Question

Explain the following observations. (i) The C-O single bond is weaker than the Si-O single bond whereas the double bonds show the reverse order of strength. (ii) The C-C bond and the \(\mathrm{F}-\mathrm{F}\) bonds are both considerably weaker than the C-F single bond. (iii) The N-N single bond is weaker than the P-P single bond. (iv) The Si-Cl bond is easily hydrolysed although the C-F bond is resistant to attack by water.

Short answer

The Si-O single bond is stronger than the C-O single bond because of Si's larger size, while the C=O double bond is stronger due to higher electronegativity differences and more effective π bonding. The C-F bond is stronger than C-C and F-F bonds due to larger electronegativity differences, creating polar bonds with stronger electrostatic interactions. The N-N bond is weaker than the P-P bond due to lone pair-lone pair repulsion in N-N bonds. The Si-Cl bond is more easily hydrolyzed than the C-F bond because of Si's larger size and lower electronegativity, allowing water molecules to approach and attack the bond more easily.

Step-by-step solution

Observation (i) - C-O single bond vs. Si-O single bond, and their double bonds

The strength of a bond depends on several factors, including the bond order (single/double), bond length, and electronegativity of the atoms involved. In the case of C-O and Si-O single bonds, the Si-O bond is stronger because silicon (Si) is larger than carbon (C), resulting in a longer bond length and therefore a weaker bond. However, when comparing double bonds, the C=O bond is stronger due to the higher electronegativity difference between carbon and oxygen compared to silicon and oxygen. Moreover, the presence of d-orbitals in Si is less effective in π bonding, which contributes to the weaker Si=O bond.

Observation (ii) - C-C bond, F-F bond, and C-F bond

The C-C and F-F bonds are both weaker than the C-F bond mainly due to differences in electronegativity between the atoms involved. In C-C and F-F bonds, the atoms involved have similar electronegativities (C: 2.55, F: 3.98), which results in weak bonds as the electrons are shared evenly between the two atoms. In contrast, the C-F bond exhibits a larger electronegativity difference (C: 2.55 and F: 3.98), which results in a polar bond, with electron density shifting towards the more electronegative fluorine atom. This leads to stronger electrostatic interactions between the atoms, resulting in a stronger bond.

Observation (iii) - N-N single bond vs. P-P single bond

The N-N single bond is weaker than the P-P single bond due to the presence of lone pair-lone pair repulsion in N-N bonds. Both nitrogen atoms in the N-N bond have two lone pairs of electrons, which generate significant repulsion between them, weakening the bond. In contrast, phosphorus atoms in a P-P bond have only one lone pair each, resulting in less lone pair-lone pair repulsion and thus a stronger bond.

Observation (iv) - Si-Cl bond vs. C-F bond reactivity

The Si-Cl bond is easier to hydrolyze than the C-F bond due to the larger size and lower electronegativity of silicon compared to carbon. The larger size of silicon allows water molecules to more easily approach and attack the Si-Cl bond, leading to hydrolysis. In contrast, the smaller size and higher electronegativity difference in C-F bonds make it harder for water molecules to approach and "attack" the bond, resulting in lower reactivity and resistance to hydrolysis.