Practicing Success
Which of the following acid has the smallest dissociation constant? |
CH3CHFCOOH FCH2CH2COOH BrCH2CH2COOH CH3CHBrCOOH |
BrCH2CH2COOH |
The correct answer is option 3. BrCH2CH2COOH. Let us explain why BrCH₂CH₂COOH has the smallest dissociation constant among the given acids by analyzing the effects of the substituents on the carboxylic acid group. Factors Affecting Acid Strength: Inductive Effect: Electronegative substituents (like halogens) withdraw electron density through the sigma bonds, stabilizing the conjugate base and increasing the acid strength. Position of Substituents: The closer the electronegative substituent to the carboxyl group, the stronger the inductive effect. Given Acids: 1. CH₃CHFCOOH (2-fluoropropanoic acid): Substituent: Fluorine on the α-carbon. Effect: Strong inductive electron-withdrawing effect due to fluorine being very electronegative and close to the carboxyl group. This makes the acid quite strong. 2. FCH₂CH₂COOH (3-fluoropropanoic acid): Substituent: Fluorine on the β-carbon. Effect: Fluorine still withdraws electron density but less effectively because it is farther from the carboxyl group compared to CH₃CHFCOOH. Thus, the acid is weaker than CH₃CHFCOOH. 3. BrCH₂CH₂COOH (3-bromopropanoic acid): Substituent: Bromine on the β-carbon. Effect: Bromine is less electronegative than fluorine and is on the β-carbon. The inductive effect is weaker than fluorine’s and less effective due to the position, making the acid weaker. 4. CH₃CHBrCOOH (2-bromopropanoic acid): Substituent: Bromine on the α-carbon. Effect: Bromine’s inductive effect is stronger when closer to the carboxyl group (α-carbon), making this acid stronger than BrCH₂CH₂COOH but weaker than CH₃CHFCOOH due to bromine being less electronegative than fluorine. Comparison: CH₃CHFCOOH: Strongest acid due to the strong inductive effect of fluorine on the α-carbon. FCH₂CH₂COOH: Weaker than CH₃CHFCOOH but still relatively strong due to fluorine on the β-carbon. CH₃CHBrCOOH: Weaker than both fluorine-containing acids because bromine is less electronegative but still stronger than BrCH₂CH₂COOH due to the α-position. BrCH₂CH₂COOH: Weakest acid due to bromine’s weak inductive effect from the β-position. Conclusion: BrCH₂CH₂COOH has the smallest dissociation constant because the bromine atom's inductive effect is weaker and less effective due to its position on the β-carbon. Therefore, it stabilizes the conjugate base the least, making the acid the weakest among the given options. |