Question

Which of the following is most acidic?
a.) Benzyl alcohol
b.) Cyclohexanol
c.) Phenol
d.) M-chlorophenol

Answer 1

In order to solve the given problem we will first determine what are the basic characteristics on which acidic nature of an organic compound depends further on the basic of the given criteria we will compare the acidic nature of each of the organic compounds from one another. On the basis of our comparison of the acidic behavior we will select the most acidic compound amongst the given four options.

Complete step by step answer:
In order to find the answer we have different characteristics on which the acidic nature of alcohol depends.
Some of these characteristics are:
+R effect
–I effect
Type of group attached to the alcohol
Tendency to remove H ion
The above all the options are alcohol.

Let us first compare option A, B and C.
As option A and B are both alcohols.
We know that the alcohols as well as the phenol are acidic in nature but they both are weak acids.
The alcohols are less acidic than phenols so it is very difficult to quickly detach the H ion because the formed phenoxide ion is balanced to certain degree. This is the negative charge around the ring on the oxygen atom.
Phenol is more acidic than alcohol due to resonance stabilization of the phenoxide ion. The presence of the electron withdrawing group increases phenol acidity by stabilizing the phenoxide ion, while the presence of the electron releasing group reduces phenol acidity by destabilizing the phenoxide ion.
So phenol is more acidic than benzyl alcohol and cyclohexanol.
Now let us compare the acidic nature of phenol and m-chlorophenol.

As we know that chlorine (a halogen) is attached to m-chlorophenol apart from the structure of phenol.
Also we know that due to the presence of halogen we have +R effect and –I effect and –I effect is dominating. Due to electron withdrawing nature of –Cl group or halogen group it is easier to remove hydrogen from m-chlorophenol as compared to phenol.
So, the acidity of m-chlorophenol is higher than phenol.

Finally we can conclude that m-chlorophenol is most acidic amongst them all.
Hence, M-chlorophenol is most acidic. So, the correct answer is “Option D”.

Note: In order to solve such types of problems students must be aware of the different characteristics on which acidic and basic nature of a compound depends, some of which are listed above in the solution. Students should also remember about the electron donating and electron withdrawing nature of different groups.