Product obtained after nitration of nitrobenzene is. Explain why does nitro group nitrobenzene direct the incoming group to m-position? Electrophilic substitution in benzoic acid takes place at meta position why? Disclaimer The questions posted on the site are solely user generated, Doubtnut has no ownership or control over the nature and content of those questions. Latest Videos. By using this site you agree to the use of cookies.
HCl through the formation of intermediate nitrosobenzene and N-phenyl hydroxylamine. Nitrobenzene is first reduced to nitrosobenzene and then to N-phenyl hydroxylamine by the neutral reducing agent like Zn and aq. NH 4 Cl. Nitrobenzene undergoes the bimolecular reduction in basic medium to form successively azoxybenzene, azobenzene, and hydrazo benzene depending on the nature of the reducing agent used.
Electrophile: It is defined as an electron deficient species which accepts an electron pair from an electron rich center of a substrate. The reaction involving substitution of hydrogen of the benzene ring by the initial attack of an electrophile is called electrophilic aromatic substitution reaction which is common in the reaction of aromatic compounds. So, it is an electron rich system and is thus a good site for the attack of electrophile.
Nitrobenzene undergoes electrophilic aromatic substitution in which hydrogen is replaced by various electrophiles. Electrophilic aromatic substitution reaction in benzene occurs in meta position which is explained by resonance. Nitrobenzene may be represented by following resonating structures.
Because of the resonance effect, the nitro group -NO 2 withdraws electrons from the benzene ring and thus decreases electron density more at ortho and para position so that incoming electrophiles attack at electron rich meta position and is thus called meta director. Nitro group -NO 2 , being electron withdrawing group withdraws electrons from the benzene ring and thus deactivates it for further electrophilic aromatic substitution and is thus called ring deactivator.
Nitrobenzene undergoes electrophilic aromatic substitution like halogenation, nitration, sulphonation on metal positions. I Halogenation: The substitution of hydrogen of the benzene ring by the halogens is called halogenation.
Nitrobenzene undergoes bromination with bromination in the presence of Ferric bromide FeBr 3 to m-Bromo nitrobenzene. II Nitration: Nitrobenzene, on nitration with the mixture of conc. H 2 SO 4 form m- dinitrobenzene. Nitrobenzene undergoes sulphonation with conc. So let's go ahead and once again show our nitrobenzene and our nitronium ion. And this time we will do a meta attack. So if I want to show a nitro group adding onto the meta position I would once again use these pi electrons.
So nucleophilic attack pushes these electrons off. And so we're going to once again show the resulting carbocation. So we have a nitro group right here. And this time we're showing the nitro group adding on meta.
And once again there's a hydrogen attached to our ring. And these pi electrons here are forming the bonds between this carbon and our nitrogen, taking a bond away from this carbon. So that carbon gets a plus 1 formal charge. And we still have, of course, pi electrons in our ring. And so that's our first resonance structure. We can draw another one. I could take these pi electrons and move them over to here.
So let's go ahead and show the next resonance structure with our ring. Our nitro group here, our nitro group in the meta position, and hydrogen also attached to that carbon. Pi electrons here, and pi electrons have moved over to here. So let me highlight those. So these pi electrons have moved over to here, taking a bond away from this carbon. So we get a plus 1 formal charge here. We can draw another resonance structure taking these pi electrons and moving them over to here.
So let's go ahead and do that. We have our ring once again. We have a nitro group in the top carbon. We have a nitro group in the meta position. Once again, we have hydrogen, we have these pi electrons, and we now have moved the pi electrons over to here, so let me highlight those. So in red, these pi electrons have moved over to this position, taking a bond away from this carbon. So we get a plus 1 formal charge on that carbon. And so these are the three resonance structures that show up for a meta attack.
And notice, we don't have a destabilizing one. So in our three resonance structures, none of them have the two positive charges right next to each other as we saw in the previous example. So it's not so much that the sigma complex for a meta attack is extra stable.
It's just that the sigma complex for a meta attack doesn't have any destabilizing like charges repelling each other. And so because there is no destabilization the meta sigma complex becomes the most stable one and the one that's most likely to form in your mechanism.
The second step of alkene addition reactions proceeds by the first mode, and any of these three reactions may exhibit molecular rearrangement if an initial unstable carbocation is formed.
The carbocation intermediate in electrophilic aromatic substitution the benzenonium ion is stabilized by charge delocalization resonance so it is not subject to rearrangement. In principle it could react by either mode 1 or 2, but the energetic advantage of reforming an aromatic ring leads to exclusive reaction by mode 2 ie. Ring Substitution Reactions of Benzene Derivatives When substituted benzene compounds undergo electrophilic substitution reactions of the kind discussed above, two related features must be considered:.
The first is the relative reactivity of the compound compared with benzene itself. Experiments have shown that substituents on a benzene ring can influence reactivity in a profound manner. 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.
In contrast, a nitro substituent decreases the ring's reactivity by roughly a million. This activation or deactivation of the benzene ring toward electrophilic substitution may be correlated with the electron donating or electron withdrawing influence of the substituents, as measured by molecular dipole moments. 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.
The influence a substituent exerts on the reactivity of a benzene ring may be explained by the interaction of two effects:.
