Study Notes on The Chemistry of Aryl Halides, Vinylic Halides, and Phenols

Chapter 18: The Chemistry of Aryl Halides, Vinylic Halides, and Phenols

Definitions:
- Aryl Halides: Halogen bound to carbon of a benzene ring (or other aromatic ring).
- Vinylic Halides: Halogen bound to carbon of a double bond.

Aryl and vinylic halides do not participate in SN2 reactions unlike simple alkyl halides.

During an SN2 reaction, the carbon rehybridizes from sp² to sp in the transition state.
- This rehybridization incurs a high energy cost of approximately 21 kJ/mol more than the sp³ to sp² shift observed in alkyl halides.
The geometry of vinylic halides:
- The carbon bearing the halogen is sp² hybridized and arranged trigonal planar; bonds and π bond occupy distinct planes (C–X bond in the alkene's plane, π bond in a p orbital perpendicular).
- Nucleophiles must attack from the backside (180° opposite of the leaving group), but this is geometrically unfavorable at a vinylic carbon.
Approach must occur from the opposite side of the C–X carbon and in the alkene’s plane, leading to significant steric hindrance.

Aryl halides face similar hybridization issues and van der Waals repulsions as vinylic halides.
The required stereochemical inversion necessitates a backside approach through the plane of the benzene ring, which is geometrically unfeasible.

Base-promoted β-elimination reactions of vinylic halides can prepare alkynes.
Such reactions may require harsh conditions (e.g., high heat or strong bases).

For vinylic halides to undergo an SN1 or E1 reaction, ionization to form a vinylic cation is required.
- Characteristics of Vinylic Cations:
- Unstable due to carbocation nature; electron-deficient carbon part of C=C.
- Linear geometry with sp hybridization.
Vinylic and aryl cations are less stable compared to alkyl carbocations due to non-conjugated vacant 2p orbitals.

Substitution can occur, but not via SN1 or SN2 mechanisms—requires para and/or ortho nitro groups.

Reaction type: Nucleophilic Aromatic Substitution (NAS or SNAr) and follows second order rate laws:
- Rate expression: ext{rate} = k[ ext{aryl halide}][ ext{nucleophile}]
Reactivities:
- ext{Ar—F} >> ext{Ar—Cl} ext{ and } ext{Ar—Br} ext{ and } ext{Ar—I}
- This reverses the trend seen for SN2 and SN1 reactions of alkyl halides.

The aromatic ring temporarily loses aromaticity, forming a resonance-stabilized intermediate known as the Meisenheimer complex.
SNAr mechanism:
1. Step 1 (slow, rate-determining): Nucleophile adds to the aromatic ring, creating the Meisenheimer complex.
2. Step 2 (fast): The leaving group departs, restoring aromaticity.
Fluorine’s role: Fluorine is a strong electron-withdrawing group which enhances the electrophilicity of the carbon attached to it, facilitating nucleophilic attack despite being a poor leaving group.

Transition metals are found in the d block or the B groups of the periodic table, filling their n s and (n − 1) d orbitals.
Group characteristics:
- Grouped together due to similar energy levels of valence electrons.
- Example: Nickel (Ni) possesses the electronic configuration [Ar]4s^{2}3d^{8}.

Surrounding groups of transition metals are termed ligands which are all Lewis bases, forming coordination compounds.
Ligand classification can involve:
1. Classifying ligands.
2. Specifying formal charge.
3. Calculating oxidation state.
4. Counting electrons around the metal.

L-type ligands: Neutral ligands that retain bonding electrons when dissociated from the metal.
X-type ligands: One electron remains with the metal and the other with the ligand when dissociated, resulting in a negative charge.
Certain ligands exhibit both L-type and X-type properties (e.g., allyl and cyclopentadienyl).

Ligands can be indexed with their respective electron counts and behaviors:
- L-type ligands (e.g., ammine, aquo): 2 electrons.
- X-type ligands (e.g., halides like F⁻, Cl⁻): 2 electrons and gain a negative charge upon dissociation.
- Cyclopentadienyl (Cp): Functions as an L₂X-type ligand, participating in diverse bonding modes.

The oxidation state of metal M (in a metal complex) is calculated using the equation:
- ext{Oxidation state of M} = ( ext{number of X-type ligands}) + Q_{M}
- Example: In [ ext{Pt}( ext{Cl}){6}]^{2-}, where there are 6 X-type ligands and Q{M} = -2, the oxidation state of Pt is +4.

Involves the coupling of an alkene to an aryl group using a Pd(0) catalyst.
Mechanics:
1. The reaction utilizes β-elimination on noncyclic alkenes.
2. Internal rotation can occur following syn insertion.
3. Cyclic alkenes are unable to internally rotate, hence the stereochemistry is influenced by the reaction pathway.

Definition: This reaction couples an aryl or vinyl boronic acid with another aryl or vinyl group, also catalyzed by Pd(0) and requires a base (e.g., NaOH or Na₂CO₃).
Can be employed in producing conjugated dienes or aryl-substituted alkenes.

Phenols can ionize similar to alcohols but have greater acidity than non-aromatic alcohols.
Reasons for Increased Acidity:
- Resonance stabilization of the resultant phenoxide ion ( ext{C}6 ext{H}5 ext{O}⁻).
- The presence of substituents can influence phenol acidity through polar and resonance effects.

The equilibrium of phenol with NaOH heavily favors products, forming phenoxides which can act as nucleophiles (akin to alkoxides), illustrating the Williamson Ether Synthesis.

The hydroxyl group (-OH) strongly activates the aromatic ring towards electrophilic substitution, making additional catalysts like FeBr₃ unnecessary.
Bromination in water occurs more readily due to:
1. Ionized phenol (partially) presents increased nucleophilicity (phenoxide > phenol).
2. Protonated derivatives create better electrophiles.
3. Neutral intermediates are derived from phenoxide, improving reaction rates compared to carbocation intermediates.

Reactions like nitration could be reactive; however harsh conditions might oxidize phenolic compounds instead.
Common synthetic paths suggest alternative approaches via nucleophilic aromatic substitution (NAS) to yield certain products.