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Alcohols, Phenols and Ethers
Acidity of Alcohols
1 question from 1 paper, each with its answer.
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Concepts tested
- Relative acidity of carboxylic acids, water, alcohols and alkynes
All Questions
2003 AIPMT 1 question
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Which of the following orders of acid strength is correct :Carboxylic acids are the strongest because the carboxylate ion is stabilised by resonance ($pK_a \approx 4.8$ for acetic acid).
Water ($pK_a \approx 15.7$) is a stronger acid than alcohols ($pK_a \approx 16$–18), because the alkyl group (+I effect) increases the electron density on the oxygen of $RO^-$ and destabilises it.
Ethyne ($pK_a \approx 25$) is a much weaker acid than water and alcohols, although it is acidic because of the $sp$ hybridised carbon.
This order is confirmed by reactions: water reacts with sodium acetylide to give ethyne, and alkoxides are formed from alcohols but are destroyed by water.
So the correct order is $RCOOH \gt HOH \gt ROH \gt HC \equiv CH$.