See image — GOC and Organic Chemistry Basics Chemistry Question

💡 Solution & Explanation

# Analysis of pKa Values for Carboxylic Acids **Step 1: Identify the compounds and their structures** - (a) $O_2N-CH_2-COOH$ (nitromethylacetic acid) - (b) $HOOC-CH_2-COOH$ (malonic acid) - (c) $CH_3-CH_2-COOH$ (propionic acid) **Step 2: Understand pKa trends** Lower pKa = stronger acid (more easily ionized). Carboxylic acid strength increases with: - Electron-withdrawing groups (make $COOH$ more acidic) - Resonance stabilization of conjugate base ($COO^-$) **Step 3: Analyze each acid** **(a) Nitro compound:** $NO_2$ is highly electron-withdrawing → strongly stabilizes $COO^-$ → **lowest pKa** ≈ 1.5 **(b) Malonic acid:** Second $COOH$ group is electron-withdrawing AND its anion is stabilized by resonance through the central $CH_2$ → intermediate pKa ≈ 2.4 **(c) Propionic acid:** Alkyl group ($CH_3$) is weakly electron-donating → weakest acid → **highest pKa** ≈ 4.9 **Step 4: Order by increasing pKa** $$a < b < c$$ **(a) is correct** because electron-withdrawal strength follows: $NO_2$ >> $COOH$ >> $CH_3$, making acids progressively weaker in that order.