Role of Back Bonding in Organic Chemistry
While often discussed in inorganic contexts ($BF_3$), Back Bonding plays a crucial role in explaining the stability, acidity, and structure of various organic intermediates and compounds, particularly those containing halogens, silicon, phosphorus, or sulfur.
1. Stability of Dihalocarbenes
Carbenes ($:CX_2$) are electron-deficient intermediates with a sextet of electrons. Halogens attached to the carbene carbon can donate lone pairs into the empty p-orbital of carbon, stabilizing the Singlet State.
Order of Stability
$:CF_2 > :CCl_2 > :CBr_2 > :CI_2$
Reason: Efficient $2p\pi-2p\pi$ overlap in $:CF_2$ (Carbon 2p - Fluorine 2p) provides strong back bonding. In $:CCl_2$, the overlap is $2p-3p$, which is weaker.
2. Anomalous Acidity of Haloforms
Normally, acidity increases with the electronegativity of the substituent ($F > Cl > Br$). However, Chloroform ($CHCl_3$) is more acidic than Fluoroform ($CHF_3$).
1. $CCl_3^-$ (from Chloroform)
2. $CF_3^-$ (from Fluoroform)
Explanation: In $CCl_3^-$, the negative charge on Carbon (2p orbital) can delocalize into the empty 3d-orbitals of Chlorine ($2p\pi-3d\pi$ back bonding).
In $CF_3^-$, Fluorine has no d-orbitals (2nd period), so back bonding is impossible. The strong repulsion between the lone pairs on Fluorine and the carbanion destabilizes $CF_3^-$.
Acidity Order: $CHCl_3 > CHF_3$
3. Structure of Ylides (Wittig Reagent)
Phosphorus Ylides (e.g., $Ph_3P=CH_2$) are stable intermediates used in the Wittig reaction.
The stability arises from back bonding between the filled $2p$ orbital of Carbon (anionic center) and the empty $3d$ orbital of Phosphorus ($C_{2p} \to P_{3d}$), giving the $P-C$ bond significant double bond character.
4. Basicity of Silyl Amines
Compare Trimethylamine $(CH_3)_3N$ and Trisilylamine $(SiH_3)_3N$.
- $(CH_3)_3N$: No back bonding. Nitrogen is $sp^3$, Pyramidal. The lone pair is available. Strong Base.
- $(SiH_3)_3N$: Silicon has empty 3d orbitals. Nitrogen donates its lone pair to Si ($2p\pi-3d\pi$ back bonding). Nitrogen becomes $sp^2$, Planar. The lone pair is delocalized. Weak Base.
5. Bond Angles in Ethers
The $C-O-C$ angle in Dimethyl ether is $\approx 111^\circ$. However, in Disilyl ether ($SiH_3-O-SiH_3$), the angle expands significantly to $\approx 144^\circ$.
Reason
Oxygen donates its lone pairs into the empty d-orbitals of both Silicon atoms ($O \to Si$ back bonding). This increases the $Si-O$ bond order and causes the bond angle to widen to minimize repulsion, approaching linearity ($180^\circ$).
6. Summary of Effects
| Phenomenon | Cause (Back Bonding Type) | Effect |
|---|---|---|
| Stability of $:CF_2$ | $2p\pi-2p\pi$ ($F \to C$) | Singlet stable |
| Acidity of $CHCl_3$ | $2p\pi-3d\pi$ ($C^- \to Cl$) | Increased Acidity |
| Planarity of $(SiH_3)_3N$ | $2p\pi-3d\pi$ ($N \to Si$) | Planar Geometry |
| Stability of Ylides | $2p\pi-3d\pi$ ($C^- \to P$) | Short P=C bond |
Organic Back Bonding Quiz
Test your concepts on stability and reactivity. 10 MCQs with explanations.
No comments:
Post a Comment