The correct increasing order of acidity of the following alkynes is :
(1) `CH_(3) - C -= C - CH_(3)`
(2) `CH_(3) - C -= CH`
(3) `CH -= CH`

Options

A. ` 1 lt 2 lt 3`
B. `2 lt 3 lt 1 `
C. `3 lt 2 lt 1 `
D. `1 lt 3 lt 2 `

Explanation

To determine the increasing order of acidity of the given alkynes, we need to analyze the stability of the conjugate bases formed when these alkynes lose a proton (H⁺). The more stable the conjugate base, the stronger the acid.

### Step-by-step Solution:

1. **Identify the Alkynes**:
- (1) `CH₃ - C ≡ C - CH₃` (2-butyne)
- (2) `CH₃ - C ≡ CH` (1-butyne)
- (3) `CH ≡ CH` (ethyne)

2. **Understanding Acidity**:
- The acidity of alkynes can be compared based on the stability of their conjugate bases. When an alkyne loses a proton, it forms a carbanion (negatively charged carbon species).

3. **Conjugate Base Stability**:
- For (1) `CH₃ - C ≡ C - CH₃`, the conjugate base is `CH₃ - C ≡ C⁻ - CH₃`. The negative charge is on a sp-hybridized carbon, which is relatively stable due to its high s-character (50%).
- For (2) `CH₃ - C ≡ CH`, the conjugate base is `CH₃ - C ≡ C⁻`. Again, the negative charge is on an sp-hybridized carbon, which is stable.
- For (3) `CH ≡ CH`, the conjugate base is `C⁻ ≡ C`. The negative charge is also on an sp-hybridized carbon, making it stable.

4. **Comparing the Effects of Substituents**:
- The presence of electron-donating groups (like the methyl group in (1) and (2)) can destabilize the negative charge due to the +I (inductive) effect.
- In (1), there are two methyl groups which will exert a stronger +I effect compared to (2) which has only one methyl group. Thus, (1) will be less acidic than (2).
- (3) has no substituents, so it will be the most acidic among the three because there are no destabilizing effects.

5. **Final Order of Acidity**:
- Based on the stability of the conjugate bases, we can conclude:
- (1) `CH₃ - C ≡ C - CH₃` is the least acidic,
- (2) `CH₃ - C ≡ CH` is more acidic than (1),
- (3) `CH ≡ CH` is the most acidic.
- Therefore, the correct increasing order of acidity is:
- **(1) < (2) < (3)**