The first is the inductive effect of the substituent. Most elements other than metals and carbon have a significantly greater electronegativity than hydrogen. Consequently, substituents in which nitrogen, oxygen and halogen atoms form sigma-bonds to the aromatic ring exert an inductive electron withdrawal, which deactivates the ring left-hand diagram below. The second effect is the result of conjugation of a substituent function with the aromatic ring.
This conjugative interaction facilitates electron pair donation or withdrawal, to or from the benzene ring, in a manner different from the inductive shift. Finally, polar double and triple bonds conjugated with the benzene ring may withdraw electrons, as in the right-hand diagram.
Note that in the resonance examples all the contributors are not shown. In both cases the charge distribution in the benzene ring is greatest at sites ortho and para to the substituent.
In the case of the nitrogen and oxygen activating groups displayed in the top row of the previous diagram, electron donation by resonance dominates the inductive effect and these compounds show exceptional reactivity in electrophilic substitution reactions. 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.
Alkyl substituents such as methyl increase the nucleophilicity of aromatic rings in the same fashion as they act on double bonds. The second factor that becomes important in reactions of substituted benzenes concerns the site at which electrophilic substitution occurs.
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.
Again we find that the nature of the substituent influences this product ratio in a dramatic fashion. Bromination of nitrobenzene requires strong heating and produces the meta-bromo isomer as the chief product. Some additional examples of product isomer distribution in other electrophilic substitutions are given in the table below. 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 C 6 H 5 -Y.
The high reactivity of anisole, for example, requires that the first two reactions be conducted under very mild conditions low temperature and little or no catalyst.
The nitrobenzene reactant in the third example is very unreactive, so rather harsh reaction conditions must be used to accomplish that reaction. 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. Thus, substituents that activate the benzene ring toward electrophilic attack generally direct substitution to the ortho and para locations.
With some exceptions, such as the halogens, deactivating substituents direct substitution to the meta location. The following table summarizes this classification. 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.
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 first thing to recognize is that the proportions of ortho, meta and para substitution in a given case reflect the relative rates of substitution at each of these sites.
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. Since there are six equivalent carbons in benzene, the total rate would be 6. If we examine the nitration of toluene, tert-butylbenzene, chlorobenzene and ethyl benzoate in the same manner, we can assign relative rates to the ortho, meta and para sites in each of these compounds.
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.
The overall relative rates of reaction, referenced to benzene as 1. Clearly, the alkyl substituents activate the benzene ring in the nitration reaction, and the chlorine and ester substituents deactivate the ring.
From rate data of this kind, it is a simple matter to calculate the proportions of the three substitution isomers. Toluene gives Equivalent rate and product studies for other substitution reactions lead to similar conclusions.
The manner in which specific substituents influence the orientation of electrophilic substitution of a benzene ring is shown in the following interactive diagram. 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.
It is not surprising, therefore, that there is a rough correlation between the rate-enhancing effect of a substituent and its site directing influence.
The exact influence of a given substituent is best seen by looking at its interactions with the delocalized positive charge on the benzenonium intermediates generated by bonding to the electrophile at each of the three substitution sites. This can be done for seven representative substituents by using the selection buttons underneath the diagram. In the case of alkyl substituents, charge stabilization is greatest when the alkyl group is bonded to one of the positively charged carbons of the benzenonium intermediate.
This happens only for ortho and para electrophilic attack, so such substituents favor formation of those products. Interestingly, primary alkyl substituents, especially methyl, provide greater stabilization of an adjacent charge than do more substituted groups note the greater reactivity of toluene compared with tert-butylbenzene.
Structures in which like-charges are close to each other are destabilized by charge repulsion, so these substituents inhibit ortho and para substitution more than meta substitution.
Consequently, meta-products preominate when electrophilic substitution is forced to occur. Halogen X , OR and NR 2 substituents all exert a destabilizing inductive effect on an adjacent positive charge, due to the high electronegativity of the substituent atoms. By itself, this would favor meta-substitution; however, these substituent atoms all have non-bonding valence electron pairs which serve to stabilize an adjacent positive charge by pi-bonding, with resulting delocalization of charge.
Consequently, all these substituents direct substitution to ortho and para sites. The conditions commonly used for the aromatic substitution reactions discussed here are repeated in the table on the right.
The electrophilic reactivity of these different reagents varies. Also, as noted earlier, toluene undergoes nitration about 25 times faster than benzene, but chlorination of toluene is over times faster than that of benzene.
From this we may conclude that the nitration reagent is more reactive and less selective than the halogenation reagents. Both sulfonation and nitration yield water as a by-product. This does not significantly affect the nitration reaction note the presence of sulfuric acid as a dehydrating agent , but sulfonation is reversible and is driven to completion by addition of sulfur trioxide, which converts the water to sulfuric acid. The reversibility of the sulfonation reaction is occasionally useful for removing this functional group.
The Friedel-Crafts acylation reagent is normally composed of an acyl halide or anhydride mixed with a Lewis acid catalyst such as AlCl 3. Such electrophiles are not exceptionally reactive, so the acylation reaction is generally restricted to aromatic systems that are at least as reactive as chlorobenzene.
Carbon disulfide is often used as a solvent, since it is unreactive and is easily removed from the product.
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