The correct basic strength is
Concept: Basic strength of amines and phosphines is determined by the availability of the lone pair of electrons on the central atom (Nitrogen or Phosphorus) for donation. Factors affecting basicity include inductive effects, steric hindrance, and the ability of the central atom to delocalize its lone pair.
Why (B) is correct:
1. Comparing and : In (trimethylamine), the methyl groups () are electron-donating due to their positive inductive effect (+I effect). This increases the electron density on the nitrogen atom, making its lone pair more available for donation and thus increasing basicity. In contrast, in , silicon has vacant d-orbitals. The lone pair on nitrogen can be delocalized into the vacant d-orbitals of silicon through pπ-dπ backbonding. This delocalization reduces the availability of the lone pair on nitrogen, making a much weaker base than .
2. Comparing and : Phosphorus is a larger atom than nitrogen and its 3p orbital is larger and more diffuse than nitrogen's 2p orbital. The extent of pπ-dπ backbonding between the lone pair on phosphorus and the vacant d-orbitals of silicon is less effective compared to nitrogen due to the larger size difference and poorer orbital overlap. Therefore, the lone pair on phosphorus in is more available for donation than the lone pair on nitrogen in . This makes a stronger base than .
3. Overall Order: Combining these factors, is the strongest base due to the +I effect and no pπ-dπ backbonding. is weaker than but stronger than because pπ-dπ backbonding is less effective in phosphines. is the weakest base due to significant pπ-dπ backbonding. Thus, the correct order is .
Option Analysis:
- A) : Incorrect. is the weakest base.
- B) : Correct, as explained above.
- C) : Incorrect. The order is reversed for the last two compounds.
- D) : Incorrect. This order is completely reversed.
Correct Answer: (B)