Question:medium
Correct order of acidic strength:
Correct order of acidic strength:
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Carboxylic acids are generally stronger than phenols, and phenols are stronger than alcohols. Remember the sequence: $RCOOH>ArOH>H_2O>ROH$.
Updated On: Apr 20, 2026
- Phenol>Ethanol>Acetic acid
- Acetic acid>Phenol>Ethanol
- Ethanol>Phenol>Acetic acid
- Phenol>Acetic acid>Ethanol
- Acetic acid>Ethanol>Phenol
Show Solution
The Correct Option is B
Solution and Explanation
Step 1: Understanding the Concept:
The fundamental principle for comparing the acidic strength of different organic compounds is to assess the stability of their corresponding conjugate bases. An acid is considered strong if it easily donates a proton (\(\text{H}^+\)). It will readily do so if the resulting negative species (the conjugate base) is stable. A more stable conjugate base corresponds to a stronger parent acid.
Step 2: Key Formula or Approach:
Determine the structure of the conjugate base for each given compound by removing an acidic proton. Then, evaluate and rank their stabilities based on electronic effects, primarily resonance delocalization and inductive effects.
Step 3: Detailed Explanation:
Let's examine the three compounds and their conjugate bases:
1. Acetic acid (\(\text{CH}_3\text{COOH}\)):
When it loses a proton, it forms the acetate ion (\(\text{CH}_3\text{COO}^-\)). This conjugate base is extremely stable because the negative charge is delocalized equally over two highly electronegative oxygen atoms via resonance. These equivalent resonance structures provide significant stabilization.
2. Phenol (\(\text{C}_6\text{H}_5\text{OH}\)):
Losing a proton yields the phenoxide ion (\(\text{C}_6\text{H}_5\text{O}^-\)). This ion also enjoys resonance stabilization; the negative charge initially on the oxygen atom can be delocalized into the \(\pi\)-electron system of the aromatic benzene ring. However, in these resonance structures, the negative charge is distributed onto carbon atoms, which are significantly less electronegative than oxygen. Consequently, this resonance is less effective at stabilizing the charge than the equivalent resonance seen in the acetate ion. Therefore, phenol is a weaker acid than acetic acid.
3. Ethanol (\(\text{C}_2\text{H}_5\text{OH}\)):
Deprotonation of ethanol gives the ethoxide ion (\(\text{C}_2\text{H}_5\text{O}^-\)). This ion lacks any resonance stabilization. Furthermore, the attached ethyl group (\(-\text{CH}_2\text{CH}_3\)) exerts an electron-donating inductive effect (+I effect). This pushes additional electron density onto the already negatively charged oxygen atom, intensifying the charge and destabilizing the ion. Because its conjugate base is the least stable, ethanol is the weakest acid of the three.
Synthesizing these observations, the order of conjugate base stability is:
Acetate ion > Phenoxide ion > Ethoxide ion
This directly translates to the order of acidic strength for their parent molecules:
Acetic acid > Phenol > Ethanol
Step 4: Final Answer:
The correct decreasing order of acidic strength is Acetic acid > Phenol > Ethanol.
The fundamental principle for comparing the acidic strength of different organic compounds is to assess the stability of their corresponding conjugate bases. An acid is considered strong if it easily donates a proton (\(\text{H}^+\)). It will readily do so if the resulting negative species (the conjugate base) is stable. A more stable conjugate base corresponds to a stronger parent acid.
Step 2: Key Formula or Approach:
Determine the structure of the conjugate base for each given compound by removing an acidic proton. Then, evaluate and rank their stabilities based on electronic effects, primarily resonance delocalization and inductive effects.
Step 3: Detailed Explanation:
Let's examine the three compounds and their conjugate bases:
1. Acetic acid (\(\text{CH}_3\text{COOH}\)):
When it loses a proton, it forms the acetate ion (\(\text{CH}_3\text{COO}^-\)). This conjugate base is extremely stable because the negative charge is delocalized equally over two highly electronegative oxygen atoms via resonance. These equivalent resonance structures provide significant stabilization.
2. Phenol (\(\text{C}_6\text{H}_5\text{OH}\)):
Losing a proton yields the phenoxide ion (\(\text{C}_6\text{H}_5\text{O}^-\)). This ion also enjoys resonance stabilization; the negative charge initially on the oxygen atom can be delocalized into the \(\pi\)-electron system of the aromatic benzene ring. However, in these resonance structures, the negative charge is distributed onto carbon atoms, which are significantly less electronegative than oxygen. Consequently, this resonance is less effective at stabilizing the charge than the equivalent resonance seen in the acetate ion. Therefore, phenol is a weaker acid than acetic acid.
3. Ethanol (\(\text{C}_2\text{H}_5\text{OH}\)):
Deprotonation of ethanol gives the ethoxide ion (\(\text{C}_2\text{H}_5\text{O}^-\)). This ion lacks any resonance stabilization. Furthermore, the attached ethyl group (\(-\text{CH}_2\text{CH}_3\)) exerts an electron-donating inductive effect (+I effect). This pushes additional electron density onto the already negatively charged oxygen atom, intensifying the charge and destabilizing the ion. Because its conjugate base is the least stable, ethanol is the weakest acid of the three.
Synthesizing these observations, the order of conjugate base stability is:
Acetate ion > Phenoxide ion > Ethoxide ion
This directly translates to the order of acidic strength for their parent molecules:
Acetic acid > Phenol > Ethanol
Step 4: Final Answer:
The correct decreasing order of acidic strength is Acetic acid > Phenol > Ethanol.
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