Positive attacking species

The function of the acid catalyst is to generate a positive attacking species. In the Friedel-Crafts reaction, A1C13 functions as an acid catalyst by increasing the concentration of the carbocation, R+, which is the attacking species as shown in the following equations ... 

In this reaction, AICI3 functions as a catalyst by generating a positive attacking species, R+ (see Chapter 5) ... 

The N02+ ion is the attacking species involved in nitration reactions, so H2S04 increases the concentration of a positive attacking species and is, therefore, functioning as an acid catalyst (see Chapter 5). 

The add catalyst, H2SO4, functions to generate a. positive attacking species, which is generally the function of an acid catalyst. While we will not show specific examples here, it is the function of a base catalyst to generate a negative attacking species. 

Electron distribution governs the electrostatic potential of molecules. The electrostatic potential describes the interaction of energy of the molecular system with a positive point charge. Electrostatic potential is useful for finding sites of reaction in a molecule positively charged species tend to attack where the electrostatic potential is strongly negative (electrophilic attack). 

The most complete discussion of the electrophilic substitution in pyrazole, which experimentally always takes place at the 4-position in both the neutral pyrazole and the cation (Section 4.04.2.1.1), is to be found in (70JCS(B)1692). The results reported in Table 2 show that for (29), (30) and (31) both tt- and total (tt cr)-electron densities predict electrophilic substitution at the 4-position, with the exception of an older publication that should be considered no further (60AJC49). More elaborate models, within the CNDO approximation, have been used by Burton and Finar (70JCS(B)1692) to study the electrophilic substitution in (29) and (31). Considering the substrate plus the properties of the attacking species (H", Cl" ), they predict the correct orientation only for perpendicular attack on a planar site. For the neutral molecule (the cation is symmetrical) the second most reactive position towards H" and Cl" is the 5-position. The activation energies (kJmoF ) relative to the 4-position are H ", C-3, 28.3 C-5, 7.13 Cr, C-3, 34.4 C-5, 16.9. 

Several studies338,340-342 show that the chlorination does not proceed, as assumed previously343, by proton abstraction followed by reaction of the carbanion thus formed, with electrophilic chlorine. A mechanism involving a chlorooxosulfonium ion formed by attack of a positive chlorine species on sulfur was shown to be more likely344. 

That is, the more reactive an attacking species, the less preference it has for the para position compared to the meta. If we combine the Hammett-Brown a p relationship with the linearity between log Sf and log p and between log Sf and log m] , it is possible to derive the following expressions ... 

When a heteroatom, such as N, O, or a halogen, is present in a molecule containing an aromatic ring or a double bond, lithiation is usually quite regio-selective. The lithium usually bonds with the sp carbon closest to the hetero atom, probably because the attacking species coordinates with the hetero atom. Such reactions with compounds such as anisole are often called directed metala-tions. In the case of aromatic rings, this means attack at the ortho position.Two examples are... 

Electrophiles are positively charged ions or neutral molecules that are deficient in electrons. Examples include H, NO and SO3. They are capable of accepting a pair of electrons from an electron pair donor to form a covalent bond. Electrophilic means electron-loving and so electrophiles will be attracted to and will attack species with a negative or partial negative charge. 

A rapid-reaction technique was used to study the pH dependence of the reversible addition of water across the 3,4-double bond of eighteen quinazolines and four triazanaphthalenes. The pH range of 0-13 was covered, at 20°. When the rate constants for hydration were plotted against pH, a paraboloid curve was obtained with the minimum rate near neutrality. It was calculated that there is a strong acceleration of hydration in acidic solution due to the successive formation of mono-and dications (the attacking species is the water molecule). The increasing rate of hydration in alkaline solution was seen as the catalytic effect of the hydroxyl ion on the neutral species.30 The kinetics of dehydration in neutral solution proved to be 105 times faster than those for hydration. For quinazoline, the two curves crossed at pH 3.5, below which hydration ran much the faster. Substituent and positional effects, particularly the slowing effect of a substituent in the 4-position, were quantified.30... 

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