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Monday, December 21, 2020

Which is/are the correct statement/s about structure of methyl isothiocyanate (H3CNCS) and Silyl isothiocyanate(SiH3NCO)?

(A) CNC bond angle in H₃CNCS is >120 and hybridization is closed to sp2

(B) Si-N-C bond angle is 180 in H₃CNCS
(C) Both have Back Bonding
(D) Skeleton Si-N-C-S is linear but molecule are non planer.

SOLUTION:

The hybridization of Nitrogen in Methyl isothiocyanate (H₃CNCS) is Sp2. Thus bond angle (< C-N-C) is expected to be 120°. But it is slightly greater than 120° due to resonating structure. The resonating structure has N as sp hybridized. Hence bond angle of the overall structure is found to be about 142°

But in case Silyl isothiocyanate (SiH₃NCO), the structure is found to be (sp hybridized) and planar due to back bond between Si-N. In which the lone pair of electrons on N are donated to the vacant 3d orbitals of Si through back bonding (2pπ-3dπ back bond).

Hence Options (A, B, D) are correct.

Thursday, December 17, 2020

Why Guanadine is a strongest base among neutral compounds?

Basicity of Guanidine:

Guanidine is the strongest base among neutral compounds:

The remarkable basicity of guanidine is attributed to the fact that the positive charge on

the guanidinium ion is delocalized equally over the three nitrogen atoms, as shown by

these three equivalent resonating structures:

Basicity of nitrogen can be increased by attachment to pi-donors (NH₂) group. These two pi-donating NH₂ groups donate electron density to the (pi-accepting) C=NH. Hence, the guanidinium ion is a highly stable cation.

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