CHEMCA
EXAM MASTER REVIEW SHEET
Organic Chemistry: Haloarenes
1. Low Reactivity of Haloarenes
Haloarenes are much less reactive towards Nucleophilic Substitution ($S_N$) than haloalkanes.
Lone pair on halogen is in conjugation with the ring, giving the $C-X$ bond partial double bond character.
Harder to break than a single bond.
Carbon is $sp^2$ hybridized (more s-character) compared to $sp^3$ in haloalkanes.
Bond is shorter, stronger, and more electronegative.
2. Important Preparations
From Diazonium Salts (BDC)
• Sandmeyer Reaction: $ArN_2^+Cl^- \xrightarrow{CuCl/HCl} ArCl + N_2 \uparrow$
• Gattermann Reaction: $ArN_2^+Cl^- \xrightarrow{Cu \text{ powder}/HCl} ArCl$
• Balz-Schiemann: $ArN_2^+Cl^- \xrightarrow{HBF_4, \Delta} ArF$ (For Fluorides).
Direct Halogenation
Benzene + $Cl_2 \xrightarrow{FeCl_3/Anhyd. AlCl_3} Chlorobenzene$.
• Electrophilic Aromatic Substitution (EAS) mechanism.
• Reaction with $I_2$ requires $HIO_3$ or $HNO_3$ (Oxidizing agents).
3. Nucleophilic Substitution under Drastic Conditions
Chlorobenzene $\xrightarrow{623K, 300atm, NaOH} \text{Phenol}$.
Reactivity towards $S_N$ increases significantly when EWGs like $-NO_2$ are present at Ortho and Para positions.
Reactivity: 2,4,6-trinitro > 2,4-dinitro > 4-nitro > Chlorobenzene
*No effect at meta position due to resonance geometry.*
4. Electrophilic Substitution Reactions
Halogens are Deactivating but Ortho-Para Directing.
| Reaction | Reagents | Major Product |
|---|---|---|
| Nitration | Conc. $HNO_3 + H_2SO_4$ | 1-chloro-4-nitrobenzene (Para) |
| Sulphonation | Fuming $H_2SO_4$ | 4-chlorobenzenesulphonic acid |
| F.C. Alkylation | $CH_3Cl + Anhyd. AlCl_3$ | 1-chloro-4-methylbenzene |
5. Coupling Reactions
Mix of Alkyl halide and Aryl halide.
$Ar-X + 2Na + R-X \xrightarrow{Ether} Ar-R + 2NaX$
Useful for making substituted benzenes.
Two Aryl halides coupling.
$2Ar-X + 2Na \xrightarrow{Ether} Ar-Ar + 2NaX$
Example: Formation of Diphenyl.
Ullmann Reaction
Coupling of Iodoarenes using Copper powder and heat to form diaryls.
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