The lone pair of amines makes them basic. They react with acids to form acid-base salts. Amines are more basic than alcohols, ethers and water. When an amine is dissolved in water, an equilibrium is established , where water acts as an acid and transfer a proton to the amine. The basic strength of an amine can be measured by basicity constant `K_(b)`.
Arylamines are less basic than alkylamines because the lone pair of nitrogen is delocalised with the aromatic ring and are less available for donation.
Substituted arylamines can be either more basic or less basic than aniline , depending on the substituted . ERG substituents, such as `-CH_(3), -NH_(2) and -OCH_(3)` increases the basicity and EWG substituents , such as `-Cl, -NO_(2) and -CN` decreases basicity. While `sp^(2)-` hybridized nitrogen atom in pyridine is less basic then the `sp^(3)` -hybridized nitrogen in an alkylamine.
`pK_(b)` order of the following compound is :
(I) `NH_(2)OH" "(II) NH_(2)NH_(2)" "(III) NH_(3)" "(IV)H_(2)O`

Options

A. `IV gt I gt II gt III`
B. `III gt II gt I gt IV`
C. `I gt IV gt II gt III`
D. `III gt I gt II gt IV`

Answer

To determine the pK_b order of the given compounds (I) NH₂OH, (II) NH₂NH₂, (III) NH₃, and (IV) H₂O, we need to analyze their basicity. Here’s a step-by-step breakdown:

### Step 1: Understand Basicity
Basicity refers to the ability of a compound to accept protons (H⁺ ions). The stronger the base, the more readily it accepts a proton, resulting in a lower pK_b value.

### Step 2: Analyze Each Compound
1. **(I) NH₂OH (Hydroxylamine)**: This compound has a nitrogen atom with a lone pair that can accept a proton. The presence of the -OH group can also influence its basicity, but it is still a good base.

2. **(II) NH₂NH₂ (Hydrazine)**: Hydrazine has two amine groups (NH₂), which makes it a stronger base than hydroxylamine because it can donate a lone pair from either nitrogen atom to accept a proton.

3. **(III) NH₃ (Ammonia)**: Ammonia is a well-known base. It has one nitrogen atom with a lone pair available for protonation, making it less basic than hydrazine but more basic than hydroxylamine.

4. **(IV) H₂O (Water)**: Water can act as an acid, donating a proton, but it is less basic than the amines and hydroxylamine since it does not have a lone pair available for protonation in the same way.

### Step 3: Rank the Basicity
Based on the analysis:
- **Most Basic**: NH₂NH₂ (Hydrazine)
- **Next**: NH₃ (Ammonia)
- **Then**: NH₂OH (Hydroxylamine)
- **Least Basic**: H₂O (Water)

### Step 4: Determine pK_b Order
Since pK_b is inversely proportional to basicity, we can now write the order of pK_b values:
- **pK_b Order**: H₂O > NH₂OH > NH₃ > NH₂NH₂

### Final Answer
The pK_b order of the compounds is:
(IV) H₂O > (I) NH₂OH > (III) NH₃ > (II) NH₂NH₂