Question

Least Aridic Strength

• A. R- $\overset{O}{\underset{||}{C}}$ - OCH$_{3}$
• B. R- $\overset{O}{\underset{||}{C}}$-CH$_{2}$-$\overset{O}{\underset{||}{C}}$-R
• C. R-$\overset{O}{\underset{||}{C}}$-CH$_{2}$-NO$_{2}$

Answer 1

Answer: (A)

R- $\overset{O}{\underset{||}{C}}$ - OCH$_{3}$

Answer 2

Acidity depends on the stability of the conjugate base.

1. Compound (A) (Ester): Alpha-hydrogens are deprotonated. The resulting carbanion is stabilized by resonance with one carbonyl group. Esters are relatively weak acids (pKa ~25).

2. Compound (B) ($\beta$-diketone): The methylene protons between two carbonyl groups are deprotonated. The resulting carbanion is highly stabilized by resonance with both carbonyl groups, delocalizing the negative charge over two oxygen atoms. $\beta$-diketones are moderately strong acids (pKa ~9-11).

3. Compound (C) (Nitro-ketone): The methylene protons between a carbonyl and a nitro group are deprotonated. The resulting carbanion is very strongly stabilized by resonance with both the carbonyl and the highly electron-withdrawing nitro group, delocalizing the negative charge over the carbonyl oxygen and two nitro oxygens. Nitro-ketones are very strong acids (pKa ~4-6).

Comparing the stability of the conjugate bases: (C) > (B) > (A).

Therefore, the order of acidic strength is (C) > (B) > (A).

The least acidic compound is (A).