What will be the decreasing order of basic strength of the following conjugate bases?
\( \text{OH}^-, \, \text{R}^-, \, \text{CH}_3\text{COO}^-, \, \text{Cl}^- \)
\( \text{OH}^-, \, \text{R}^-, \, \text{CH}_3\text{COO}^-, \, \text{Cl}^- \)
- \( \text{Cl}^- > \text{OH}^- > \text{R}^- > \text{CH}_3\text{COO}^- \)
- \( \text{R}^- > \text{OH}^- > \text{CH}_3\text{COO}^- > \text{Cl}^- \)
- \( \text{OH}^- > \text{R}^- > \text{CH}_3\text{COO}^- > \text{Cl}^- \)
- \( \text{Cl}^- > \text{R}^- > \text{OH}^- > \text{CH}_3\text{COO}^- \)
The Correct Option is B
Approach Solution - 1
Understanding Basic Strength in Terms of Conjugate Acids:
The basic strength of a conjugate base is inversely related to the strength of its conjugate acid. A strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.
Order of Acidity of Conjugate Acids:
To determine the basic strength of the conjugate bases, analyze the acidity of their conjugate acids:
- For \( \text{OH}^- \): The conjugate acid is \( \text{H}_2\text{O} \), which is a weak acid.
- For \( \text{RO}^- \): The conjugate acid is \( \text{R-OH} \) (an alcohol), which is generally weaker than water as an acid.
- For \( \text{CH}_3\text{COO}^- \): The conjugate acid is \( \text{CH}_3\text{COOH} \) (acetic acid), which is stronger than water and alcohols.
- For \( \text{Cl}^- \): The conjugate acid is \( \text{HCl} \), a strong acid.
Determine the Order of Basic Strength:
Based on the above analysis, the decreasing order of basic strength is:
\[ \text{RO}^- > \text{OH}^- > \text{CH}_3\text{COO}^- > \text{Cl}^- \]
Conclusion:
The correct order of decreasing basic strength is:
\[ \text{RO}^- > \text{OH}^- > \text{CH}_3\text{COO}^- > \text{Cl}^- \]
which corresponds to Option (2).
Approach Solution -2
We are asked to find the decreasing order of basic strength of the following conjugate bases:
\( \text{OH}^-, \, \text{R}^-, \, \text{CH}_3\text{COO}^-, \, \text{Cl}^- \).
Here, \( \text{R}^- \) represents an alkyl group conjugate base, for example, \( \text{CH}_3^- \).
Step 2: Understanding basic strength of conjugate bases.
Basic strength of a conjugate base is related to the stability of the base and acidity of its conjugate acid. The weaker the acid, the stronger the conjugate base.
- \( \text{Cl}^- \) is the conjugate base of hydrochloric acid (HCl), which is a strong acid, making \( \text{Cl}^- \) a very weak base.
- \( \text{CH}_3\text{COO}^- \) is the conjugate base of acetic acid, a weak acid, so acetate ion has moderate basic strength.
- \( \text{OH}^- \) is the conjugate base of water, which is very weakly acidic, making hydroxide ion a relatively strong base.
- \( \text{R}^- \) (like \( \text{CH}_3^- \)) is the conjugate base of an alkane (\( \text{R-H} \)), which is an extremely weak acid. Therefore, the alkyl anion is a very strong base.
Step 3: Comparison of acidity of the parent acids.
Acidity order (from weakest acid to strongest):
\[ \text{R-H} < \text{H}_2\text{O} < \text{CH}_3\text{COOH} < \text{HCl} \] Corresponding conjugate bases have the opposite order in basic strength:
\[ \text{R}^- > \text{OH}^- > \text{CH}_3\text{COO}^- > \text{Cl}^- \]
Step 4: Reasoning based on stability.
- \( \text{Cl}^- \) is stabilized by resonance with the strong acid environment, so it's very weakly basic.
- \( \text{CH}_3\text{COO}^- \) has resonance stabilization which lowers its basicity.
- \( \text{OH}^- \) is less stabilized and hence more basic than acetate.
- \( \text{R}^- \) has a negative charge localized on carbon, no resonance stabilization, making it very unstable and hence a very strong base.
Step 5: Conclusion - Final order of basic strength.
Based on acidity, stability, and resonance effects, the decreasing order of base strength is:
\[ \boxed{ \text{R}^- > \text{OH}^- > \text{CH}_3\text{COO}^- > \text{Cl}^- } \] This means the alkyl anion is the strongest base, followed by hydroxide ion, acetate ion, and chloride ion.
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