The decreasing basic order of the following is:
i. `PhSO_(3)^(o-)`, ii. `C_(2)H_(5)SO_(3)^(o-)`, iii. `C_(2)H_(5)COO^(o-)`, iv. `overset(o-)(C)N`, v. `overset(o-)(O)H`

Options

A. `(v) gt (iv) gt (iii) gt (ii) gt (i)`
B. `(i) gt (ii) gt (iii) gt (iv) gt (v)`
C. `(iv) gt (v) gt (iii) gt (ii) gt (i)`
D. `(i) gt (ii) gt (iii) gt (v) gt (iv)`

Explanation

To determine the decreasing basic order of the given species, we will analyze the conjugate acids of each species and their relative acidity. The stronger the conjugate acid, the weaker the corresponding base will be. Here’s a step-by-step solution:

### Step 1: Identify the given species
The species provided are:
1. \( \text{PhSO}_3^{-} \) (Phenyl sulfonate)
2. \( \text{C}_2\text{H}_5\text{SO}_3^{-} \) (Ethyl sulfonate)
3. \( \text{C}_2\text{H}_5\text{COO}^{-} \) (Ethanoate)
4. \( \text{CN}^{-} \) (Cyanide)
5. \( \text{OH}^{-} \) (Hydroxide)

### Step 2: Write the conjugate acids
To find the basicity order, we need to write the conjugate acids of each species by adding a proton (\( H^{+} \)):
1. \( \text{PhSO}_3^{-} + H^{+} \rightarrow \text{PhSO}_3H \) (Benzene sulfonic acid)
2. \( \text{C}_2\text{H}_5\text{SO}_3^{-} + H^{+} \rightarrow \text{C}_2\text{H}_5\text{SO}_3H \) (Ethyl sulfonic acid)
3. \( \text{C}_2\text{H}_5\text{COO}^{-} + H^{+} \rightarrow \text{C}_2\text{H}_5\text{COOH} \) (Propanoic acid)
4. \( \text{CN}^{-} + H^{+} \rightarrow \text{HCN} \) (Hydrogen cyanide)
5. \( \text{OH}^{-} + H^{+} \rightarrow \text{H}_2\text{O} \) (Water)

### Step 3: Analyze the acidity of the conjugate acids
Now, we will analyze the acidity of these conjugate acids:
- **Benzene sulfonic acid** is more acidic than **ethyl sulfonic acid** due to the electron-withdrawing effect of the phenyl group.
- **Ethyl sulfonic acid** is less acidic than both sulfonic acids but more acidic than **propanoic acid**.
- **Propanoic acid** is more acidic than **hydrogen cyanide**.
- **Hydrogen cyanide** is more acidic than **water**.

### Step 4: Determine the basicity order
Since stronger acids correspond to weaker bases, we can now deduce the basicity order from the acidity of the conjugate acids:
1. \( \text{OH}^{-} \) (most basic, conjugate acid is water)
2. \( \text{CN}^{-} \) (second most basic, conjugate acid is HCN)
3. \( \text{C}_2\text{H}_5\text{COO}^{-} \) (third, conjugate acid is propanoic acid)
4. \( \text{C}_2\text{H}_5\text{SO}_3^{-} \) (fourth, conjugate acid is ethyl sulfonic acid)
5. \( \text{PhSO}_3^{-} \) (least basic, conjugate acid is benzene sulfonic acid)

### Final Order of Basicity
Thus, the decreasing basic order is:
\[ \text{OH}^{-} > \text{CN}^{-} > \text{C}_2\text{H}_5\text{COO}^{-} > \text{C}_2\text{H}_5\text{SO}_3^{-} > \text{PhSO}_3^{-} \]