Positively charged intermediate

It is clear from the results that there is no kinetic isotope effect when deuterium is substituted for hydrogen in various positions in hydrazobenzene and 1,1 -hydrazonaphthalene. This means that the final removal of hydrogen ions from the aromatic rings (which is assisted either by the solvent or anionic base) in a positively charged intermediate or in a concerted process, is not rate-determining (cf. most electrophilic aromatic substitution reactions47). The product distribution... 

Electrophilic substitution begins like electrophilic addition, with an attack on a region of high electron density to form a positively charged intermediate ... 

Notice that there are three curved arrows here. For some reason, students drawing this mechanism commonly forget to draw the third curved arrow (the one that shows the expulsion of Br ). The product of this hrst step is a bridged, positively charged intermediate, called a bromonium ion ( onium because there is a positive charge). In the second step of our mechanism, the bromonium ion gets attacked by Br (formed in the hrst step) ... 

Because the organosilane reduction of ketones passes through a positively charged intermediate via the complexation or protonation of the carbonyl oxygen, the presence of suitably placed C=C functions can lead to cyclizations with the hydride of the silane adding to the C=C group. This strategy applies to... 

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. 

Electrophilic aromatic substitution normally proceeds via a positively charged intermediate 71 (known as a Wheland intermediate or cr-complex) (equation 27)84. 

The rate-determining step in the electrophilic substitution is the formation of the positively charged intermediate, and so the rate of the reaction is determined by the energy level of the transition state leading to that intermediate. The transition state resembles... 

In general, one speaks of a neighboring group effect only when the cyclic and usually positively charged intermediate cannot be isolated but is subject to an SN2-like ring opening by the... 

For many decades chemists had been interested in whether the positively charged intermediate of the SN reaction in Figure 2.28 was a carbenium or a carbonium ion. Also the existence of a rapidly equilibrating mixture of two carbenium ions was considered. It is now known with certainty that this intermediate is a carbonium ion it is known as a nonclassical carboca-tion. In this carbonium ion, there is a bond between the centers Cl, C2, and C6, that consists of two sp2 AOs and one sp3 AO (see MO diagram, lower right, Figure 2.28). It accommodates two electrons. There are many examples of nonclassical carbocations. 

Because of the drastically reduced water activity in aqueous medium of 4 mol.dm 3 LiCl, equilibrium (1) is lying to the left, in favour of the mobile intermediates X1 The concentration of X,-OH is then decreased drastically. This can be the cause why the second decomposition step consists of the reaction between two positively charged intermediates X (instead of the reaction between a charged mobile intermediate X and an uncharged immobile intermediate X,-OH found in aqueous medium of low electrolyte concentration). 

For many decades chemists had been interested in whether the positively charged intermediate of the SN reaction in Figure 2.25 was a carbenium or a carbonium... 

Pyridine is. in fact, more nucleophilic than the alcohol, and it attacks the acyl chloride rapidly, forming a highly electrophilic (because of the positive charge) intermediate. It is then this intermediate that subsequently reacts with the alcohol to give the ester. Because pyridine is acting as a nucleophile to speed up the reaction, yet is unchanged by the reaction, it is called a nucleophilic catalyst. 

We ll start with the acid-catalysed reaction, because it is more similar to the examples we have just been discussing—opening happens at the more substituted end. Protonation by acid produces a positively charged intermediate that bears some resemblance to the corresponding bromonium ion. The two alkyl groups make possible a build-up of charge on the carbon at the tertiary end of the proto-nated epoxide, and methanol attacks here, just as it does in the bromonium ion. 

Resonance structures explain that bromination occurs in the ortho and para positions of the rings. The positively charged intermediate formed from ortho or para attack can be stabilized by resonance contributions from the second ring of biphenyl, but this stabilization is not possible for meta attack. 

Attack occurs on the unsubstituted ring because bromine is a deactivating group. Attack occurs at the ortho and para positions of the ring because the positively charged intermediate can be stabilized by resonance contributions from bromine and from the second ring (Problem 16.43). 

More recently, cationic intermediates have been observed in the Heck reactions of arene diazonium salts catalyzed by triolefinic macrocycle Pd(0) complexes [17,59], o-iodophenols and enoates to form new lactones [60], and o-iodophenols with olefins (the oxa-Heck reaction) [61 ]. In the first case ions were formed by oxidation of the analyte at the capillary, or by association of [NH4] or Na". In the two other cases ionization occurred through the more typical loss of a halide ligand. The oxa-Heck reaction provides a good example of how these experiments are typically performed and the type of information that can be obtained. The oxyarylations of olefins were performed in acetone, catalyzed by palladium, and required the presence of sodium carbonate as base. Samples from the reaction mixtures were diluted with acetonitrile and analyzed by ESI(+)-MS. Loss of iodide after oxidative addition of o-iodophenol to palladium afforded positively-charged intermediates. Species consistent with oxidative addition, such as [Pd(PPh3)2(C6H50)], and the formation of palladacycles of the type seen in Scheme 8 were observed. Based on this, a mechanism for the reaction was proposed (Scheme 8). 

Electron-releasing substituents stabilize the positively charged intermediate and facilitate attack by an electrophile, which is directed to the onho and para positions. 

For S g1 The carbocation reacts with a nucleophile. Nucleophilic attack of CH3OH on the carbocation generates a positively charged intermediate that loses a proton to afford the neutral Snjl product. 

Remembering that Xj actually represents a positively charged surface intermediate, Eq (46) may be considered to represent the neutralization of this positive charge by reaction with H2O. It is reasonable to assume that further electrochemical reaction will, for electrostatic reasons, rather occur between the neutral entity X)-OH and the positively charged intermediate Xj than between two positive Xj intermediates (note that the stabilization kinetics yield no information on chemical... 

Electrophihc aromatic substitutions are unhke nucleophilic substitutions in that the large majority proceed by just one mechanism with respect to the substrate. In this mechanism, which we call the arenium ion mechanism, the electrophile (which can be viewed as a Lewis acid) is attacked by the 71-electrons of the aromatic ring (behaving as a Lewis base in most cases) in the first step. This reaction leads to formation of a new C—X bond and a new sp carbon in a positively charged intermediate called an arenium ion, where X is the electrophile. The positively charged intermediate (the arenium ion) is resonance stabilized, but not aromatic. Loss of a proton from the sp carbon that is adjacent to the positive carbon in the arenium ion, in what is effectively an El process (see p. 1487), is driven by rearomatization of the ring from the arenium ion to give the aromatic substitution product. A proton... 

If, on the other hand, aqueous acetic acid is added to the reaction mixture containing the positively charged intermediate, the product is a monoester of the diol, which, on hydrolysis, gives a diol. Such a reaction mimics a syn addition (Woodward reaction) [781, 783]. 

---