Discussion
Which of the following is the weakest acid?
Answer: D
The weakest acid is Ethanol.
Acid strength depends on how stable the anion is after the proton leaves. When phenol loses a proton, the negative charge on the phenoxide ion is delocalised into the benzene ring by resonance. The ethoxide ion has no such resonance, and the ethyl group's +I effect concentrates the charge on oxygen further, so ethanol (pKa about 16) is a much weaker acid than any of the phenols (pKa about 7 to 10).
Among the phenols, p-nitrophenol is the strongest, since the nitro group stabilises the phenoxide further. The cresols carry an electron-donating methyl group and are slightly weaker than phenol itself, but they are still far stronger acids than ethanol.
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