Ethylbenzene bromination

The relative rates of reaction of ethane toluene and ethylbenzene with bromine atoms have been measured The most reactive hydrocarbon undergoes hydrogen atom abstraction a million times faster than does the least reactive one Arrange these hydrocarbons in order of decreasing reactivity... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

Purification of industrial oils, kerosene/jet fuel, lubricating oils Mono- dicumyldiphenylamine Mono- dioctyldiphenylamine Dimer fatty acids Purification of xylenes Improvement of bromine number of recycle cumene in phenol plants Improvement of bromine number of recycle ethylbenzene in styrene plants based on liquid pha.se oxidation Alkylation of xylenes with diisobutylenes to mono-/ rr-butyI derivatives Phenyl xylyl ethane... 

Benzene, toluene, ethylbenzene and chlorobenzene have been shown to be suitable substrates under these conditions, and the reaction rates are 50 to 70 times faster than for uncatalysed reactions. This reaction can be further enhanced by use of methanol as a co-catalyst, which allows bromination of anilines in quantitative yield with complete selectivity for the para- isomer [54],... 

Benzyl chloride undergoes further chlorination to give di- and tri-chloro derivatives, though it is possible to control the extent of chlorination by restricting the amount of chlorine used. As indicated above, it is easier to mono-brominate than it is to mono-chlorinate. The particular stabilization conferred on the benzylic radical by resonance is underlined by the reaction of ethylbenzene with halogens. 

Direct bromination of toluene and ethylbenzene form the corresponding benzyl bromides in high yield. The observed selectivity in SC-CO2 is similar to that observed in conventional organic solvents. Also, SC-CO2 is an effective alternative to carbon tetrachloride for use in the classical Ziegler bromination with N-bromosuccinimide. Reaction yields are high, side products are minimized, and bromine-atom selectivities are observed. Thus, SC-CO2 must be useful as a viable, environmentally benign substitute for many of the solvents typically used for free-radical reactions (Tanko and Blackert, 1994). 

Substrates that carry a replaceable benzylic hydrogen atom, or a similar hydrogen that gives rise to a stabilized radical, can be selectively chlorinated or brominated. Ethylbenzene leads to only... 

In 1994, the free-radical bromination of toluene (and other alkylaro-matics) in sc C02 was reported (Tanko and Blackert, 1994 Tanko et al., 1994). Product yields were similar to those obtained with conventional solvents. The relative reactivity of the secondary hydrogens of ethylbenzene versus the primary hydrogens of toluene on a per hydrogen basis, r(2°/l°), were assessed via competition experiments and (a) did not vary with pressure, (b) were nearly identical to what is observed in conventional solvents. 

The stability of free radicals is reflected in their ease of formation. Toluene, which forms a benzyl radical, reacts with bromine 64,000 times faster than does ethane, which forms a primary alkyl radical. Ethylbenzene, which forms a secondary benzylic radical, reacts 1 million times faster than ethane. 

Shown next is the reaction of ethylbenzene with bromine, catalyzed by ferric bromide. As with toluene, the rates of formation of the ortho- and para-substituted isomers are gready enhanced with respect to the meta isomer. 

Propose a mechanism for the bromination of ethylbenzene shown above. 

In the circumstances where activated ring systems are used as substrates, nuclear bromination is sometimes a problem. Two substrates worth mentioning which are mildly activated and have been used in the photolytic hydrogen peroxide/hydrogen bromide system are ethylbenzene and 4-t-butyltoluene. The ethylbenzene has been oxidized to acetophenone in 57% yield and 61% selectivity, the remainder being the intermediate product 1-bromoethylbenzene. The preparation of 4-t-butylbenzaldehyde from the toluene affords a yield of 58% (Figure 3.77). 

