What is the order of decreasing acidity for the molecules phenol, ethanoic acid and ethanol? Why?

Ethanoic acid is the most acidic of the three. This is because once ethanoic acid has lost a proton the negative charge can be spread over two oxygen atoms via two resonance forms. Oxygen atoms are electronegative, therefore happy to stabilise the negative charge resulting in a relatively stable ion. The next most acidic is phenol. Although more resonance forms can be drawn (4 in total) the majority of them (3) have the negative charge on the more electropositive atom carbon. This ion is therefore less stable than the ion formed from ethanoic acid as oxygen stabilises a negative charge better than carbon. Phenol is therefore less able to form the ion, therefore less acidic. The least acidic of the three is ethanol. This is because no resonance forms can be drawn for the resulting ion. This inability to delocalise the charge over the ion makes this the least stable ion. If the ion is the most unstable, ethanol must have the most difficulty to remove it's most acidic proton, making it the least acidic molecule of the three.

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Answered by Dan T. Chemistry tutor

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