Question

Which of the following is the least reactive in electrophilic substitution? (1) Aniline (2) Nitrobenzene (3) Aniline hydrochloride (4) Acetanilide

Short answer

Aniline hydrochloride is the least reactive in electrophilic substitution.

Step-by-step solution

- Understand Electrophilic Substitution

Electrophilic substitution is a reaction where an electrophile replaces a hydrogen atom in an aromatic ring. The reactivity in electrophilic substitution depends on the electron density of the aromatic ring, which is influenced by the substituents attached to it.

- Analyze the Substituents

Examine the substituents on each of the given compounds: (1) Aniline: -NH2 group (electron-donating)(2) Nitrobenzene: -NO2 group (electron-withdrawing)(3) Aniline hydrochloride: -NH3+ group (electron-withdrawing)(4) Acetanilide: -NHCOCH3 group (moderately electron-donating)

- Compare Electron-Density Effects

Electron-donating groups (EDGs) increase electron density on the aromatic ring, making it more reactive towards electrophiles. Electron-withdrawing groups (EWGs), on the other hand, decrease electron density, making the ring less reactive. The order of decreasing reactivity is: Aniline (NH2) > Acetanilide (NHCOCH3) > Nitrobenzene (NO2) > Aniline hydrochloride (NH3+).

- Determine the Least Reactive Compound

Based on the effects of substituents: - Aniline is highly reactive due to the strong electron-donating effect of the NH2 group.- Acetanilide is also reactive but less so than aniline because the NHCOCH3 group donates electrons less strongly.- Nitrobenzene is less reactive due to the electron-withdrawing NO2 group.- Aniline hydrochloride is the least reactive because the NH3+ group strongly withdraws electron density from the ring.

Key concepts

electron-donating groups

Electron-donating groups (EDGs) are substituents that increase the electron density around the aromatic ring. They do this by donating electrons through various mechanisms.
One common mechanism is resonance, where the group donates electron pairs into the ring. Another way is through inductive effects, where electronegative atoms attached to the group pull electron density.
Some examples of electron-donating groups include:

• -NH2 (amino group)
• -OH (hydroxyl group)
• -OCH3 (methoxy group)

***Example: Aniline***
Aniline has an -NH2 group, which is a strong electron-donating group. Because of this, the electron density on the aromatic ring increases, making it highly reactive in electrophilic substitution reactions.
Increased electron density makes it easier for the electrophile to find and react with the aromatic ring.
Matching the high reactivity makes aniline very useful in many chemical processes that require fast reactions.

electron-withdrawing groups

Electron-withdrawing groups (EWGs) decrease the electron density on the aromatic ring. They do this by pulling electrons away from the ring, through either resonance or inductive effects.
These groups make the ring less attractive to electrophiles, thus reducing its reactivity.
Common examples of electron-withdrawing groups include:

• -NO2 (nitro group)
• -CN (cyano group)
• -COOH (carboxyl group)

***Example: Nitrobenzene***
Nitrobenzene has an -NO2 group, which is a strong electron-withdrawing group. The nitro group significantly reduces the electron density on the aromatic ring, making it less reactive towards electrophiles.
This decreased reactivity is important in many applications where stability rather than reactivity is desired.
***Example: Aniline Hydrochloride***
Aniline hydrochloride has an -NH3+ group, which also withdraws electron density from the ring. This makes aniline hydrochloride even less reactive in electrophilic substitution than nitrobenzene.
Understanding how different groups affect electron density is crucial for predicting and controlling reaction outcomes.

aromatic substitution

Aromatic substitution is a type of reaction where one substituent on an aromatic ring is replaced by another. In electrophilic aromatic substitution (EAS), the substituent being replaced is a hydrogen atom.
The general mechanism involves the following steps:

• Step 1: Formation of the electrophile
• Step 2: Attack of the electrophile on the aromatic ring, forming a sigma complex
• Step 3: Loss of a proton (reformation of the aromatic system)

### Factors Affecting ReactivityReactivity in electrophilic aromatic substitution is primarily affected by the nature of the substituents already present on the ring. Electron-donating groups increase reactivity by stabilizing the positively charged intermediate formed during the reaction.
On the other hand, electron-withdrawing groups decrease reactivity by destabilizing this intermediate
### Example ComparisonConsider the following compounds and their reactivity in electrophilic substitution:

• Aniline (very reactive due to -NH2 group)
• Acetanilide (moderately reactive due to -NHCOCH3 group)
• Nitrobenzene (less reactive due to -NO2 group)
• Aniline hydrochloride (least reactive due to -NH3+ group)

Understanding these concepts is essential for predicting how different aromatic compounds will behave in chemical reactions.
This knowledge can be applied to design syntheses and to choose the best conditions for a given reaction.