Which electrophilic substitution is reversible

In the eclipsed conformation of ethane, the dihedral angle between the hydrogen atoms of adjacent methyl groups is KEAM In the following reaction, major product obtained is AMU Total number of optically active compounds obtained when n-pentane is subjected to monochlorination is KEAM Which one of the following conformation of cyclohexane is chiral?

AIIMS Chemistry Most Viewed Questions. Identify a molecule which does not exist. Identify the incorrect match. Which of the following set of molecules will have zero dipole moment?

On electrolysis of dil. Because sulfonation is a reversible reaction, it can also be used in further substitution reactions in the form of a directing blocking group because it can be easily removed. The sulfonic group blocks the carbon from being attacked by other substituents and after the reaction is completed it can be removed by reverse sulfonation.

Benzenesulfonic acids are also used in the synthesis of detergents, dyes, and sulfa drugs. Bezenesulfonyl Chloride is a precursor to sulfonamides, which are used in chemotherapy. Draw an energy diagram for the nitration of benzene. Draw the intermediates, starting materials, and products. Label the transition states. For questions 1 and 2 see Electrophilic Aromatic Substitution for hints. Sulfuric acid is needed in order for a good electrophile to form.

Sulfuric acid protonates nitric acid to form the nitronium ion water molecule is lost. The nitronium ion is a very good electrophile and is open to attack by benzene. Without sulfuric acid the reaction would not occur. Nitration of Benzene The source of the nitronium ion is through the protonation of nitric acid by sulfuric acid, which causes the loss of a water molecule and formation of a nitronium ion.

Sulfuric Acid Activation of Nitric Acid The first step in the nitration of benzene is to activate HNO 3 with sulfuric acid to produce a stronger electrophile, the nitronium ion. Mechanism Resonance forms of the intermediate can be seen in the generalized electrophilic aromatic substitution.

Sulfonation of Benzene Sulfonation is a reversible reaction that produces benzenesulfonic acid by adding sulfur trioxide and fuming sulfuric acid.

Mechanism To produce benzenesulfonic acid from benzene, fuming sulfuric acid and sulfur trioxide are added. 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 predominate 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. If the substrate is a very reactive benzene derivative, such as anisole, carboxylic esters or acids may be the source of the acylating electrophile.

Some examples of Friedel-Crafts acylation reactions are shown in the following diagram. The first demonstrates that unusual acylating agents may be used as reactants. The second makes use of an anhydride acylating reagent, and the third illustrates the ease with which anisole reacts, as noted earlier.

The H 4 P 2 O 7 reagent used here is an anhydride of phosphoric acid called pyrophosphoric acid. Finally, the fourth example illustrates several important points. Since the nitro group is a powerful deactivating substituent, Friedel-Crafts acylation of nitrobenzene does not take place under any conditions.

However, the presence of a second strongly-activating substituent group permits acylation; the site of reaction is that favored by both substituents. A common characteristic of the halogenation, nitration, sulfonation and acylation reactions is that they introduce a deactivating substituent on the benzene ring.

As a result, we do not normally have to worry about disubstitution products being formed. Friedel-Crafts alkylation, on the other hand, introduces an activating substituent an alkyl group , so more than one substitution may take place.