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Monday, December 21, 2020

What are the conditions for Back bonding?

(1) Both of the atoms bonded with back Bonding are must be present in 2nd-2nd or 2nd-3rd period.

(2) One of the atoms has lone pair and another have vacant Orbital and direction of back Bonding depends upon vacant Orbital.

(3) The donor atom must have localized donatable electron pair. In general these are later half second period P - block elements (F, O, N and C).

(4) The acceptor atom must have low energy empty orbital which generally are np or nd orbitals. Small and similar sized orbital’s favour overlap.

EFFECTS OF BACK BONDING:

(1) It always leads to an increase in bond order between the participating atoms.

(2) It always leads to an increase in bond strength between participating atoms.

(3) It always leads to a decrease in bond length between participating atoms.

What is the Back Bonding?

(1) Back bonding is a type of weaker π bond which is formed by sideways overlapping of filled orbital with empty orbital present on adjacent bonded atoms in a molecule.
(2) It is also considered as intermolecular Lewis acid-base interaction as it is a π bond.
(3) Back bonding is found to be effective and considerable in following type of overlapping.

Why trimethyl amine {(CH3)3 N:} is pyramidal while trisilyl amine {(SiH3)3N:} is trigonal planer?

Trimethyl amine {(CH₃)₃N:} has sp3 hybridization at N atom and three bond pair and one lone pair electron. Thus trimethyl amine is pyramidal.

But in trisilyl amine {(SiH₃)₃N:} there is 2pπ—3dπ back bonding between lone pair orbital of nitrogen into vacant orbital of silicon. Hence trisilyl amines is sp2, planer & is less basic than trimethyl amine.

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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