CHEMCA
EXAM MASTER FORMULA SHEET
Organic Chemistry: Haloarenes
1. Low Reactivity towards Nucleophilic Substitution
Haloarenes (Aryl Halides) are extremely unreactive towards normal $S_N1$ and $S_N2$ reactions due to four primary reasons:
The lone pair of electrons on the halogen atom is in conjugation with the $\pi$-electrons of the benzene ring.
Result: The $\ce{C-X}$ bond acquires partial double bond character ($\approx 169 \text{ pm}$), making it shorter and harder to break.
The carbon atom attached to the halogen is $sp^2$ hybridized (33.3% s-character) compared to $sp^3$ (25% s-character) in haloalkanes.
Result: The $sp^2$ carbon is more electronegative, holding the electron pair of the $\ce{C-X}$ bond more tightly.
If the halide leaves, it forms a Phenyl cation ($\ce{C6H5+}$). This cation cannot be stabilized by resonance because the positive charge is in an $sp^2$ orbital orthogonal to the $\pi$-system.
Rules out $S_N1$ mechanism.
The benzene ring is an electron-rich arenium cloud. It strongly repels the approaching electron-rich nucleophile.
Rules out backside attack ($S_N2$ mechanism).
2. Methods of Preparation
Direct Halogenation (EAS)
Benzene + Halogen in the presence of a Lewis Acid catalyst (acts as a halogen carrier to generate $X^+$).
*Iodination requires oxidizing agents like $\ce{HNO3}$ or $\ce{HIO3}$ to oxidize the formed HI back to $I_2$, making the reaction irreversible.*
From Diazonium Salts (Most Important)
Benzene Diazonium Chloride (BDC) is highly reactive. Nitrogen ($N_2$) is an excellent leaving group.
-
Sandmeyer:
$\ce{ArN2+Cl- ->[CuCl/HCl] ArCl + N2}$
$\ce{ArN2+Cl- ->[CuBr/HBr] ArBr + N2}$ -
Gattermann:
$\ce{ArN2+Cl- ->[Cu \text{ powder}/HCl] ArCl + N2}$
Does not require Copper catalyst. Simply warm BDC with aqueous Potassium Iodide.
BDC treated with Fluoroboric acid to form insoluble diazonium fluoroborate, which decomposes on heating.
3. Physical Properties (Anomalous Trends)
For isomeric dihalobenzenes, the para-isomer has a significantly higher melting point than ortho and meta isomers.
Reason: Para-isomer is highly symmetrical, fits perfectly into the crystal lattice, leading to stronger intermolecular forces.
Dipole moment depends on the vector sum of individual bond moments. The angle $\theta$ dictates the resultant $\mu = \sqrt{\mu_1^2 + \mu_2^2 + 2\mu_1\mu_2\cos\theta}$.
Para isomer is completely non-polar because the vectors cancel each other out exactly.
4. Nucleophilic Aromatic Substitution ($S_NAr$)
Requires drastic conditions (Addition-Elimination mechanism) UNLESS strongly activated by electron-withdrawing groups.
Requires extremely high temperature and pressure.
The presence of Electron Withdrawing Groups (like $-NO_2$) at Ortho and/or Para positions dramatically increases reactivity. They stabilize the intermediate carbanion (Meisenheimer complex) through resonance.
1 $NO_2$ (Para)
$\ce{NaOH, 443K}$
2 $NO_2$ (Ortho, Para)
$\ce{Na2CO3(aq), 368K}$
3 $NO_2$ (o, p, o')
$\ce{Warm H2O}$
*Meta-$NO_2$ has almost NO effect on reactivity because the negative charge in the intermediate never reaches the meta position.*
When very strong bases like $\ce{NaNH2}$ in liquid $\ce{NH3}$ are used on unactivated haloarenes, substitution occurs via an extremely unstable Benzyne intermediate.
- 1. Elimination: Strong base removes ortho-proton, halide leaves $\to$ Benzyne forms.
- 2. Addition: Nucleophile attacks Benzyne to form aniline.
- Cine Substitution: The incoming nucleophile can attack the carbon holding the halogen OR the adjacent ortho carbon (50/50 mix if labelled).
5. Electrophilic Substitution Reactions
| Reaction | Reagents Added | Major Product (Para) |
|---|---|---|
| Nitration | Conc. $\ce{HNO3}$ + Conc. $\ce{H2SO4}$ | 1-chloro-4-nitrobenzene |
| Halogenation | $\ce{Cl2}$ + Anhydrous $\ce{FeCl3}$ | 1,4-dichlorobenzene |
| Sulphonation | Fuming $\ce{H2SO4}$ ($\ce{SO3/H2SO4}$) | 4-chlorobenzenesulphonic acid |
| F.C. Alkylation | $\ce{CH3Cl}$ + Anhydrous $\ce{AlCl3}$ | 1-chloro-4-methylbenzene |
| F.C. Acylation | $\ce{CH3COCl}$ + Anhydrous $\ce{AlCl3}$ | 4-chloroacetophenone |
Para isomer is usually the major product due to steric hindrance at the ortho position.
6. Coupling Reactions with Metals
Coupling of an Alkyl halide with an Aryl halide using Sodium metal. Useful for making alkyl benzenes.
Coupling of two Aryl halides using Sodium metal to form diphenyl (biphenyl).
Ullmann Reaction
Coupling of Iodobenzene using Copper powder ($\ce{Cu}$) in a sealed tube at high heat to form Diphenyl.
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