重拾化學情懷系列１：為何PHENOL是弱酸？

Unlike alcohols (which also contain an -OH group) phenol is a weak acid. A hydrogen ion can break away from the -OH group and transfer to a base.
For example, in solution in water:

圖片參考：http://www.chemguide.co.uk/organicprops/phenol/phenolh2oeqm.gif

Phenol is a very weak acid and the position of equilibrium lies well to the left.
Phenol can lose a hydrogen ion because the phenoxide ion formed is stabilised to some extent. The negative charge on the oxygen atom is delocalised around the ring. The more stable the ion is, the more likely it is to form.
One of the lone pairs on the oxygen atom overlaps with the delocalised electrons on the benzene ring.

圖片參考：http://www.chemguide.co.uk/organicprops/phenol/phenate1.gif

This overlap leads to a delocalisation which extends from the ring out over the oxygen atom. As a result, the negative charge is no longer entirely localised on the oxygen, but is spread out around the whole ion.

圖片參考：http://www.chemguide.co.uk/organicprops/phenol/phenate2.gif

Spreading the charge around makes the ion more stable than it would be if all the charge remained on the oxygen.
However, oxygen is the most electronegative element in the ion and the delocalised electrons will be drawn towards it. That means that there will still be a lot of charge around the oxygen which will tend to attract the hydrogen ion back again.
That's why phenol is only a very weak acid.

大話兄!!你真的很博學喎,化學都識;老實說,飛天兄的答案我曾看過,是在這裡:http://www.chemguide.co.uk/organicprops/phenol/acidity.html
只不過很耐了!!中六,中七的化學知識小弟真的很久沒掂了!!所以不敢隨便以網上資料作答案!!(畢竟內裡的理念是很深的!!)