Question

Certain activated benzene rings can be chlorinated by hypochlorous acid, HOCL, and this reaction is catalyzed by \(\mathrm{H}^{+}\). Can you suggest a possible function of \(\mathrm{H}^{+}\) ?

Short answer

The possible function of \(\mathrm{H}^{+}\) (a proton) in the chlorination reaction of an activated benzene ring using hypochlorous acid (HOCl) is to protonate the oxygen atom in the hypochlorous acid, making it a better electrophile (an electron-seeking species): \[ \mathrm{H}^{+} + \mathrm{HOCl} \rightarrow \mathrm{HOClH}^{+} \] The protonated hypochlorous acid, \(\mathrm{HOClH}^{+}\), is more attractive to the electron-rich activated benzene ring, leading to an electrophilic substitution reaction, where the activated benzene ring donates its electrons to form a bond with the chlorine atom and releases the proton back into the medium. This catalytic role of the proton helps increase the rate of the chlorination reaction.

Step-by-step solution

Understand the reaction and its components

First, let's get familiar with the main components of the reaction. An activated benzene ring is a benzene molecule that has one or more activating groups, which makes it more reactive than benzene itself. Hypochlorous acid (HOCl) is a weak, unstable acid and a good source of chlorine atoms. The presence of a proton (\(\mathrm{H}^{+}\)) is said to catalyze this reaction.

Propose a possible role of the proton in the reaction mechanism

A catalyst is a substance that speeds up the rate of a reaction without being consumed in the process. In this case, the proton (\(\mathrm{H}^{+}\)) is acting as a catalyst. The possible role of \(\mathrm{H}^{+}\) might be to stabilize the reaction intermediates, making it easier for the reaction to proceed.

Determine the point in the reaction mechanism where the proton plays a role

One possible function of the proton is to protonate the oxygen atom in the hypochlorous acid (HOCl), making it a better electrophile (an electron-seeking species): \[ \mathrm{H}^{+} + \mathrm{HOCl} \rightarrow \mathrm{HOClH}^{+} \]

Explain how the catalytic role of the proton helps the electrophilic substitution reaction

The protonated hypochlorous acid, \(\mathrm{HOClH}^{+}\), is a better electrophile due to its increased positive charge and, as a result, the electron-rich activated benzene ring is more attracted to it. This leads to an electrophilic substitution reaction, where the activated benzene ring donates its electrons to form a bond with the chlorine atom of hypochlorous acid, while the oxygen-hydrogen moiety leaves (with the proton returning to the medium): \[ \text{Activated benzene ring} + \mathrm{HOClH}^{+} \rightarrow \text{Chlorinated benzene} + \mathrm{H}_{2}\mathrm{O} + \mathrm{H}^{+} \] This catalytic role of the proton helps facilitate this electrophilic substitution reaction and thus increases the rate at which the benzene undergoes chlorination.

Key concepts

Activated Benzene

An activated benzene is a benzene molecule that is out of the ordinary in its reactivity. Benzene, as a ring of carbon atoms connected by alternating double bonds, is generally stable. However, when benzene has one or more substituents attached, known as activating groups, it becomes more reactive. These groups can donate electrons into the benzene ring. This donation enhances the electron density of the ring.
This increased electron density makes activated benzene more likely to participate in reactions. Such activities are specifically called electrophilic aromatic substitutions. This is because the benzene focuses its high electron density on attracting positively charged species, i.e., electrophiles, during the reaction. The more electron-donating groups a benzene ring has, the more activated it becomes. Therefore, it is more inclined towards participating in chemical reactions like the chlorination mentioned in the exercise.

Catalysis in Organic Reactions

In organic chemistry, catalysis is a fascinating tool. It represents the idea of speeding up reactions without the catalyst consuming itself. Catalysts, like acids or bases, help in lowering the energy barrier of a reaction. This means that the reaction takes place faster than it normally would.
In the case of the activated benzene reacting with hypochlorous acid, ext{H}^{+} ions play the role of a catalyst. They accelerate the process of chlorination. Catalysis is crucial because it makes reactions feasible under milder conditions. Without a catalyst, reactions would require more energy input and possibly harsher conditions to occur at an accelerated rate. This elegant technique allows chemists to save time and resources while conducting reactions more efficiently.
In this scenario, the presence of ext{H}^{+} helps the hypochlorous acid become a better electrophile quickly, enabling the activated benzene to react with it swiftly.

Reaction Mechanism

A reaction mechanism is like a roadmap showing how molecules transform during a chemical reaction. It provides detailed steps from reactants to products, highlighting the conversion process in stages. Understanding mechanisms allows chemists to predict how changes in conditions or structures can affect the course of a reaction.
In the chlorination of activated benzene, the reaction mechanism involves several key stages. First, the proton ( ext{H}^{+}) protonates hypochlorous acid, converting it into a more active form:

1. Protonation: \ ext{H}^{+}\( + \)HOCl \rightarrow HOClH ext{+}\
2. Electrophile Attack: The activated benzene, rich in electrons, attacks the chlorinated species resulting in electrophilic substitution.

This stepwise process ensures a logical transformation where each stage builds up to the formation of the final chlorinated benzene product. Clear mechanisms are fundamental to understand reactions, allowing chemists to alter or improve these processes for practical uses.

Electrophiles

Electrophiles are chemical species that crave electrons. As electron-seeking entities, they play a leading role in reactions involving electron-rich counterparts, like nucleophiles. In the context of electrophilic aromatic substitution, electrophiles are attracted to benzene derived substrates.
In this exercise, hypochlorous acid (HOCl) becomes a better electrophile when it gets combined with a proton ( ext{H}^{+} ). This enhances its positive nature, making it more appealing to the electron-dense activated benzene ring.
The hypochlorous acid molecule is unstable alone, but when protonated it becomes the HOClH ext{+} ion. This ion then functions as the electrophile, ready to accept electrons and react with the activated benzene, thereby forming chlorinated benzene. Hence, understanding electrophiles can help comprehend the driving forces of chemical reactions and appreciate why substrates like benzene willingly give up their electrons to these attracting species.

Protonation

Protonation is the process of adding a proton (H⁺) to an atom, molecule, or ion. It is a simplified version of the acid-base reaction. This addition typically affects a molecule's reactivity dramatically.
In many cases, such as the chlorination of benzene, protonation can transform a species into a stronger electrophile or facilitate other reaction steps. When hypochlorous acid (HOCl) encounters ext{H}^{+} , it becomes protonated, forming HOClH ext{+} .
This protonated form is more positive in charge and thus more reactive. It interacts efficiently with electron-rich substrates like activated benzene, establishing a pathway to new bonds forming. Protonation essentially primes a molecule for reaction, enabling or speeding up chemical processes where otherwise inert substances could participate. Thus, grasping the concept of protonation can reveal a lot about how and why certain molecules behave in reactions.