Question

What is the order of reverse hyperconjugation in CCL3 , cf3 and cI3 . And explain in detail also

Answer 1

CF3 > CCl3 > CI3

Answer 2

Reverse hyperconjugation (also known as negative hyperconjugation) is a stabilizing interaction involving the delocalization of electron density from a filled orbital (like a lone pair, a $\pi$ bond, or an adjacent C-H $\sigma$ bond) into an adjacent empty or partially filled antibonding $\sigma^*$ orbital. For $\text{CX}_3$ groups, it refers to their electron-withdrawing nature due to the low-lying $\sigma^*$ orbitals of the C-X bonds, which can accept electron density from an adjacent electron-rich center.

The strength of reverse hyperconjugation primarily depends on the energy of the $\sigma^*$ antibonding orbital:

1. Electronegativity of X: A more electronegative atom X pulls electron density more strongly from carbon, polarizing the C-X bond (C$^{\delta+}$ - X$^{\delta-}$). This strong polarization lowers the energy of the $\sigma^*$ orbital, making it a better electron acceptor.
2. Orbital Overlap: Effective overlap between the carbon 2p orbital and the valence p orbital of X is crucial. Good overlap leads to well-separated bonding and antibonding orbitals, potentially lowering the $\sigma^*$ energy.

Let's analyze the given $\text{CX}_3$ groups:

• $\text{CF}_3$ (Trifluoromethyl group):

  • Fluorine (F) is the most electronegative element. This makes the C-F bond highly polarized.
  • The high electronegativity of F significantly lowers the energy of the $\sigma^*$ (C-F) antibonding orbitals.
  • The C(2p)-F(2p) orbital overlap is very effective due to similar principal quantum numbers.
  • These low-lying, well-overlapped $\sigma^*$ orbitals are excellent electron acceptors, making $\text{CF}_3$ exhibit the strongest reverse hyperconjugation.

• $\text{CCl}_3$ (Trichloromethyl group):

  • Chlorine (Cl) is less electronegative than fluorine but more electronegative than iodine.
  • The C-Cl bond is polarized, but less so than the C-F bond. Consequently, the $\sigma^*$ (C-Cl) orbitals are higher in energy than $\sigma^*$ (C-F) orbitals.
  • The C(2p)-Cl(3p) orbital overlap is reasonable.
  • Therefore, $\text{CCl}_3$ shows weaker reverse hyperconjugation compared to $\text{CF}_3$.

• $\text{CI}_3$ (Triiodomethyl group):

  • Iodine (I) is the least electronegative among the halogens considered.
  • The C-I bond is the least polarized, resulting in higher energy $\sigma^*$ (C-I) orbitals.
  • Crucially, there is a significant size mismatch and poor overlap between the carbon 2p orbital and the iodine 5p orbital. This poor overlap further contributes to the high energy of the $\sigma^*$ (C-I) orbital.
  • Thus, $\text{CI}_3$ exhibits the weakest reverse hyperconjugation effect.

Order of Reverse Hyperconjugation:

Based on the electronegativity of the halogen and the effectiveness of orbital overlap, the energy of the $\sigma^*$ antibonding orbital follows the order: $\text{Energy of } \sigma^*(\text{C-F}) < \text{Energy of } \sigma^*(\text{C-Cl}) < \text{Energy of } \sigma^*(\text{C-I})$

A lower energy $\sigma^*$ orbital implies a stronger ability to accept electron density, leading to stronger reverse hyperconjugation.

Therefore, the order of reverse hyperconjugation is: $\text{CF}_3 > \text{CCl}_3 > \text{CI}_3$

This order is consistent with the observed electron-withdrawing nature of these groups; $\text{CF}_3$ is a much stronger electron-withdrawing group than $\text{CCl}_3$ or $\text{CI}_3$.

Explanation of the solution: Reverse hyperconjugation strength depends on the energy of the $\sigma^*$ antibonding orbital of the C-X bond. Lower $\sigma^*$ energy means stronger interaction. The $\sigma^*$ energy is lowered by higher electronegativity of X and good orbital overlap. Fluorine is the most electronegative and forms C-F bonds with excellent orbital overlap, leading to the lowest energy $\sigma^*$ (C-F) orbitals. Iodine is the least electronegative, and the C-I bond has poor orbital overlap due to size mismatch, resulting in the highest energy $\sigma^*$ (C-I) orbitals. Thus, the order of reverse hyperconjugation is $\text{CF}_3 > \text{CCl}_3 > \text{CI}_3$.