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  4. Which one of the following compounds is most acidic?

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Q. Which one of the following compounds is most acidic?

AFMCAFMC 2012

Solution:

Phenols are much more acidic than alcohol due to the stabilisation of phenoxide ion resonance.
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Phenoxide ion is stablised due to the following resonating structures.
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While, in alcohols
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Ortho nitrophenol is most acidic because in it $- NO _{2}$ electron attracting group is attached at ortho position which helps in stabilising the negative charge on the oxygen of phenoxide ion. Hence, due to this reason, acidic character of phenol is increased, while on attachment of - $CH _{3}$ group (electron donating group) acidic strength of phenol is decreased in cresol due to the destabilisation of phenoxide ion.