Rate determining step, electrophilic aromatic

It is not surprising that the formation of the cationic intermediate is the rate-determining step, as aromaticity is temporarily lost in this step. The mechanism of the fast proton loss from the intermediate is shown in three ways just to prove that it doesn t matter which of the delocalized structures you choose. A useful piece of advice is that, when you draw the intermediate in any electrophilic aromatic substitution, you should always draw in the hydrogen atom at the point of substitution, just as we have been doing. 

TWo types of rate expressions have been found to describe the kinetics of most aromatic nitration reactions. With relatively unreactive substrates, second-order kinetics, first-order in the nitrating reagent and first-order in the aromatic, are observed. This second-order relationship corresponds to rate-limiting attack of the electrophile on the aromatic reactant. With more reactive aromatics, this step can be faster than formation of the active electrq)hile. When formation of the active electrophile is the rate-determining step, the concentration of the aromatic reactant no longer appears in the observed rate expression. Under these conditions, different aromatic substrates undergo nitration at the same rate, corresponding to the rate of formation of the active electrophile. 

The active electrophile is formed by a subsequent reaction, often involving a Lewis acid. As discussed above with regard to nitration, the formation of the active electrophile may or may not be the rate-determining step. Scheme 10.1 indicates the structure of some of the electrophihc species that are involved in typical electrophilic aromatic substitution processes and the reactions involved in their formation. 

The rate-determining step is the electrophilic aromatic substitution as in the closely related Friedel-Crafts reaction. Both reactions have in common that a Lewis acid catalyst is used. For the Blanc reaction zinc chloride is generally employed, and the formation of the electrophilic species can be formulated as follows ... 

Novolacs are prepared with an excess of phenol over formaldehyde under acidic conditions (Fig. 7.6). A methylene glycol is protonated by an acid from the reaction medium, which then releases water to form a hydroxymethylene cation (step 1 in Fig. 7.6). This ion hydroxyalkylates a phenol via electrophilic aromatic substitution. The rate-determining step of the sequence occurs in step 2 where a pair of electrons from the phenol ring attacks the electrophile forming a car-bocation intermediate. The methylol group of the hydroxymethylated phenol is unstable in the presence of acid and loses water readily to form a benzylic carbo-nium ion (step 3). This ion then reacts with another phenol to form a methylene bridge in another electrophilic aromatic substitution. This major process repeats until the formaldehyde is exhausted. 

Systematic studies of the selectivity of electrophilic bromine addition to ethylenic bonds are almost inexistent whereas the selectivity of electrophilic bromination of aromatic compounds has been extensively investigated (ref. 1). This surprising difference arises probably from particular features of their reaction mechanisms. Aromatic substitution exhibits only regioselectivity, which is determined by the bromine attack itself, i.e. the selectivity- and rate-determining steps are identical. 

Most aromatic substitution reactions conform to a simple mechanism. In the rate-determining step, a new bond is formed between an aromatic carbon atom and the electrophilic reagent yielding an intermediate... 

The most widely accepted mechanism for electrophilic aromatic substitution involves a change from sp2 to sps hybridization of the carbon under attack, with formation of a species (the Wheland or a complex) which is a real intermediate, i.e., a minimum in the energy-reaction coordinate diagram. In most of cases the rate-determining step is the formation of the a intermediate in other cases, depending on the structure of the substrate, the nature of the electrophile, and the reaction conditions, the decomposition of such an intermediate is kinetically significant. In such cases a positive primary kinetic isotope effect and a base catalysis are expected (as Melander43 first pointed out). 

Let us once more view the mechanism of the classic Ar-SE reaction of Figure 5.1. In the rate-determining step, an electrophile reacts with an aromatic compound. A carbenium ion is produced, which is called the sigma complex. Therein a positive charge is delocalized over the... 

Nitration of an aromatic compound, ArH, takes, place by electrophilic attack by NO, to form ArHNOt, followed by the decomposition of this activated complex to give ArNO, and H. The decomposition of the activated complex is the rate determining step( l, 2). It can be shown that the nitration reaction is a second order reaction, proportional to the concentrations of the organic species and the nitronium ion in the acid phase. In the model presented here molefractions are used instead of concentrations. 

Because of the presence of nitrogen in the aromatic ring, electrons in pyridine are distributed in such a way that their density is higher in positions 3 and 5 (the P-positions). In these positions, electrophilic substitutions such as halogenation, nitration, and sulfonation take place. On the contrary, positions 2, 4, and 6 (a- and y-positions, respectively) have lower electron density and are therefore centers for nucleophilic displacements such as hydrolysis or Chichibabin reaction. In the case of 3,5-dichlorotrifluoropyridine, hydroxide anion of potassium hydroxide attacks the a- and y-positions because, in addition to the effect of the pyridine nitrogen, fluorine atoms in these position facilitate nucleophilic reaction by decreasing the electron density at the carbon atoms to which they are bonded. In a rate-determining step, hydroxyl becomes attached to the carbon atoms linked to fluorine and converts the aromatic compound into a nonaromatic Meisenheimer complex (see Surprise 67). To restore the aromaticity, fluoride ion is ejected in a fast step, and hydroxy pyridines I and J are obtained as the products [58],... 

A C—H bond is broken faster than is a C—D bond. This rate difference (isotope effect, kH/kD) is observed only if the C—H (or C—D) bond is broken in the rate-determining step. If no difference is observed, as is the case for most aromatic electrophilic substitutions, C—H bond-breaking must occur in a fast step (in this case the second step). Therefore, the first step, involving no C—H bond-breaking, is rate-determining. This slow step requires the loss of aromaticity, the fast second step restores the aromaticity. 

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