a) The hard-soft acid-base theory developed by Pearson has been used to predict

Question

a) The hard-soft acid-base theory developed by Pearson has been used to predict the position of equilibrium for many reactions. Explain the position of the equilibria for the following reactions: TiBr4 4 HgFTiF4 + 4 HgBr 2 CuCN + Cu12 > 2 Cul + Cu(CN)2 b) Use the values for the E & C parameters given below to rank the basicities of NH3, (CH3)2NH, (CH3)3P, and (CH3)2S towards H20 and B(C2H5)3 EB CB EA H20 1.54 0.13 CA NH3 (CH3)2NH 1.80 4.21 B(C2Hs)3 1.70 2.71 2.31 2.04 (CH3)3P 1.46 3.44 (CH3)2S 0.25 3.75

Explanation / Answer

a) According to the soft acid-base concept, a soft acid forms strong bond with soft base and a hard acid forms stronger bond with hard base.

In the first reaction,

TiBr4 + 4HgF ---> TiF4 + 4HgBr

Ti4+ is hard acid, Hg+ is soft acid

Br- is borderline base

F- is a hard base

So the reaction is favourable in forward direction, with TiF4 more stable as combination of hard acid and hard base as compared to TiBr4.

In the second reaction,

2CuCN + CuI2 --> 2CuI + Cu(CN)2

Cu+ is a soft acid, Cu2+ is a hard acid

CN- is a soft base, I- is a soft base

So here the reaction is favourable in forward direction as CuI is more stable than CuCN due to size of soft acid and soft base matching with each other.

b) Using the values for E and C given above,

basicity of amines would be with respect to its reaction with acids H2O and B(C2H5)2 is,

In decreasing order : (CH3)3NH > NH3 > (CH3)3P > (CH3)2S

S is more electronegative and less likely to donate its lone pair as compared to more electron rich N atom. As the alkyl groups increases on N, the basicity also increases.

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