Carbenes: Formation, Singlet vs Triplet & Reactions
Carbenes are neutral, divalent carbon intermediates with six valence electrons. They are highly reactive and electron-deficient, acting as electrophiles in organic synthesis.
1. Singlet vs Triplet Carbenes
The reactivity and stereochemistry of carbenes depend heavily on their spin state.
- Spin State: Paired electrons ($\uparrow\downarrow$).
- Hybridization: $sp^2$ (Bent shape).
- Magnetism: Diamagnetic.
- Spin State: Unpaired electrons ($\uparrow \uparrow$).
- Hybridization: $sp$ (Linear).
- Magnetism: Paramagnetic (Biradical).
Stability Order (Exam Favorite)
Generally, Triplet > Singlet due to less inter-electronic repulsion (Hund's Rule).
Exception (Halocarbenes): For $:CX_2$ (e.g., $:CCl_2$), the Singlet is more stable due to $p\pi-p\pi$ back-bonding from the halogen lone pair to the empty carbon orbital.
2. Methods of Formation
Carbenes are typically generated in situ via elimination or thermal decomposition.
Common Methods
- From Chloroform ($\alpha$-Elimination): Used in Reimer-Tiemann reaction.
$CHCl_3 + OH^- \rightarrow :CCl_2 + H_2O + Cl^-$ - From Diazomethane: Photolysis or Thermolysis.
$CH_2N_2 \xrightarrow{h\nu \text{ or } \Delta} :CH_2 + N_2$ - Simmons-Smith Reagent:
$CH_2I_2 + Zn-Cu \rightarrow I-CH_2-ZnI$ (Carbenoid)
Figure 1: Formation Pathways for Dichlorocarbene and Methylene
3. Factors Affecting Stability
The stability of carbenes is influenced by the electronic nature of substituents attached to the carbene carbon.
Figure 2: Stability Trends and Back-Bonding Effects
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