Bromination of methoxybenzene (anisole) is very fast and gives mainly the para-bromo isomer, accompanied by 10% of the ortho-isomer and only a trace of the meta-isomer. . ctivating Substituents ortho & para-Orientation, Deactivating Substituents meta-Orientation, Deactivating Substituents ortho & para-Orientation. The remarkable stability of the unsaturated hydrocarbon benzene has been discussed in an earlier section. Reagent : Concentrated nitric acid, HNO3 Condition : Reflux at a temperature lower than 50 ºC and the presence of concentrated sulfuric acid, H2SO4 as catalyst Product : Nitrobenzene; Nitration happens when one or more hydrogen atoms in benzene is replaced by a nitro group, NO2. An analogous addition reaction between benzene and bromine would be endothermic by +2kcal. For example, a hydroxy or methoxy substituent increases the rate of electrophilic substitution about ten thousand fold, as illustrated by the case of anisole in the virtual demonstration (above). If a benzene already has a first substituent in it, second incoming substituent is affected by the nature of the first substituent, Take methylbenzene as an example, since methyl group is electron-donating and 2,4-directing, the incoming halogen is substituted at the 2 or 4 position. Therefore, by adding heat to benzenesulfonic acid in diluted aqueous sulfuric acid the reaction is reversed. Note that in the resonance examples all the contributors are not shown. The balance between inductive electron withdrawal and p-π conjugation is such that the nitrogen and oxygen substituents have an overall stabilizing influence on the benzenonium intermediate and increase the rate of substitution markedly; whereas halogen substituents have an overall destabilizing influence. Water is added to isolate the acyl benzene final product. The reactivity of benzene is also affected by the presence of other group(s) in the benzene This is known as substituent group effect, and it will be explained later. Mechanism Step 2: Pi electrons of benzene react with the acylium ion to form the sigma complex, resonance stabilized acylbenzenium intermediate: Mechanism Step 3: Deprotonation of the sigma comlex to restore aromaticity. Sulfonation of benzene is a reversible reaction. As noted on the opening illustration, the product-determining step in the substitution mechanism is the first step, which is also the slow or rate determining step. This can be seen in seven representative substituents beneath the following diagram. Electrophilic aromatic substitution is a general method of derivatizing benzene. For example, a hydroxy or methoxy substituent increases the rate of electrophilic substitution about ten thousand fold, as illustrated by the case of anisole in the virtual demonstration (above). Reagent : Concentrated nitric acid, HNO3. Again we find that the nature of the substituent influences this product ratio in a dramatic fashion. Benzene and ethanoyl chloride with anhydride aluminium chloride Benzene and with Acetic anhydride with aluminium chloride These observations, and many others like them, have led chemists to formulate an empirical classification of the various substituent groups commonly encountered in aromatic substitution reactions. The periodic table—the transition metals, Topic 11: Measurement and data processing, 3. • As we will see, there are many reactions, depending upon the particular electrophile, they all use the same mechanism. However, each time a nitro group substitutes, the rate of reaction of the next substitution decreases because nitro group is electron-withdrawing. For example, electrophilic chlorination of toluene occurs hundreds of times faster than chlorination of benzene, but the relative rates are such that the products are 60% ortho-chlorotoluene, 39% para and 1% meta-isomers, a ratio similar to that observed for nitration. Aluminium chloride polarises the Cl-Cl bond to create the electrophile Cl⁺. The reaction of a substituted ring with an activating group is faster than benzene. Halogenation of Benzene. Benzene reacts with halogens (salt former) like chlorine and bromine and these reactions are called electrophilic substitution reaction in the presence of catalyst of Lewis acid like aluminum chloride, sulfur dichloride, ferric chloride or iron . Bromination of nitrobenzene requires strong heating and produces the meta-bromo isomer as the chief product. The chemical reactivity of benzene contrasts with that of the alkenes in that substitution reactions occur in preference to addition reactions, as illustrated in the following diagram (some comparable reactions of cyclohexene are shown in the green box). The LibreTexts libraries are Powered by MindTouch® and are supported by the Department of Education Open Textbook Pilot Project, the UC Davis Office of the Provost, the UC Davis Library, the California State University Affordable Learning Solutions Program, and Merlot. Clearly, the alkyl substituents activate the benzene ring in the nitration reaction, and the chlorine and ester substituents deactivate the ring. The reactions mentioned above belong to the category of halogenation reaction. Since a mono-substituted benzene ring has two equivalent ortho-sites, two equivalent meta-sites and a unique para-site, three possible constitutional isomers may be formed in such a substitution. Equivalent rate and product studies for other substitution reactions lead to similar conclusions. For example: With benzene:. Toluene gives 58.5% ortho-nitrotoluene, 37% para-nitrotoluene and only 4.5% of the meta isomer.
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