Haloarenes — Revision Notes

⚡ 30-Second Revision

Haloarenes: — Halogen directly attached to aromatic ring ($sp^2$ carbon).
C-X Bond: — Partial double bond character due to resonance. Shorter, stronger than in haloalkanes.
Reactivity (Nucleophilic Substitution): — Less reactive than haloalkanes.

- Activated by EWGs ($-\text{NO}_2$) at ortho/para positions (SNAr mechanism). - Reactivity order for SNAr: $\text{F} > \text{Cl} \approx \text{Br} > \text{I}$.

Reactivity (Electrophilic Substitution): — Deactivating but ortho-para directing.
Preparation:

- Sandmeyer: $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{CuCl/HCl}} \text{ArCl}$; $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{CuBr/HBr}} \text{ArBr}$. - Balz-Schiemann: $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{HBF}_4} \text{ArN}_2^+\text{BF}_4^- \xrightarrow{\text{heat}} \text{ArF}$. - Direct Halogenation: $\text{ArH} + \text{X}_2 \xrightarrow{\text{Lewis acid}} \text{ArX}$.

Reactions with Metals:

- Wurtz-Fittig: $\text{ArX} + \text{RX} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{ArR}$. - Fittig: $2\text{ArX} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{Ar-Ar}$. - Grignard: $\text{ArX} + \text{Mg} \xrightarrow{\text{dry ether}} \text{ArMgX}$.

2-Minute Revision

Haloarenes are aromatic compounds with a halogen directly bonded to an $sp^2$ carbon of the ring. Their C-X bond has partial double bond character due to resonance, making it stronger and shorter than in haloalkanes.

This resonance, along with the $sp^2$ hybridization of the carbon, makes haloarenes significantly less reactive towards nucleophilic substitution (SN1/SN2) under normal conditions. However, nucleophilic aromatic substitution (SNAr) can occur, especially when strong electron-withdrawing groups (like $-\text{NO}_2$) are present at ortho or para positions, stabilizing the intermediate carbanion.

For electrophilic aromatic substitution, halogens are deactivating but ortho-para directing. Key preparation methods include the Sandmeyer reaction for chlorides and bromides, and the Balz-Schiemann reaction for fluorides, both starting from diazonium salts.

Direct halogenation of benzene with a Lewis acid also yields haloarenes. Important reactions with metals include Wurtz-Fittig (aryl + alkyl halide), Fittig (two aryl halides), and the formation of Grignard reagents.

Remember the distinct reactivity patterns and named reactions for NEET.

5-Minute Revision

Haloarenes, or aryl halides, are organic compounds where a halogen atom is directly attached to an aromatic ring. The defining feature is the C-X bond, which exhibits partial double bond character due to the resonance of the halogen's lone pair electrons with the aromatic $\pi$-system.

This makes the C-X bond shorter, stronger, and less polar than in haloalkanes, where the carbon is $sp^3$ hybridized and no such resonance occurs. This structural difference is critical for understanding their chemical behavior.

Reactivity:

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Nucleophilic Substitution: — Haloarenes are generally unreactive towards SN1 and SN2 reactions due to the strong C-X bond, the instability of the phenyl carbocation, and repulsion from the electron-rich ring. However, they undergo Nucleophilic Aromatic Substitution (SNAr) via an addition-elimination mechanism (Meisenheimer complex) when activated by strong electron-withdrawing groups (EWGs) like $-\text{NO}_2$ at ortho or para positions. The more EWGs, the higher the reactivity. The reactivity order for halogens in SNAr is $\text{F} > \text{Cl} \approx \text{Br} > \text{I}$.
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Electrophilic Aromatic Substitution (EAS): — Halogens are deactivating groups (due to their strong -I effect) but are ortho-para directing (due to their +R effect). This means reactions like nitration, halogenation, and Friedel-Crafts will be slower than with benzene, and the incoming electrophile will primarily attack the ortho and para positions, with the para isomer usually being the major product due to steric hindrance.

Preparation Methods:

Sandmeyer Reaction: — For aryl chlorides and bromides. Aniline $\rightarrow$ Diazonium salt $\xrightarrow{\text{CuX/HX}}$ Aryl halide. Example: $\text{C}_6\text{H}_5\text{NH}_2 \xrightarrow{\text{NaNO}_2/\text{HCl}} \text{C}_6\text{H}_5\text{N}_2^+\text{Cl}^- \xrightarrow{\text{CuCl/HCl}} \text{C}_6\text{H}_5\text{Cl}$.
Balz-Schiemann Reaction: — For aryl fluorides. Diazonium salt $\xrightarrow{\text{HBF}_4} \text{ArN}_2^+\text{BF}_4^- \xrightarrow{\text{heat}} \text{ArF}$.
Direct Halogenation: — Benzene $\xrightarrow{\text{Cl}_2/\text{FeCl}_3}$ Chlorobenzene.

