Aromatic nucleus

Gattermann s reaction A variation of the Sandmeyer reaction copper powder and hydrogen halide are allowed to react with the diazonium salt solution and halogen is introduced into the aromatic nucleus in place of an amino group. 

Brominarion of the aromatic nucleus is now regarded as replacement of a hydrogen atom of the intact nucleus as a result of an attack by a polarised complex with a positive end. Iron acts as a carrier by forming FcBrj, which as a Lewis acid forms a polarised complex with one mol. of Bri ... 

The mechanism of the reaction is generally considered to proceed by way of carbonhim ions (alkyl cations) which attack the aromatic nucleus ... 

The mechanism of the aromatic substitution may involve the attack of the dectrophilic NOj" " ion upon the nucleophilic aromatic nucleus to produce the carboniiim ion (I) the latter transfers a proton to the bisulphate ion, the most basic substance in the reaction mixture... 

The meta directing groups include those in which the atom directly attached to the aromatic nucleus is either positively charged or strongly unsaturated. These are, in order of decreasing effectiveness —... 

The observation of nitration nitrosation for mesitylene is important, for it shows that this reaction depends on the reactivity of the aromatic nucleus rather than on any special properties of phenols or anilines. 

If acetoxylation were a conventional electrophilic substitution it is hard to understand why it is not more generally observed in nitration in acetic anhydride. The acetoxylating species is supposed to be very much more selective than the nitrating species, and therefore compared with the situation in (say) toluene in which the ratio of acetoxylation to nitration is small, the introduction of activating substituents into the aromatic nucleus should lead to an increase in the importance of acetoxylation relative to nitration. This is, in fact, observed in the limited range of the alkylbenzenes, although the apparently severe steric requirement of the acetoxylation species is a complicating feature. The failure to observe acetoxylation in the reactions of compounds more reactive than 2-xylene has been attributed to the incursion of another mechan-104... 

The TT-inductive effect describes how an inductive substituent might selectively influence the electron distribution at the o- and -positions of the aromatic nucleus. A familiar example is represented by the... 

QUANTITATIVE CORRELATIONS OF SUBSTITUENT EFFECTS The theories outlined above are concerned with the way in which substituents modify the reactivity of the aromatic nucleus. An alternative approach to the effects of substituents is provided by quantitative... 

The value of k was fixed at 0-5 and the n electron energy when the orbital representing the attacking reagent was positioned near to a particular position in the aromatic nucleus was computed, using values of h var3nng from — 3 to +3. 

It IS possible to replace ammo substituents on an aromatic nucleus by hydrogen by reducing a diazonium salt with hypophosphorous acid (H3PO2) or with ethanol These... 

Reactions at the aromatic nucleus that are quite different from the usual mild condensations and rearrangements which apparendy generate the typical alkaloids already discussed must be iavolved. Securinine (137) is reported to stimulate respiration and increase cardiac output, as do many other alkaloids, but it also appears generally to be less toxic (98). 

Chloromethylation of the aromatic nucleus occurs readily with alkyl and alkoxy substituents accelerating the reaction and halo, chloromethyl, carboxyl, and nitro groups retarding it. 

Ketone Synthesis. In the Friedel-Crafts ketone synthesis, an acyl group is iatroduced iato the aromatic nucleus by an acylating agent such as an acyl haUde, acid anhydride, ester, or the acid itself. Ketenes, amides, and nittiles also may be used aluminum chloride and boron ttitiuotide are the most common catalysts (see Ketones). 

Make acid yields coumaUc acid when treated with fuming sulfuric acid (19). Similar treatment of malic acid in the presence of phenol and substituted phenols is a facile method of synthesi2ing coumarins that are substituted in the aromatic nucleus (20,21) (see Coumarin). Similar reactions take place with thiophenol and substituted thiophenols, yielding, among other compounds, a red dye (22) (see Dyes and dye intermediates). Oxidation of an aqueous solution of malic acid with hydrogen peroxide (qv) cataly2ed by ferrous ions yields oxalacetic acid (23). If this oxidation is performed in the presence of chromium, ferric, or titanium ions, or mixtures of these, the product is tartaric acid (24). Chlorals react with malic acid in the presence of sulfuric acid or other acidic catalysts to produce 4-ketodioxolones (25,26). 

Alkylphenols undergo a variety of chemical transformations, involving the hydroxyl group or the aromatic nucleus that convert them to value-added products. 

The unshared pairs of electrons on hydroxyl oxygens seek electron deficient centers. Alkylphenols tend to be less nucleophiUc than aUphatic alcohols as a direct result of the attraction of the electron density by the aromatic nucleus. The reactivity of the hydroxyl group can be enhanced in spite of the attraction of the ring current by use of a basic catalyst which removes the acidic proton from the hydroxyl group leaving the more nucleophiUc alkylphenoxide. 

The aromatic nucleus of alkylphenols can undergo a variety of aromatic electrophiUc substitutions. Electron density from the hydroxyl group is fed iato the ring. Besides activating the aromatic nucleus, the hydroxyl group controls the orientation of the incoming electrophile. 

Vanillin is a compound that possesses both a phenoHc and an aldehydic group. It is capable of undergoing a number of different types of chemical reactions. Addition reactions are possible owing to the reactivity of the aromatic nucleus. 

Aromatic carboxyUc acids are produced annually in amounts of several million metric tons. Several aromatic acids occur naturally, eg, benzoic acid (qv), sahcyhc acid (qv), cinnamic acid (qv), and gaUic acids, but those used in commerce are produced synthetically. These acids are generally crystalline sohds with relatively high melting points, attributable to the rigid, planar, aromatic nucleus (see Phthalic acids). 

Cost. The catalytically active component(s) in many supported catalysts are expensive metals. By using a catalyst in which the active component is but a very small fraction of the weight of the total catalyst, lower costs can be achieved. As an example, hydrogenation of an aromatic nucleus requires the use of rhenium, rhodium, or mthenium. This can be accomplished with as fittie as 0.5 wt % of the metal finely dispersed on alumina or activated carbon. Furthermore, it is almost always easier to recover the metal from a spent supported catalyst bed than to attempt to separate a finely divided metal from a liquid product stream. If recovery is efficient, the actual cost of the catalyst is the time value of the cost of the metal less processing expenses, assuming a nondeclining market value for the metal. Precious metals used in catalytic processes are often leased. 

Esters with aromatic nucleus in -1.174-1-0.376N -140.04-1- 13.869N Add to values of AN, AB calculated for ester... 

Primary amines with NH2 group on aromatic nucleus 15.04-N 0 The AN value is not a correction to regular amine value to find AB, use primary amine valiie" ... 

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