Textbook Question
Rank the compounds in each set in order of increasing acid strength.
(b) CH3CH2CH2CHBrCOOH, CH3CH2CHBrCH2COOH, CH3CHBrCH2CH2COOH
1333
views
3. Acids and Bases
Ranking Acidity
4:13 minutesProblem 11
Textbook Question
p-Nitrophenol (pKa = 7.2) is ten times more acidic than m-nitrophenol (pKa = 8.4) Explain.
Verified step by step guidance1
Step 1: Understand the relationship between pKa and acidity. A lower pKa value indicates a stronger acid. Since p-nitrophenol has a pKa of 7.2 and m-nitrophenol has a pKa of 8.4, p-nitrophenol is more acidic.
Step 2: Analyze the structures of p-nitrophenol and m-nitrophenol. Both molecules contain a phenol group (-OH attached to a benzene ring) and a nitro group (-NO2). The position of the nitro group relative to the hydroxyl group differs: para (p) for p-nitrophenol and meta (m) for m-nitrophenol.
Step 3: Consider the electron-withdrawing effect of the nitro group. The nitro group is highly electronegative and withdraws electron density from the benzene ring through both inductive and resonance effects. In p-nitrophenol, the nitro group is positioned para to the hydroxyl group, allowing resonance stabilization of the phenoxide ion formed after deprotonation.
Step 4: Examine resonance stabilization in p-nitrophenol. When p-nitrophenol loses a proton, the negative charge on the oxygen can delocalize through the benzene ring and interact with the nitro group via resonance. This delocalization stabilizes the phenoxide ion, making p-nitrophenol more acidic.
Step 5: Compare with m-nitrophenol. In m-nitrophenol, the nitro group is not in a position to participate in resonance stabilization of the phenoxide ion. The electron-withdrawing effect is limited to inductive effects, which are weaker than the combined inductive and resonance effects in p-nitrophenol. This explains why m-nitrophenol is less acidic than p-nitrophenol.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
Video duration:
4mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Acidity and pKa
Acidity in organic chemistry is often measured using the pKa value, which indicates the strength of an acid in solution. A lower pKa value corresponds to a stronger acid, meaning it more readily donates protons (H+). In this context, p-nitrophenol has a pKa of 7.2, making it a stronger acid than m-nitrophenol, which has a pKa of 8.4.
Recommended video:
Guided course
07:45
Identifying pKa values
Resonance Stabilization
Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, leading to a more stable overall structure. In p-nitrophenol, the nitro group is positioned para to the hydroxyl group, allowing for effective resonance that stabilizes the negative charge on the phenoxide ion formed after deprotonation. This stabilization is less effective in m-nitrophenol due to its meta positioning.
Recommended video:
Guided course
03:43The radical stability trend.
Inductive Effect
The inductive effect refers to the electron-withdrawing or electron-donating effects of substituents on a molecule, influencing its reactivity and acidity. The nitro group in p-nitrophenol exerts a strong electron-withdrawing inductive effect, enhancing the acidity by stabilizing the negative charge on the conjugate base. In contrast, the inductive effect in m-nitrophenol is less pronounced, contributing to its lower acidity.
Recommended video:
Guided course
01:47Understanding the Inductive Effect.
Related Videos
Related Practice