`{:("Oxalic acid",pK_1),("Malonic acid",pK_2),("Heptanedioic acid",pK_3):}`
where `pK_1,pK_2 ,pK_3` are first ionization constants. Correct order is :

Options

A. `pK_1 gt pK_2 gt pK_3`
B. `pK_1 lt pK_2 lt pK_3`
C. `pK_3 gt pK_2 = pK_q`
D. `pK_3 gt pK_1 gt pK_2`

Explanation

To determine the correct order of the first ionization constants (pK values) for oxalic acid, malonic acid, and heptanedioic acid, we need to analyze their acidic strengths based on their structures and the effects of substituents.

### Step-by-Step Solution:

1. **Identify the Structures of the Acids**:
- **Oxalic Acid**: It has the structure HOOC-COOH (two carboxylic acid groups).
- **Malonic Acid**: It has the structure HOOC-CH2-COOH (two carboxylic acid groups with a methylene (-CH2-) group in between).
- **Heptanedioic Acid**: It has the structure HOOC-(CH2)5-COOH (two carboxylic acid groups with a longer alkyl chain).

2. **Understand Acidic Strength**:
- The acidic strength of a compound is determined by its ability to donate protons (H+ ions). Stronger acids have lower pKa values.
- The pKa is inversely related to the acid strength: a lower pKa indicates a stronger acid.

3. **Analyze the Effects of Substituents**:
- **Oxalic Acid**: No electron-donating groups are present, making it the strongest acid among the three.
- **Malonic Acid**: The presence of a methylene (-CH2-) group introduces a slight electron-donating effect, making it less acidic than oxalic acid.
- **Heptanedioic Acid**: The long alkyl chain (five carbon atoms) has a significant electron-donating (+I) effect, which decreases its acidic strength, making it the weakest acid of the three.

4. **Order of Acidic Strength**:
- Based on the analysis, the order of acidic strength is:
- **Oxalic Acid (most acidic) > Malonic Acid > Heptanedioic Acid (least acidic)**

5. **Corresponding pKa Values**:
- Since stronger acids have lower pKa values, the order of pKa values will be the opposite of the order of acidic strength:
- **pK1 (Oxalic Acid) < pK2 (Malonic Acid) < pK3 (Heptanedioic Acid)**

### Final Answer:
The correct order of the first ionization constants (pKa values) is:
**pK1 < pK2 < pK3**