Benzenes addition reactions

Benzene can undergo addition reactions which successively saturate the three formal double bonds, e.g. up to 6 chlorine atoms can be added under radical reaction conditions whilst catalytic hydrogenation gives cyclohexane. 

Toluene, an aLkylben2ene, has the chemistry typical of each example of this type of compound. However, the typical aromatic ring or alkene reactions are affected by the presence of the other group as a substituent. Except for hydrogenation and oxidation, the most important reactions involve either electrophilic substitution in the aromatic ring or free-radical substitution on the methyl group. Addition reactions to the double bonds of the ring and disproportionation of two toluene molecules to yield one molecule of benzene and one molecule of xylene also occur. 

These effects can be attributed mainly to the inductive nature of the chlorine atoms, which reduces the electron density at position 4 and increases polarization of the 3,4-double bond. The dual reactivity of the chloropteridines has been further confirmed by the preparation of new adducts and substitution products. The addition reaction competes successfully, in a preparative sense, with the substitution reaction, if the latter is slowed down by a low temperature and a non-polar solvent. Compounds (12) and (13) react with dry ammonia in benzene at 5 °C to yield the 3,4-adducts (IS), which were shown by IR spectroscopy to contain little or none of the corresponding substitution product. The adducts decompose slowly in air and almost instantaneously in water or ethanol to give the original chloropteridine and ammonia. Certain other amines behave similarly, forming adducts which can be stored for a few days at -20 °C. Treatment of (12) and (13) in acetone with hydrogen sulfide or toluene-a-thiol gives adducts of the same type. 

The classification of hydrocar bons as aliphatic or ar omatic took place in the 1860s when it was aheady apparent that there was something special about benzene, toluene, and their- derivatives. Their molecular- for-mulas (benzene is CgHg, toluene is CyKj ) indicate that, like alkenes and alkynes, they are unsaturated and should undergo addition reactions. Under conditions in which bromine, for example, reacts rapidly with alkenes and alkynes, however, benzene proved to be inert. Benzene does react with Br-2 in the presence of iron(III) bromide as a catalyst, but even then addition isn t observed. Substitution occurs instead ... 

Benzene rings do not readily undergo eleetrophilic addition reactions, presumably because the loss of resonance makes these reaetions unfavorable. 

Unsaturated hydrocarbons undergo a variety of reactions. Experimentally, alkenes and alkynes undergo addition reactions, whereas aromatic molecules, such as benzene, undergo substitution reactions instead. Why ... 

Depending on the electronic state of azafulvalene and the reaction conditions, simple nucleophiles such as amines or alcohols show a different behavior. Upon heating methanol reacted with azafulvalenes as electron-rich olefins by addition to the central double bond (64BSF2857 67LA155). Using the TAF 77 (Ar = Ph), the addition reaction in a neutral benzene-ethanol solution required several days to obtain a minor amount of 147, while the reaction proceeded rapidly in the presence of a catalytic amount of potassium hydroxide (79JOC1241). Tlie yellow-colored adduct 147 can be reconverted to the quinoid starting material by irradiation (Scheme 58). 

The corresponding reaction Scheme, Scheme (16) proposed for benzene solution is similar, the only additional reaction being the deactivation of the exciplex (CM ) by the banzene molecule (B). 

It is important to note that benzene does not behave like a typical cyclic olefin in that the benzene ring undergoes ionic substitution rather than addition reactions the ring also resists hydrogenation and is chemically more inert. Despite this, it is still a common practice to represent benzene with three double bonds as if it were 2,4,6-cyclohexatriene,... 

Benzene shows neither the typical reactivity nor the usual addition reaction of ethylene. Benzene does react with bromine, Brs, but in a different type of reaction ... 

Since both oxepin and its valence isomer benzene oxide contain a x-tb-diene structure they are prone to Diels-Alder addition reactions. The dienophiles 4-phenyl- and 4-methyl-4//-l,2,4-triazole-3,5-dione react with substituted oxepins at room temperature to give the 1 1 adducts 7 formed by addition to the diene structure of the respective benzene oxide.149 190,222... 

Besides simple alkyl-substituted sulfoxides, (a-chloroalkyl)sulfoxides have been used as reagents for diastereoselective addition reactions. Thus, a synthesis of enantiomerically pure 2-hydroxy carboxylates is based on the addition of (-)-l-[(l-chlorobutyl)sulfinyl]-4-methyl-benzene (10) to aldehydes433. The sulfoxide, optically pure with respect to the sulfoxide chirality but a mixture of diastereomers with respect to the a-sulfinyl carbon, can be readily deprotonated at — 55 °C. Subsequent addition to aldehydes afforded a mixture of the diastereomers 11A and 11B. Although the diastereoselectivity of the addition reaction is very low, the diastereomers are easily separated by flash chromatography. Thermal elimination of the sulfinyl group in refluxing xylene cleanly afforded the vinyl chlorides 12 A/12B in high chemical yield as a mixture of E- and Z-isomers. After ozonolysis in ethanol, followed by reductive workup, enantiomerically pure ethyl a-hydroxycarboxylates were obtained. 

Arenes are unsaturated but, unlike the alkenes, they are not very reactive. Whereas alkenes commonly take part in addition reactions, arenes undergo predominantly substitution reactions, with the TT-bonds of the ring left intact. For example, bromine immediately adds to a double bond of an alkene but reacts with benzene only in the presence of a catalyst—typically, iron(III) bromide—and it does not affect the bonding in the ring. Instead, one of the bromine atoms replaces a hydrogen atom to give bromobenzene, C H Br ... 

