Question

A dichlorobenzene which on nitration gives only one mononitro derivative is

Short answer

The given dichlorobenzene is para-dichlorobenzene, as it is the only arrangement of chlorine atoms on the benzene ring that results in only one mononitro derivative upon nitration.

Step-by-step solution

Understand Electrophilic Aromatic Substitution

Electrophilic aromatic substitution is a reaction where an electrophile (in this case, the nitro group) substitutes a hydrogen atom on an aromatic ring (benzene). The reaction of benzene with the nitro group requires the activation of the ring by another group, called a directing group. In our problem, the directing groups are the chlorine atoms.

Consider the Directing Effects of Chlorine

Chlorine is a deactivating ortho and para directing group, which means that when it is attached to a benzene ring, it directs the incoming electrophile to ortho (adjacent) and para (opposite) positions. In the case of a dichlorobenzene, we must examine the various possible positional arrangements of chlorine atoms on the benzene ring and determine which one will give only one mononitro derivative.

Evaluate the Various Positions of Chlorine

To determine the position of chlorines on benzene, we must evaluate the three possible dichlorobenzene arrangements: ortho-dichlorobenzene, meta-dichlorobenzene, and para-dichlorobenzene. 1. Ortho-dichlorobenzene: In this arrangement, the chlorine atoms are adjacent to each other. This arrangement would give three possible mononitro derivatives, corresponding to three available electrophilic attack positions (two ortho, one para). 2. Meta-dichlorobenzene: In this arrangement, the chlorine atoms are in meta position relative to each other. This arrangement would give two possible mononitro derivatives, corresponding to the two available electrophilic attack positions (para to both chlorines). 3. Para-dichlorobenzene: In this arrangement, the chlorine atoms are in the opposite position to each other. This arrangement would give only one mononitro derivative, corresponding to the single available electrophilic attack position (ortho to both chlorines).

Identify the Dichlorobenzene

Since we are looking for a dichlorobenzene that gives only one mononitro derivative upon nitration, the para-dichlorobenzene matches this criterion. The other arrangements (ortho and meta) would yield more than one mononitro derivative. Thus, the given dichlorobenzene is para-dichlorobenzene.

Key concepts

Dichlorobenzene Isomers

When discussing dichlorobenzene isomers, we are looking at the different ways two chlorine atoms can be arranged on a benzene ring. There are three main types of isomers for dichlorobenzene: ortho, meta, and para. These isomers differ based on the positioning of the chlorine atoms relative to each other on the benzene ring.

Understanding these positions:

• Ortho-dichlorobenzene: The two chlorine atoms are adjacent to each other. This is a 1,2-arrangement on the benzene ring.


• Meta-dichlorobenzene: The chlorine atoms are separated by one carbon atom, forming a 1,3-arrangement.


• Para-dichlorobenzene: The chlorine atoms are opposite each other, resulting in a 1,4-arrangement.

These isomeric forms affect the chemical properties and reactivity in specific reactions, such as electrophilic aromatic substitution reactions like nitration.

Directing Effects in Benzene

Directing effects in benzene are crucial in understanding how substituents influence the positioning of incoming groups. In electrophilic aromatic substitution, substituents already attached to a benzene ring can affect both the reactivity of the ring and the orientation of new substituents.

Key aspects:

• Substituents can be broadly classified into activating or deactivating groups based on how they affect the reactivity of the benzene ring. Activating groups increase reactivity, while deactivating groups decrease it.


• Substituents are also classified as ortho/para directors or meta directors depending on where they direct incoming electrophiles. Ortho/para directors push new groups to adjacent or opposite positions, while meta directors lead incoming groups to the third position away from themselves.


• Chlorine is a unique case as it is an ortho/para directing deactivating group. It directs electrophiles to ortho and para positions, yet decreases the overall reactivity because of its electronegative withdrawing nature.

These directing effects help predict the outcomes of reactions involving benzene and guide the synthesis of desired aromatic compounds.

Nitration Reaction

The nitration reaction is a classic example of electrophilic aromatic substitution where a nitro group ( O_2N ) is introduced to an aromatic ring, such as benzene. This reaction is typically performed using a mixture of concentrated nitric acid and sulfuric acid, which generates the nitronium ion ( NO_2^+ ) - the active electrophile in the reaction.

In the context of dichlorobenzene, understanding which isomer configuration leads to a specific nitration outcome is essential.

• For ortho-dichlorobenzene, the reaction would lead to three distinct mononitro derivatives because of multiple possible ortho and para positions.


• In meta-dichlorobenzene, two mononitro derivatives can form due to available para positions relative to each chlorine.


• Para-dichlorobenzene, however, offers only one nitration product. This is because there is only one position that is ortho to any chlorine and para to the other. Therefore, only one mononitro derivative results.

This means that among the isomers, para-dichlorobenzene specifically gives rise to a singular mononitro product upon nitration, exemplifying selective synthesis in aromatic chemistry.