Which of the following compounds will not undergo Azo coupling reaction?
Nitrobenzene
Understanding Azo Coupling Reactions
Azo coupling is a type of electrophilic aromatic substitution reaction where a diazonium cation acts as the electrophile and attacks an activated aromatic ring. For the reaction to occur, the aromatic ring needs to be activated by the presence of electron-donating groups (EDGs). These groups increase the electron density of the aromatic ring, making it more susceptible to electrophilic attack, especially at the ortho and para positions relative to the activating group.
Analyzing Compounds for Azo Coupling Potential
Let's examine each given compound to determine if it has the necessary activation for Azo coupling reaction:
- Aniline ($\text{C}_6\text{H}_5\text{NH}_2$): Aniline has an amino group ($\text{-NH}_2$) attached to the benzene ring. The amino group is a strong electron-donating group due to the resonance effect and inductive effect (to a lesser extent). It activates the benzene ring towards electrophilic substitution, particularly at the ortho and para positions. Therefore, aniline readily undergoes Azo coupling.
- Phenol ($\text{C}_6\text{H}_5\text{OH}$): Phenol has a hydroxyl group ($\text{-OH}$) attached to the benzene ring. The hydroxyl group is also a strong electron-donating group due to resonance. It activates the benzene ring towards electrophilic substitution, primarily at the ortho and para positions. Thus, phenol participates in Azo coupling.
- Anisole ($\text{C}_6\text{H}_5\text{OCH}_3$): Anisole has a methoxy group ($\text{-OCH}_3$) attached to the benzene ring. The methoxy group is an electron-donating group due to the resonance effect (though slightly weaker than $-\text{NH}_2$ or $-\text{OH}$). It activates the benzene ring towards electrophilic substitution, mainly at the ortho and para positions. Anisole also undergoes Azo coupling.
- Nitrobenzene ($\text{C}_6\text{H}_5\text{NO}_2$): Nitrobenzene has a nitro group ($\text{-NO}_2$) attached to the benzene ring. The nitro group is a strong electron-withdrawing group due to both resonance and inductive effects. It deactivates the benzene ring towards electrophilic substitution and directs incoming electrophiles to the meta position. Since Azo coupling is an electrophilic aromatic substitution that requires an activated ring (and typically occurs at ortho/para positions), nitrobenzene will not undergo this reaction under normal conditions.
Identifying the Compound That Will Not Undergo Azo Coupling
Based on the analysis of the substituents and their activating/deactivating effects on the benzene ring, nitrobenzene, having a strongly deactivating nitro group, will not undergo the Azo coupling reaction.
| Compound | Substituent | Electronic Effect | Activation/Deactivation | Azo Coupling |
|---|---|---|---|---|
| Aniline | $-\text{NH}_2$ | Strong EDG (Resonance) | Strong Activation | Yes |
| Phenol | $-\text{OH}$ | Strong EDG (Resonance) | Strong Activation | Yes |
| Anisole | $-\text{OCH}_3$ | EDG (Resonance) | Activation | Yes |
| Nitrobenzene | $-\text{NO}_2$ | Strong EWG (Resonance + Inductive) | Strong Deactivation | No |
Revision Table: Substituent Effects on Electrophilic Aromatic Substitution
| Substituent Type | Effect on Reactivity | Directing Effect | Examples |
|---|---|---|---|
| Activating (Ortho/Para Directors) | Increase reactivity | Ortho and Para | $-\text{NH}_2$, $-\text{NR}_2$, $-\text{OH}$, $-\text{OR}$, $-\text{NHCOR}$, Alkyl groups ($\text{-R}$) |
| Deactivating (Ortho/Para Directors) | Decrease reactivity | Ortho and Para | Halogens ($\text{-F}$, $\text{-Cl}$, $\text{-Br}$, $\text{-I}$) |
| Deactivating (Meta Directors) | Decrease reactivity significantly | Meta | $-\text{NO}_2$, $-\text{SO}_3\text{H}$, $-\text{CN}$, $-\text{CHO}$, $-\text{COR}$, $-\text{COOH}$, $-\text{COOR}$, $-\text{NR}_3^+$ |
Additional Information: Azo Coupling Details
Azo coupling reaction is the reaction between a diazonium salt and another aromatic compound (the coupling component) which must be activated by an electron-donating group. The reaction typically takes place under slightly acidic, neutral, or alkaline conditions, depending on the coupling component.
- Diazonium Salt: A diazonium salt is formed by treating a primary aromatic amine with nitrous acid ($\text{HNO}_2$, usually generated from $\text{NaNO}_2$ and $\text{HCl}$) at low temperatures (0-5 °C). This process is called diazotization. The diazonium cation ($\text{Ar-N}_2^+$) acts as the electrophile.
- Coupling Component: This is the aromatic compound that reacts with the diazonium salt. It must contain a strong electron-donating group like $-\text{OH}$, $-\text{NR}_2$, $-\text{NH}_2$, $-\text{OR}$, etc. The coupling usually occurs at the para position if available, and if not, at an ortho position to the activating group.
- Product: The product of Azo coupling is an azo compound, which contains the azo group ($\text{-N}=\text{N}-$) linking the two aromatic rings ($\text{Ar}_1\text{-N}=\text{N-Ar}_2$). Azo compounds are often brightly colored and are used as dyes and pigments.
Since nitrobenzene has a deactivating group, it cannot act as the coupling component in an Azo coupling reaction.
Important Questions from The d-and f-block Elements
Which of the following is incorrect?
Increasing order of oxidation states of transition metal oxides will be:
(A) TiO₂
(B) MnO-₄
(C) VO₂⁺
(D) CrO₄²⁻
(E) Ni (CO)₄
Choose the correct answer from the options given below:
Sulphate of magnesium of the following is:
Indium is mainly refined by:
Which one is the correct electronic configuration of sulphur?
