The correct order of the following compounds showing increasing tendency towards nucleophilic substi

Q: Why doesn't SN1 or SN2 happen here?

Aryl halides don't undergo SN1 or SN2 due to the $sp^2$ hybridization of the carbon atom and the partial double bond character of the C-X bond, which makes it very hard to break.

Q: Why are ortho and para positions special?

When the EWG is at the ortho or para position, the resonance structures of the intermediate place the negative charge directly on the carbon attached to the nitro group, allowing for maximum stabilization.

Q: Does the meta position help?

A meta-nitro group helps slightly through the $-I$ (inductive) effect, but it cannot stabilize the intermediate through the much stronger $-R$ (resonance) effect.

Q: What if we had an electron-donating group?

An electron-donating group (like $-CH_3$ or $-OH$) would push electron density into the ring, destabilizing the negatively charged intermediate and slowing down the reaction.

Q: Why is F a better leaving group than Cl in SNAr?

In $S_N\text{Ar}$, the rate-determining step is the attack of the nucleophile, not the departure of the leaving group. Fluorine's high electronegativity makes the attached carbon much more electrophilic, speeding up the attack.

Question

The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is

Options:

(A) (iv) < (iii) < (ii) < (i)

(B) (iv) < (i) < (ii) < (iii)

(C) (iv) < (i) < (iii) < (ii)

(D) (i) < (ii) < (iii) < (iv)

Sigma Percentile · Accepted Answer

### The Challenge of Aromatic Substitution
Normally, benzene rings are highly electron-rich and repel nucleophiles. Furthermore, the carbon-halogen bond in haloarenes acquires a partial double bond character due to resonance. This makes it incredibly difficult to break the bond and perform a nucleophilic substitution.

### The Magic of Electron-Withdrawing Groups
However, if we attach an electron-withdrawing group (EWG) like a nitro group ($-NO_2$) to the benzene ring, the scenario changes dramatically. The $-NO_2$ group pulls electron density away from the ring through both the inductive effect ($-I$) and the resonance effect ($-R$).

When a nucleophile attacks the carbon bearing the halogen, a negatively charged intermediate called the **Meisenheimer complex** is formed. The rate of the reaction depends entirely on the stability of this carbanion intermediate.

### Position Matters: Ortho and Para
The stabilization provided by the $-NO_2$ group is most effective when it is located at the **ortho** or **para** positions relative to the halogen. In these positions, the negative charge of the intermediate can be delocalized directly onto the oxygen atoms of the nitro group.

### Analyzing the Compounds
Let's evaluate our four compounds based on this principle:
1. **Compound (i)** is simple chlorobenzene. It has no electron-withdrawing groups to stabilize the intermediate. It will be the slowest to react.
2. **Compound (ii)** has one $-NO_2$ group at the para position. This provides significant stabilization, making it faster than chlorobenzene.
3. **Compound (iii)** has two $-NO_2$ groups (one ortho, one para). The combined electron-withdrawing effect of two groups makes the intermediate even more stable, further increasing the reaction rate.
4. **Compound (iv)** is 2,4,6-trinitrochlorobenzene. It boasts three $-NO_2$ groups (two ortho, one para). This provides maximum stabilization to the carbanion intermediate, making it the most reactive of the bunch.

### The Final Verdict
By simply counting the number of strongly electron-withdrawing $-NO_2$ groups at the ortho and para positions, we can confidently determine the order of reactivity. The correct increasing order is **(i) < (ii) < (iii) < (iv)**.

This perfectly matches option (d).

Animated Solution for Chemistry - Organic Chemistry: The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is

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Visualized Solution

$S_{N} Ar Reactivity$

$Role of Leaving Group$

$Effect of One - N O_{2} Group$

$Effect of Two - N O_{2} Groups$

$Effect of Three - N O_{2} Groups$

$Final Reactivity Order$

$Halogen Effect in S_{N} Ar$

The Sigma Insight: Haloalkanes & Haloarenes

The Challenge of Aromatic Substitution Normally, benzene rings are highly electron-rich and repel nucleophiles

The Magic of Electron-Withdrawing Groups However, if we attach an electron-withdrawing group (EWG) like a nitro group ( $- N O_{2}$ ) to the benzene ring, the scenario changes dramatically

Position Matters

Analyzing the Compounds Let's evaluate our four compounds based on this principle: 1. Compound (i) is simple chlorobenzene

The Final Verdict By simply counting the number of strongly electron-withdrawing $- N O_{2}$ groups at the ortho and para positions, we can confidently determine the order of reactivity

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Animated Solution for Chemistry - Organic Chemistry: The correct order of the following compounds showing increasing tendency towards nucleophilic substitution reaction is

Select Answer:

Visualized Solution

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } SN​Ar Reactivity

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Role of Leaving Group

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Effect of One −NO2​ Group

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Effect of Two −NO2​ Groups

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Effect of Three −NO2​ Groups

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Final Reactivity Order

.math-text-wrapper .katex { font-size: 1.05em !important; } .math-text-wrapper .katex-display { margin: 1rem 0 !important; } Halogen Effect in SN​Ar