The periodic table—the transition metals, Topic 11: Measurement and data processing, 3. Benzene reacts with acid chloride or acid anhydride and give acyl benzene in the presence of anhydrous AlCl 3. By the means of electrophilic aromatic substitution reaction, one hydrogen atom of the arene is substituted by one halogen atom. We have already analyzed the activating or deactivating properties of substituents in terms of inductive and resonance effects, and these same factors may be used to rationalize their influence on substitution orientation. The reversibility of the sulfonation reaction creates an opportunity to prepare deuterated benzene. Three canonical resonance contributors may be drawn, and are displayed in the following diagram. and methylbenzene: These reactions destroy the electron delocalisation in the original benzene ring, because those electrons are being used to form bonds with the new hydrogen atoms. Consequently, meta-products predominate when electrophilic substitution is forced to occur. Bromination of nitrobenzene requires strong heating and produces the meta-bromo isomer as the chief product. It is important to note here that the reaction conditions for these substitution reactions are not the same, and must be adjusted to fit the reactivity of the reactant C6H5-Y. Steamy fumes of hydrogen chloride is also observed. The General Mechanism for Electrophilic Aromatic Substitution: • The rate determining step(R.D.S.) It is not surprising, therefore, that there is a rough correlation between the rate-enhancing effect of a substituent and its site directing influence. These relative rates are shown (colored red) in the following illustration, and the total rate given below each structure reflects the 2 to 1 ratio of ortho and meta sites to the para position. Nitro (NO2), sulfonic acid (SO3H) and carbonyl (C=O) substituents have a full or partial positive charge on the atom bonded to the aromatic ring. Finally, the benzoic ester gave predominantly the meta-nitro product (73%) accompanied by the ortho (22%) and para (5%) isomers, as shown by the relative rates. The presence of the unpaired electrons that can be donated to the ring, stabilize the carbocation in the transition state. Halogen (X), OR and NR2 substituents all exert a destabilizing inductive effect on an adjacent positive charge, due to the high electronegativity of the substituent atoms. The manner in which specific substituents influence the orientation of electrophilic substitution of a benzene ring is shown in the following interactive diagram. In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent. The overall relative rates of reaction, referenced to benzene as 1.0, are calculated by dividing by six. Halogenation of Benzene. The reaction of a substituted ring with an activating group is faster than benzene. 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). Common benzene reactions are Nitration of Benzene. Activating groups speed up the reaction because of the resonance effect. A hydrogen ion is expelled from the ring by AlCl4⁻ and leaving its electrons in the The delocalised electron system is now restored. Since methyl group activates the ring, making the ring more reactive, the temperature has to be lowered to 30 ºC to prevent multiple substitutions, Methyl group is an electron-donating group, it activates the ring and is 2,4-directing, therefore the nitro group is substituted at the 2 or 4 positions positions. • As we will see, there are many reactions, depending upon the particular electrophile, they all use the same mechanism. The second effect is the result of conjugation of a substituent function with the aromatic ring. Finally, polar double and triple bonds conjugated with the benzene ring may withdraw electrons, as in the right-hand diagram. With some exceptions, such as the halogens, deactivating substituents direct substitution to the meta location. The three examples on the left of the bottom row (in the same diagram) are examples of electron withdrawal by conjugation to polar double or triple bonds, and in these cases the inductive effect further enhances the deactivation of the benzene ring. Benzene reacts with concentrated nitric acid at 323-333k in the presence of concentrated sulphuric acid to form nitrobenzene. Formulae, stoichiometry and the mole concept, 7. Again we find that the nature of the substituent influences this product ratio in a dramatic fashion. Compounds in which two or more benzene rings are fused together were described in an earlier section, and they present interesting insights into aromaticity and reactivity. Clearly, the alkyl substituents activate the benzene ring in the nitration reaction, and the chlorine and ester substituents deactivate the ring. Note that in the resonance examples all the contributors are not shown. If the temperature exceeds 50 ºC, 1,3-dinitrobenzene will be formed as Notice that the second nitro group is added to the 3 position of the ring. If we use the nitration of benzene as a reference, we can assign the rate of reaction at one of the carbons to be 1.0. This is because it involves breaking the delocalised electron system and thus losing its stability. Since there are six equivalent carbons in benzene, the total rate would be 6.0. A cycloalkane is formed. In the following diagram we see that electron donating substituents (blue dipoles) activate the benzene ring toward electrophilic attack, and electron withdrawing substituents (red dipoles) deactivate the ring (make it less reactive to electrophilic attack). 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. ctivating Substituents ortho & para-Orientation, Deactivating Substituents meta-Orientation, Deactivating Substituents ortho & para-Orientation. The nitrobenzene reactant in the third example is very unreactive, so rather harsh reaction conditions must be used to accomplish that reaction. This reaction is known as nitration of benzene. Although halogen atoms have non-bonding valence electron pairs that participate in p-π conjugation, their strong inductive effect predominates, and compounds such as chlorobenzene are less reactive than benzene. The destruction of the aromatic sextet causes this endothermicity. 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. For example: With benzene:. Missed the LibreFest? The most common reactions of benzene involve substitution of a proton by other groups. Get exclusive access to content from our 1768 First Edition with your subscription. Experiments have shown that substituents on a benzene ring can influence reactivity in a profound manner. Reactions of Benzene & Its Derivatives Chapter 22 Organic Lecture Series 2 Reactions of Benzene The most characteristic reaction of aromatic compounds is substitution at a ring carbon: + + Chlorobenzene Halogenation: H Cl2 Cl FeCl3 HCl + + Nitrobenzene Nitration: HNOHNO3 2 H2 SO4 H2 O This happens only for ortho and para electrophilic attack, so such substituents favor formation of those products. This conjugative interaction facilitates electron pair donation or withdrawal, to or from the benzene ring, in a manner different from the inductive shift. Aluminum bromide is used when benzene reacting bromide. Water is added to isolate the acyl benzene final product. Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. . However, each time a nitro group substitutes, the rate of reaction of the next substitution decreases because nitro group is electron-withdrawing. The delocalised electron system is partially broken, Hydrogen ion is expelled and it bonds with HSO4⁻ to regenerate the catalyst. Sulfonation of benzene is a reversible reaction. Toluene gives 58.5% ortho-nitrotoluene, 37% para-nitrotoluene and only 4.5% of the meta isomer. Therefore, 2,4,6-trinitromethylbenzene is rare. If the atom bonded to the ring has one or more non-bonding valence shell electron pairs, as do nitrogen, oxygen and the halogens, electrons may flow into the aromatic ring by p-π conjugation (resonance), as in the middle diagram. The information summarized in the above table is very useful for rationalizing and predicting the course of aromatic substitution reactions, but in practice most chemists find it desirable to understand the underlying physical principles that contribute to this empirical classification. 1 st Reaction. This addition reaction is not observed under normal reaction conditions. 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). We also acknowledge previous National Science Foundation support under grant numbers 1246120, 1525057, and 1413739. When substituted benzene compounds undergo electrophilic substitution reactions, two related features must be considered. Reactions of Fused Benzene Rings.
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