Alkyl radical addition reactions to styrene chromium tricarbonyl can be accomplished using alkyl halides (10 equiv) and (TMSlsSiH (5 equiv) in the presence of AIBN in refluxing benzene, for 18 h (Reaction 66). " These reactions are believed to proceed through intermediates in which the unpaired electron is interacting with the adjacent arene chromium tricarbonyl moiety since the analogous reaction with styrene affords only traces of addition products. 

The other bromine atom comes from another bromine-containing molecule or ion. This is clearly not a problem in reactions with benzylic species since the benzene ring is not prone to such addition reactions. If the concentration is sufficiently low, there is a low probability that the proper species will be in the vicinity once the intermediate forms. The intermediate in either case reverts to the initial species and the allylic substitution competes successfully. If this is true, it should be possible to brominate an alkene in the allylic position without competition from addition, even in the absence of NBS or a similar compound, if a very low concentration of bromine is used and if the HBr is removed as it is formed so that it is not available to complete the addition step. This has indeed been demonstrated. ... 

Like Halpem, Siekierska and Siuda with GeCl in benzene, Riedel and Merz found essentially the same distribution of radioactivity following p decay of Ge04 as by nuclear reactions, except for a uniformly higher yield of As 03. They analyse their results for this reaction as 14% failure of bond rupture, 5% radical recombination and, in benzene solution, 4% additional reaction with radiation produced radicals. 

The chemical reactions of benzene and all aromatic compounds, with few exceptions, are unlike those of unsaturated aliphatic compounds (olefins) that is, addition reactions do not occur. Instead, the hydrogens on the ring are replaced by other atoms or groups of atoms. The aromatic ring remains unchanged by these substitution reactions. All six of the hydrogens in benzene can be replaced by other atoms. 

Ti02 particles, the benzene-thermal reaction of TiCLi. and NaN3, chemical vapor reactions, plasma syntheses, or arc-melting [15-20], The optical properties of low-concentrated TiN nanoparticle systems were studied by Quinten [21], Highly dispersed TiN was used as an additive to improve the mechanical properties of titanium carbide-based cermets [22],... 

From eqns. 4.50 and 4.51 it can be seen that complex 1 is converted into complex 2 by a stronger base 2 or into complex 3 by a stronger acid 2 in other words, complexes 2 and 3 are much more stable than complex 1. Whereas reactions 4.48 and 4.49 are addition reactions, reactions 4.50 and 4.51 are exchange reactions often Lewis titrations must be carried out in completely inert solvents such as alkanes or benzene because of instability of the titrants and titrands in other media. Examples of potentiometric Lewis titration curves are given in Fig. 4.9 for CS2 and C0220, where one of their resonance structures can react as a Lewis acid with OH as a Lewis base ... 

Benzene addition to Ir(P Pr3)2Cl is an exothermic reaction (22kcal mol-1), while addition to Ir(P Pr3)2(CO)Cl is endothermic (—5kcal mol-1).501 The reaction enthalpies of substitution reactions to complexes containing the Ir(P1Pr3)2Cl fragment are supplied. Reaction of Ir(P1Pr3)2Cl with 2-pyridyl esters gives a p2 (C,0)-bound ketene, (307), where R2 = R1 = aryl or R2 = aryl,... 

R groups in which the C=0 group is conjugated with C=C (1,4-addition can also compete here, cf. p. 200), or with a benzene ring, also exhibit slower addition reactions than their saturated analogues. This is because the stabilisation, through delocalisation, in the initial carbonyl compounds (7 and 8) is lost on proceeding to the adducts (9 and 10), and to the transition states that precede them ... 

The addition of chlorine or bromine to benzene—one of the few overall addition reactions of a simple benzene nucleus—has also been shown to proceed via a radical pathway, i.e. it is catalysed by light and by the addition of peroxides, and is slowed or prevented by the usual inhibitors. With chlorine this presumably proceeds ... 

Anderson and Kemball (35) examined the reaction between gaseous deuterium and benzene catalyzed by evaporated films of iron, nickel, palladium, silver, tungsten, and platinum. The order of reactivity (estimated from the temperature at which the addition reaction achieved an initial rate of 1% per minute for a 10 mg film at certain specified reactant... 

In the two previous sections, evidence has been presented concerning the chemisorbed states formed when benzene interacts with metal surfaces. It is not the intention in this Section to discuss benzene hydrogenation in detail, but rather to enquire whether studies of this hydrogen-addition reaction provide information about the chemisorbed state of benzene. 

The chain length is therefore adversely afFected by the irradiation dose rate being inversely proportional to its square root. Wagner (1969) lists a large class of unsaturated compounds in which addition reactions can be induced by irradiation. Typical examples involving long chain lengths are for the addends HC1, Cl2, and HBr in ethylene, benzene, toluene, and so on. where the products are telomers or hexachlorides. 

These differences in the course of the reaction of Grignard and sodium compounds are not limited to carbonations but are also observed in carbonyl addition reactions. Whereas cinnamylmagnesium bromide always reacts at the secondary carbon atom next to the benzene ring, the sodium compound may react at either the secondary or the primary position, depending on the electrophilic reagent.403... 

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