

Support-bound non-benzylic alcohols can also be used to immobilize carboxylic acids as esters (Table 3.3). The advantage of this type of linker is its stability towards electrophiles. Attachment of carboxylic acids is usually realized by acylation of the resin-bound alcohol with a reactive acid derivative. 

An additional complication affecting silicon surface chemistry is the well-established fact that dimers tilt away from the symmetric position (c.f. Fig. 1(b)). Associated with dimer tilting is a charge transfer from the down atom to the up atom. Hence, the dimers exhibit somewhat zwit-terionic character, with one electron-poor atom and one electron-rich atom. Such a property of the Si(100)-(2 x 1) surface makes it possible to use nucleophilic and electrophilic attachment reactions. At temperatures less than 120 K, dimer tilting on Si(100)-(2 x 1) can be observed in STM experiments [3,9], while at higher temperatures the direction of the tilt oscillates on a time scale faster than the order milliseconds sampling times of the STM. 

An illustrative example is the electrophilic attachment of chlorine to the carbon-carbon double bond of ethylene. 

Turning to the electrophilic attachment of chlorine to ethylene, Amovilli et al in [34] recovered between 93 and 99 % of the CAS (6,5) correlation energy using four structures and ten hybrids with the following classical coupling scheme... 

Figure 11. Contour map of hybrids 4>i-5 used for the VB structure Iq obtained by analyzing the CASSCF wavefunction at the transition state geometry for the electrophilic attachment of chlorine to ethylene in aqueous solution.

Figure 13. Adiabatic ([a]) and diabatic ([b]) energy curves (hartree versus A) for the electrophilic attachment of chlorine to ethylene in vacuo (solid line) and in aqueous solution (dashed line). Curves 1-4 correspond to VB structures...

This means that the product has the electrophile attached to the carbon that would be less stable as a carbocation and the nucleophile attached to the carbon that would be more stable as a carbocation. Later, we will encounter exceptions to Markovnikov s rule. However, these exceptions are still in accord with this mechanistically based rule. 

Media pH errors and media pH span errors are common. Since electrophilic aromatic substitution is almost exclusively an acidic media process, do not make any strong bases during the mechanism. The proton on the aromatic ring becomes very acidic after the electrophile attaches and forms the carbocationic sigma-complex. However, before the electrophile attacks, the aromatic H is not acidic at all, p Ta = 43, so do not get your steps out of order and try to pull the H off first. 

However, for an agricultural chemical, structures which are more synthetically accessible are desirable, and alternative structures were sought Conceptually, it appeared that a small biologically neutral, chemical activating group which allowed either nucleophilic or electrophilic attachment of substituents would be an ideal substituent to have on the benzylic carbon. The cyano moiety seemed to have the best potential and it was found that phenylacetonitrile was... 

Because the alkenes in the preceding reactions have the same substituents on both of the sp carbons, it is easy to determine the product of the reaction The electrophile (H ) adds to one of the sp carbons, and the nucleophile (X ) adds to the other sp carbon. It doesn t make any difference which sp carbon the electrophile attaches to, because the same product will be obtained in either case. 

As a consequence of the tt electrons above and below the plane of its ring, benzene is a nucleophile. It will, therefore, react with an electrophile (Y ). When an electrophile attaches itself to a benzene ring, a carbocation intermediate is formed. 

The resonance hybrid of pyrrole indicates that there is a partial positive charge on the nitrogen. Therefore, pyrrole is protonated on C-2 rather an on nitrogen. Remember, a proton is an electrophile and, like other electrophiles, attaches to the C-2 position of pyrrole. 

The simplest reaction of alkenes is the addition of hydrogen to form alkanes. Hydrogenation is a nonpolar process, unlike many of the addition reactions, to be described later, that involve electrophiles attaching to the nucleophilic n electrons of the alkene. Hydrogenation reactions require catalysts, such as platinum oxide (Adams s catalyst), palladium on carbon (Pd-C), or Raney nickel (Ra-Ni). —, ... 

Finally for the electrophilic attachment of chlorine to ethylene we have studied the free energy and the form of the wavefunction along the reaction coordinate maintaining the geometry at a Civ symmetry. From the stationary points we have found the geometries and the free energies of the transition state and of the chloronium intermediate. 

Nucleophiles attach here —>. C = O — Electrophiles attach here... 

The oxjnnercuration reaction is thought to be a two-step process. In the first step, electrophilic attachment of the mercury ion to the alkene produces a positively charged intermediate (equation 9.39). In the second step of oxy-mercuration, a nucleophile (most likely a solvent molecule, SOH) reacts with the intermediate to produce the organomercury compoimd (equation 9.40). For reactions in water, both the organomercurial and the final product are alcohols. The reaction produces an ether if the hydroxylic solvent is an alcohol, and the reaction is called solvomercuration or alkoxymercuration. Better yields are obtained if the anion of the mercuric salt is a weaker nucleophile than is the solvent. For this reason, mercuric... 

Mn2(CO)8(PEt2Ph)2] there is kinetic evidence for this. An electrophilic attachment of Br2 at CO oxygen atoms is suggested. 

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