Which compound has the highest value of `pk_a` ?

Options

A. `Cl-CH_2-CH_2-COOH`
B. `CH_3-CH_2-COOH`
C. `CH_3-underset(Cl)underset(|)(CH)-COOH`
D. `CH_3-underset(Cl)underset(|)overset(Cl)overset(|)(C)-COOH`

Explanation

To determine which compound has the highest value of pKa, we need to analyze the acidic strength of the given compounds. The pKa value is inversely related to the acidity of a compound; thus, a higher pKa value indicates a weaker acid. Here’s a step-by-step solution:

### Step 1: Understand the Relationship Between pKa and Acid Strength
- pKa = -log(kA), where kA is the dissociation constant of an acid.
- A higher kA value means a stronger acid and a lower pKa value.
- Conversely, a lower kA value means a weaker acid and a higher pKa value.

**Hint:** Remember that pKa is a logarithmic scale; thus, it’s important to understand that higher pKa corresponds to weaker acids.

### Step 2: Identify the Factors Affecting Acidity
- The presence of electron-withdrawing groups (EWGs) increases acidity by stabilizing the negative charge on the conjugate base.
- Electron-donating groups (EDGs) decrease acidity by destabilizing the conjugate base.

**Hint:** Focus on the effects of substituents on the acidic strength of the compounds.

### Step 3: Analyze Each Compound
- **Option 1:** Contains a Cl group (an EWG), which increases acidity.
- **Option 2:** No substituents, which means it has a neutral effect on acidity.
- **Option 3:** Contains a Cl group (an EWG), which increases acidity.
- **Option 4:** Contains two Cl groups (both EWGs), which significantly increases acidity.

**Hint:** Compare the number and type of substituents on each compound to determine their relative acidity.

### Step 4: Determine the Compound with the Least Acidity
- Since Option 2 has no substituents, it will be the least acidic compared to the others, which all have electron-withdrawing groups that increase acidity.
- Therefore, Option 2 will have the highest pKa value.

**Hint:** The absence of electron-withdrawing groups typically results in a weaker acid.

### Conclusion
The compound with the highest value of pKa is **Option 2**, as it is the least acidic due to the lack of any substituents that would enhance acidity.

**Final Answer:** Option 2 has the highest value of pKa.