Question

Arrange the Hydrogen of the following compounds from most acidic to least acidic : 

Choose the correct answer from the options given below : 
 

• A. B > E > C > A > D
• B. B > C > E > D > A
• C. E > C > B > D > A
• D. C > A > E > B > D

Hint:

A fast way to order acidity is to first look at the formal charges: positively charged species ($O-H^+, N-H^+$) are almost always more acidic than neutral molecules. Then look at the functional group: Carboxylic Acids $>$ Phenols/Activated alcohols $>$ Normal Alcohols $>$ Alkynes $>$ Amines $>$ Alkanes.

Answer 1

The Correct Option is C

Step 1: Concept:
We are asked to rank the acidity of five specific protons in five different chemical environments. Acidity is measured by how easily a compound donates a proton, which corresponds to the stability of the resulting conjugate base or the formal charge of the acidic species.

Step 2: Key Formula or Approach:
Evaluate the approximate $pK_a$ values or structural stability factors for each molecule. A lower $pK_a$ means a stronger acid.
- Positively charged species with a proton on a highly electronegative atom (like $O^+$ or $N^+$) are generally very strong acids.
- Electron-withdrawing groups (EWG) via inductive or resonance effects increase acidity.
- $sp$ hybridized carbons are more electronegative than $sp^3$, making terminal alkynes moderately acidic.
- Neutral amines are very weak acids.

Step 3: Step-by-step Explanation:
Let's analyze the structures shown in the image (based on standard chemical notation inferences):

• E. Protonated ether/ketone ($R_2C=O^+-H$ or $R-O^+(H)-R$): The image shows a positively charged oxonium ion. Positively charged oxygen species are extremely acidic because Oxygen is highly electronegative and wants to pull electrons back to neutralize the charge. Expected $pK_a$ is less than 0 (e.g., protonated acetone is $\sim -7.3$). Most acidic.

• C. Dichloroacetic acid ($CHCl_2COOH$): This is a carboxylic acid. The acidity is heavily increased by the significant inductive electron-withdrawing effect ($-I$ effect) of the two adjacent chlorine atoms, which stabilize the carboxylate anion. Expected $pK_a \approx 1.3$. Second most acidic.

• B. Trifluoromethanol ($CF_3OH$): It is an alcohol, but the presence of the powerful electron-withdrawing $CF_3$ group drastically lowers its $pK_a$ compared to normal alcohols (like ethanol at $\sim 16$). Though highly unstable in reality, its theoretical $pK_a$ is around $10$, making it substantially more acidic than normal alcohols or alkynes. Third most acidic.

• D. Terminal alkyne proton ($R-C\equiv C-H$): The hydrogen is attached to an $sp$ hybridized carbon, which is more electronegative than $sp^2$ or $sp^3$ carbons due to 50% s-character. The conjugate base (acetylide anion) is relatively stable. Expected $pK_a \approx 25$. Fourth most acidic.

• A. Piperidine (neutral secondary amine): The image shows a saturated nitrogen heterocycle with an N-H bond. Neutral amines are extremely poor acids (and act primarily as bases). Deprotonating a neutral amine produces a highly unstable amide anion ($N^-$). Expected $pK_a \approx 35$. Least acidic.
Combining these evaluations, the order from most to least acidic is E > C > B > D > A.

Step 4: Final Answer:
This sequence perfectly matches option (C).