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Which of the following orders of acid strength is correct :
A
$RCOOH \gt ROH \gt HOH \gt HC \equiv CH$
B
$RCOOH \gt HOH \gt ROH \gt HC \equiv CH$
C
$RCOOH \gt HOH \gt HC \equiv CH \gt ROH$
D
$RCOOH \gt HC \equiv CH \gt HOH \gt ROH$
Detailed Solution
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 $RCOOH \gt HOH \gt ROH \gt HC \equiv CH$.
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 $RCOOH \gt HOH \gt ROH \gt HC \equiv CH$.