Equilibrium Question 887
Question: Which of the following statements is not correc'
Options:
A) $ {I^{-}} $ is a weaker base than $ {F^{-}} $
B) $ H{O^{-}} $ is a stronger base than $ H_2{N^{-}} $
C) $ HONH_2 $ is a weaker base than $ NH_3 $
D) $ F_3{C^{-}} $ is a stronger base than $ Cl_3{C^{-}} $
Show Answer
Answer:
Correct Answer: C
Solution:
[a] For elements in the same periodic group, the larger the basic site atom the more spread out (delocalized) is the charge and the weaker is the base. Thus $ {l^{-}} $ is a weaker base than $ {F^{-}} $ . Alternatively, HI is stronger acid than HF, the conjugate base follows the reverse order. [b] $ H_2O $ is a stronger acid than $ NH_3, $ thus the conjugate base $ O{H^{-}} $ is weaker base than $ NH_2^{-} $
[c] Electron - attracting inductive effect of $ -,OH $ decreases the electron density on N and thus decreases the basicity of $ -NH_2^{{}}. $
[d] In $ Cl_3{C^{-}}, $ the unshared electron pair of the carbon in the p-orbital undergoes extended $ p-d\pi $ bonding into an emtpy d-orbital of each of the three Cl atoms. $ [ \underset{\bullet \bullet }{\overset{\bullet \bullet }{\mathop{\mathbf{:}Cl}}},-\underset{\begin{smallmatrix} | \\ \underset{\bullet \bullet }{\overset{{}}{\mathop{\mathbf{:}Cl}}},\mathbf{:} \end{smallmatrix}}{\mathop{C}},=\underset{\bullet \bullet }{\overset{\bullet \bullet }{\mathop{Cl}}},\mathbf{:}{{rightarrow }^{-}}\underset{\bullet \bullet }{\overset{\bullet \bullet }{\mathop{\mathbf{:}Cl}}},=\underset{\begin{smallmatrix} | \\ \underset{\bullet \bullet }{\overset{{}}{\mathop{\mathbf{:}Cl}}},\mathbf{:} \end{smallmatrix}}{\mathop{C}},{{-}^{-}}\underset{\bullet \bullet }{\overset{\bullet \bullet }{\mathop{Cl}}},\mathbf{:}rightarrow \underset{\bullet \bullet }{\overset{\bullet \bullet }{\mathop{\mathbf{:}Cl}}},-\underset{\begin{smallmatrix} | \\ \underset{\bullet \bullet }{\overset{{}}{\mathop{\mathbf{:}Cl}}},\mathbf{:}- \end{smallmatrix}}{\mathop{C}},-\underset{\bullet \bullet }{\overset{\bullet \bullet }{\mathop{Cl}}},\mathbf{:} ] $
Resonance stabilization makes a more dominating role than the inductive effect, making $ Cl_3{C^{-}} $ a weaker base than $ F_3{C^{-}}. $
In F, there is no d orbitals, hence, no stabilization occurs in $ F_3{C^{-}}. $
