Question

Number of planar and polar molecules, which do not undergo hydrogen bonding. (neither inter molecular nor intra molecular)

$H_2SO_4, NH_3, HN_3, H_2O, H_3BO_3, cis-C_2H_2F_2$

Answer 1

1

Answer 2

The given molecules are $H_2SO_4, NH_3, HN_3, H_2O, H_3BO_3, cis-C_2H_2F_2$. We need to find the number of planar and polar molecules from this list which do not undergo hydrogen bonding (neither intermolecular nor intramolecular).

Let's analyze each molecule:

1. $H_2SO_4$: The central sulfur atom is $sp^3$ hybridized, with a tetrahedral electron geometry. The molecule has a distorted tetrahedral molecular geometry. Thus, it is not planar. It is polar and undergoes intermolecular hydrogen bonding.
2. $NH_3$: The central nitrogen atom is $sp^3$ hybridized, with a tetrahedral electron geometry and trigonal pyramidal molecular geometry. Thus, it is not planar. It is polar and undergoes intermolecular hydrogen bonding.
3. $HN_3$: Hydrazoic acid ($HN_3$) has a bent structure at the first nitrogen atom (bonded to H). The N-N-N part is linear, but the H-N-N angle is approximately 112 degrees. Thus, the molecule is not planar. It is polar and undergoes intermolecular hydrogen bonding through the N-H bond.
4. $H_2O$: Water has a bent molecular geometry, which lies in a plane. Thus, a water molecule is planar. It is polar and undergoes extensive intermolecular hydrogen bonding.
5. $H_3BO_3$: Boric acid has a central boron atom which is $sp^2$ hybridized, and the three oxygen atoms bonded to it lie in a plane. The O-H bonds are also in this plane. Thus, the entire molecule is planar. It is polar and undergoes extensive intermolecular hydrogen bonding.
6. $cis-C_2H_2F_2$: In cis-1,2-difluoroethene, the two carbon atoms of the double bond and the atoms bonded to them lie in a plane. Thus, the molecule is planar. The molecule has polar C-F bonds and C-H bonds. Due to the cis arrangement, the bond dipoles do not cancel out, resulting in a net dipole moment. Thus, it is polar. There are no O-H, N-H, or F-H bonds, so it does not undergo hydrogen bonding.

Now, let's filter the molecules based on the given conditions:

• Planar molecules: $H_2O$, $H_3BO_3$, $cis-C_2H_2F_2$.
• Polar molecules among the planar ones: $H_2O$, $H_3BO_3$, $cis-C_2H_2F_2$.
• Molecules among these that do not undergo hydrogen bonding:
  • $H_2O$: undergoes hydrogen bonding.
  • $H_3BO_3$: undergoes hydrogen bonding.
  • $cis-C_2H_2F_2$: does not undergo hydrogen bonding.

Thus, only $cis-C_2H_2F_2$ satisfies all the conditions. The number of such molecules is 1.