Reactions with Metals:

Wurtz-Fittig Reaction: — $\text{ArX} + \text{RX} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{ArR}$ (e.g., toluene from chlorobenzene and chloromethane).
Fittig Reaction: — $2\text{ArX} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{Ar-Ar}$ (e.g., biphenyl from bromobenzene).
Grignard Reagent Formation: — $\text{ArX} + \text{Mg} \xrightarrow{\text{dry ether}} \text{ArMgX}$.

Key takeaway: The resonance effect and $sp^2$ hybridization are central to understanding haloarene chemistry. Memorize named reactions and their specific reagents.

Prelims Revision Notes

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Definition & Structure: — Haloarenes (aryl halides) have a halogen (X) directly bonded to an $sp^2$ hybridized carbon of an aromatic ring. Example: Chlorobenzene.
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C-X Bond Nature:

* Partial Double Bond Character: Due to resonance of halogen's lone pair with the aromatic $\pi$-system. This makes the C-X bond shorter and stronger. * **$sp^2$ Hybridization:** Carbon attached to X is $sp^2$ hybridized, more electronegative than $sp^3$ carbon in haloalkanes. * Lower Dipole Moment: Compared to haloalkanes, due to resonance reducing net polarity.

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Reactivity towards Nucleophilic Substitution:

* Generally Low: Haloarenes are much less reactive than haloalkanes. Reasons: partial double bond character, $sp^2$ carbon, instability of phenyl carbocation, repulsion by $\pi$-electron cloud. * SNAr Mechanism: Occurs under harsh conditions or with electron-withdrawing groups (EWGs) at ortho/para positions (e.

g., $-\text{NO}_2$). EWGs stabilize the intermediate carbanion (Meisenheimer complex). * Reactivity Order (SNAr): $\text{F} > \text{Cl} \approx \text{Br} > \text{I}$ (due to rate-determining nucleophilic attack).

1
Reactivity towards Electrophilic Aromatic Substitution (EAS):

* Deactivating: Halogens are deactivating due to strong electron-withdrawing inductive effect (-I effect). * Ortho-para Directing: Halogens are ortho-para directing due to electron-donating resonance effect (+R effect) at these positions.

Para isomer is usually major due to less steric hindrance. * Examples: Halogenation ($\text{Cl}_2/\text{FeCl}_3$), Nitration ($\text{conc. HNO}_3/\text{conc. H}_2\text{SO}_4$), Sulfonation ($\text{conc.

H}_2\text{SO}_4$), Friedel-Crafts (alkylation/acylation with$\text{AlCl}_3$).

1
Methods of Preparation:

* Sandmeyer Reaction: From benzene diazonium salts. $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{CuCl/HCl}} \text{ArCl}$; $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{CuBr/HBr}} \text{ArBr}$. * Gattermann Reaction: Similar to Sandmeyer but uses Cu powder/HX (lower yield).

* Balz-Schiemann Reaction: For fluorobenzene. $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{HBF}_4} \text{ArN}_2^+\text{BF}_4^- \xrightarrow{\text{heat}} \text{ArF}$. * Iodobenzene: $\text{ArN}_2^+\text{Cl}^- \xrightarrow{\text{KI}} \text{ArI}$.

* Direct Halogenation: Benzene + $\text{X}_2$ in presence of Lewis acid (e.g., $\text{FeCl}_3$).

1
Reactions with Metals:

* Wurtz-Fittig Reaction: $\text{ArX} + \text{RX} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{ArR} + 2\text{NaX}$. * Fittig Reaction: $2\text{ArX} + 2\text{Na} \xrightarrow{\text{dry ether}} \text{Ar-Ar} + 2\text{NaX}$. * Ullmann Reaction: $2\text{ArI} + \text{Cu} \xrightarrow{\text{heat}} \text{Ar-Ar} + \text{CuI}_2$. * Grignard Reagent: $\text{ArX} + \text{Mg} \xrightarrow{\text{dry ether}} \text{ArMgX}$.

Vyyuha Quick Recall

For Haloarene Reactivity (SNAr): 'EWG's O-P-A-S'

EWG's — Electron-Withdrawing Groups (like $-\text{NO}_2$)
O-P — at Ortho and Para positions
A — Activate the ring for Nucleophilic Substitution
S — by Stabilizing the intermediate Meisenheimer complex.