Question

Phenol is less acidic than:
A. o−nitrophenol
B. p−methoxy phenol
C. 2−methylpropanol
D. 2, 2−dimethyl pentanol

Answer 1

Hint: We should know that phenol is a hydroxyl (―OH) group attached to a carbon atom which is part of an aromatic ring. To answer this question we should know that it is mildly acidic and it can cause chemical burns to our skin.

Step by step answer:
We should know that phenol is an aromatic organic compound with the molecular formula \[{{C}_{6}}{{H}_{5}}OH\]. The molecule of phenol consists of a phenyl group (−\[{{C}_{6}}{{H}_{5}}\]) bonded to a hydroxyl group (−OH). We should know that it is mostly soluble in water. We should know that compounds like phenol which contain an -OH group attached to a hydrocarbon are very weak acids. Now, we will know the reason for weak acidity of phenols. When we add phenol to water, it quickly loses hydrogen because the phenoxide ion formed is stabilised. We know that oxygen atom is more electronegative, and that’s why it draws all the electrons to itself and then there will still be a lot of charge on oxygen which then attracts the hydrogen ion back again. That is why phenol is only a very weak acid.
Now, to answer this question we should check other options also by making their structure.

The above structure is of O−nitrophenol. As we can see there is a nitro group attached to phenol. We should know that phenol is less acidic than O−nitrophenol. We should know that nitro group is an electron-withdrawing group. It means that nitro group decreases electron density on benzene ring and delocalizes negative charge of phenoxide ion. It means that nitro group is doing just opposite that of hydroxyl group. So, from this it increases acidity. Hence, O−nitrophenol is more acidic than phenol.
Now, we will look at the second option that is of P−methoxy phenol. So, first of all we will look at the structure of P−methoxy phenol.

The above structure is of p-methoxyphenol. So, we should know that a methoxy group is an electron releasing group. It increases electron density on the benzene ring and does not delocalize negative charge of phenoxide ion. It decreases acidity. So, now we should know that p−methoxyphenol is less acidic than phenol.
So, till now we know that phenol is more acidic than p-methoxyphenol but not more acidic than o−nitrophenol.

Now, we will look at two other options that are 2−methylpropanol and 2, 2−dimethyl pentanol. We should know that 2−methylpropanol is a secondary alcohol. And 2, 2−dimethyl pentanol is tertiary alcohol. We should note that primary alcohols are more acidic than secondary alcohols which are more acidic than tertiary alcohols. We should understand this concept that strength of the alcohol as an acid is dependent on the corresponding strength of its conjugate base, the alkoxide ion. A more stabilised alkoxide is a weaker conjugate base and hence the alcohol will be more acidic.
Phenols are more acidic than alcohols due to stabilisation of phenoxide ions through resonance. So, from this we can tell that phenol is more acidic than p−methoxy phenol, 2−methylpropanol and 2,2−dimethyl pentanol. But it is less acidic than o-nitrophenol. So, now we can say that option A is our correct answer.

Note: We should be careful in handling of phenols because phenol and its vapours are corrosive to the eyes, the skin, and the respiratory tract. As phenol is acidic, we should handle it carefully. It can cause chemical burns.