Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+- Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
- Toluene, p-H3C-C6H4-NO2, p-O2N-C6H4-NO2.
- Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene
- Toluene, p-H3C-C6H4-NO2, p-O2N-C6H4-NO2.
Solution and Explanation
Electrophiles are reagents that participate in a reaction by accepting an electron pair in order to bond to nucleophiles.
The higher the electron density on a benzene ring, the more reactive is the compound towards an electrophile, E+ (Electrophilic reaction).
(a) The presence of an electron withdrawing group (i.e., NO2- and Cl-) deactivates the aromatic ring by decreasing the electron density.
Since NO2- group is more electron withdrawing (due to resonance effect) than the Cl- group (due to inductive effect), the decreasing order of reactivity is as follows:
Chlorobenzene \(>\) p - nitrochlorobenzene \(>\) 2, 4 - dinitrochlorobenzene
(b) While CH3- is an electron donating group, NO2- group is electron withdrawing. Hence, toluene will have the maximum electron density and is most easily attacked by E+.
NO2- is an electron withdrawing group. Hence, when the number of NO2- substituents is greater, the order is as follows:
Toluene \(>\) p-CH3-C6H4-NO2, p -O2 N-C6H4-NO2
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Concepts Used:
Aromatic hydrocarbon
Aromatic hydrocarbons, sometimes known as arenes, are aromatic organic molecules made up entirely of carbon and hydrogen. In aromatic compounds a benzene ring which is named after the simple aromatic chemical benzene, or a phenyl group when part of a larger structure, is the configuration of six carbon atoms.
Read More: Aromaticity
Reactions of Aromatic Hydrocarbons:
1. Aromatic Substitution Reactions
This reaction involves the replacement of one substituent on the ring of an aromatic hydrocarbon, commonly a hydrogen atom, by a different substituent group.
The common types of aromatic substitution reactions are:
- Nucleophilic aromatic substitution reactions
- Electrophilic aromatic substitution reactions
- Radical nucleophilic aromatic substitution reactions
2. Coupling Reactions
In these types of reactions, the coupling of two fragments that have a radical nature is achieved with the help of a metal catalyst