Side Reactions in Organic Synthesis II: Aromatic Substitutions

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Chlorination and bromination are the most often practiced in the lab of the four possible halogenations. Halobenzenes are used for pesticides, as well as the precursors to other products.

Many COX-2 inhibitors contain halobenzene subunits. Some highly activated aromatic compounds, such as phenol and aniline, are reactive enough to undergo halogenation without a catalyst, but for typical benzene derivatives and benzene itself , the reactions are extremely slow at room temperature in the absence of a catalyst.

Side Reactions in Organic Synthesis II : Aromatic Substitutions

Usually, Lewis acids are used as catalysts, which work by helping to polarize the halogen-halogen bond, thus decreasing the electron density around one halogen atom, making it more electrophilic. Iron III bromide and iron III chloride lose their catalytic activity if they are hydrolyzed by any moisture present, including atmospheric water vapor. Therefore, they are generated in situ by adding iron fillings to bromine or chlorine. Iodination is carried out under different conditions: periodic acid is often used as a catalyst.

Iodination can also be accomplished using a diazonium reaction. Fluorination is most often done using this technique, as the use of fluorine gas is inconvenient and often fragments organic compounds. Halogenation of aromatic compounds differs from the additions to alkenes or the free-radical halogenations of alkanes, which do not require Lewis acid catalysts. The formation of the arenium ion results in the temporary loss of aromaticity, the overall result being that the reaction's activation energy is higher than those of halogenations of aliphatic compounds.

Halogenation of phenols is faster in polar solvents due to the dissociation of phenol, because the phenoxide -O - group is more strongly activating than hydroxyl itself. Aromatic sulfonation is an organic reaction in which a hydrogen atom on an arene is replaced by a sulfonic acid functional group in an electrophilic aromatic substitution.


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The electrophile of such a reaction is sulfur trioxide SO 3 , which can be released from oleum also known as fuming sulfuric acid , essentially sulfuric acid in which gaseous sulfur trioxide has been dissolved. In contrast to aromatic nitration and other electrophilic aromatic substitutions, aromatic sulfonation is reversible.

Sulfonation takes place in strongly acidic conditions, and desulfonation can occur on heating with a trace of acid. This also means that thermodynamic, rather than kinetic, control can be achieved at high temperatures.

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Hence, directive effects are not expected to play a key role in determining the proportions of isomeric products of high-temperature sulfonation. Aromatic sulfonic acids can be intermediates in the preparation of dyes and many pharmaceuticals. Sulfonation of aniline produces p-aminobenzenesulfonic acid or sulfanilic acid, which is a zwitterionic compound with an unusually high melting point.

The amide of this compound and related compounds form a large group of sulfa drugs a type of antibiotic. Nitration occurs with aromatic organic compounds via an electrophilic substitution mechanism involving the attack of the electron-rich benzene ring by the nitronium nitryl ion. The sulfuric acid is regenerated and hence acts as a catalyst. Selectivity is always a challenge in nitrations. Fluorenone nitration is selective and yields a tri-nitro compound or tetra-nitro compound by tweaking reaction conditions just slightly.

Electrophilic Aromatic Substitution Reactions Made Easy!

Another example of trinitration can be found in the synthesis of phloroglucinol. Other nitration reagents include nitronium tetrafluoroborate which is a true nitronium salt. This compound can be prepared from hydrogen fluoride, nitric acid and boron trifluoride. Aromatic nitro compounds are important intermediates for anilines; the latter may be readily prepared by action of a reducing agent. The Friedel-Crafts reactions, discovered by French alkaloid chemist Charles Friedel and his American partner, James Crafts, in , is either the alkylation or acylation of aromatic compounds catalyzed by a Lewis acid.

They are very useful in the lab for formation of carbon-carbon bonds between an aromatic nucleus and a side chain. Friedel-Crafts alkylation is an example of electrophilic substitution in aromatic compounds.

Side Reactions in Organic Synthesis II

Aromatic sulfonyl groups have a very interesting property. If treated with a strong enough acid in the absence of SO 3 , the sulfonic acid group can be removed. Does the product look familiar? Most of the time, this intermediate will just be deprotonated to regenerate the aromatic sulfonic acid.

However, in the case of SO 3 H, there is another pathway to restore aromaticity that is not energetically available in the case of most other substituents. In the presence of a high concentration of SO 3 , benzene would just attack protonated SO 3 again and re-form the sulfonic acid. Furthermore, if we vent the reaction say, by bubbling an inert gas like argon through the reaction mixture, which would eventually carry away any gaseous SO 3 along with it , then gaseous SO 3 will be slowly removed from the system.

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Answer in the comments. Note 1. This can abstract a hydrogen atom from nitroglycerin, and then — shabang! Mechanism here: back. What would happen if we try to introduce Cl- in p- methyl amino benzene? What possible difference may be there on trying to nitrate or sulfonate the same? On the other hand any procedure which requires acid will protonate the amine, resulting in an ammonium group.

What might be the major product in case of chlorination? Actually I had thought that Cl would go ortho to amino group, but one of our Chemistry faculties at school insists that Cl should go ortho to CH3 group, ie meta to NH2 in order to avoid steric repulsion with amino group… which would, in his opinion, decrease the resonance energy of the benzene ring, by sending NH2 out of the ring, ie.

Please help! This is incorrect. The repulsion difference between the amino and nitro group is marginal, so the electronic effects are predominant here and amino-group is a way stronger activating EDG than a methyl, so the substitution will go ortho to amine.


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Franck-Neumann and M. He spent one postdoctoral year in both the groups of A. Pfaltz University of Basel and A.

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Marchand University of North Texas , and then worked on the synthesis of unnatural amino acids at the Technical University of Dresden. No customer reviews. Share your thoughts with other customers.