A further useful application of SC-CO2 as a reaction medium is the free-radical side-chain bromination of alkylaromatics, replacing conventional solvents such as tetra-chloromethane or chlorofluorohydrocarbons having no abstractable hydrogen atoms [920]. For example, bromination of ethylbenzene in SC-CO2 at 40 °C and 22.9 MPa yields 95 cmol/mol (1-bromoethyl)benzene, with practically the same regioselectivity as obtained in conventional tetrachloromethane as the solvent. Even the classical Wohl-Ziegler bromination of benzylic or allylic substrates using A-bromosuccinimide (NBS) can be conducted in SC-CO2 [920]. Irradiation of a solution of toluene, NBS, and AIBN (as initiator) in SC-CO2 at 40 °C and 17.0 MPa for 4 hours gave (bromomethyl)-... 

The higher homologues of toluene such as ethylbenzene are not usually halogenated selectively and mixtures are often produced (see Chapter 3). In the case of ethylbenzene itself, the major product of chlorination is the 1-substituted product (56%). Bromine is more selective and the 1-bromo derivative 7 is formed exclusively. 

As a result, an alkyl benzene undergoes selective bromination at the weak benzylic C—H bond under radical conditions to form a benzylic halide. For example, radical bromination of ethylbenzene using either Br2 (in the presence of light or heat) or A/-bromosuccinimide (NBS, in the presence of light or peroxides) forms a benzylic bromide as the sole product. 

The mechanism for halogenation at the benzylic position resembles other radical halogenation reactions, and so it involves initiation, propagation, and termination. Mechanism 18.10 illustrates the radical bromination of ethylbenzene using Bt2 (h or A). 

These predictions are correct. Treatment of ethylbenzene with nitric acid and sulfuric acid, for instance, introduces a nitro group into the ring treatment with bromine in the presence of light introduces a bromine atom into the side chain. But because of the ethyl group, nitration takes place more readily than with benzene itself, and occurs chiefly at the positions ortho and para to the ethyl group and because of the ring, bromination takes place more readily than with ethane, and occurs exclusively on the carbon nearer the ring. Thus each portion of the molecule affects the reactivity of the other portion and determines the orientation of attack. 

An alkylbenzene with a side chain more complicated than methyl offers more than one position for attack, and so we must consider the likelihood of obtaining a mixture of isomers. Bromination of ethylbenzene, for example, could theoretically yield two products 1-bromo-l-phenylethane and 2-bromo-l-phenylethane. Despite... 

Entry 1 is a chlorination at a stereogenic tertiary center and proceeds with complete racemization. In Entry 2, a tertiary radical is generated by loss of C=0, again with complete racemization. In Entry 3, an a-methylbenzyl radical is generated by a fragmentation and the product is again racemic. Entry 4 involves a benzylic bromination by NBS. The chirality of the reactant results from enantiospecific isotopic labeling of ethylbenzene. The product, which is formed via an a-methylbenzyl radical intermediate, is racemic. 

Like radical side-chain bromination, side-chain chlorination by S02C12 and a peroxide occurs mainly on the a-carbon atom ethylbenzene gives mainly (l-chloroethyl)benzene cumol gives 90% of the a- and 10% of the /9-chloro product. Chlorine enters the jS-position of terf-butylbenzene. o- and p-Nitro-toluene cannot be converted into the corresponding benzyl chlorides by S02C12 and a peroxide.419... 

The bromination of toluene with bromine in SC-CO2, photochemically initiated through a sapphire window, led to benzyl bromide (74%) and 4-bromotoluene (11%). Ethylbenzene afforded 1-bromo-l-phenylethane in 95 % yield (Scheme 4). [23]... 

In order to demonstrate that uncomplexed bromine atoms act as chain propagators, toluene and ethylbenzene were photobromina-ted in a competition study at pressures of 75 to 423 bar and at 40 °C. Over the entire pressure range, the reactivity of the benzylic secondary C-H bond in ethylbenzene was found to be about 30 times greater than that of the corresponding primary C-H bond in toluene. The analogous value for the reactivity in CCI4 at 40 °C is 36. The bromine atoms in SC-CO2 are therefore particularly free. It would be important to determine quantum yields (chain lengths) at various pressures to learn more about mechanistic aspects and other details of the reaction. Local solvent structures on model free-radical reactions in SC-CO2 have been analyzed in some detail. [33]... 

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