Question

Which of the following is a weak acid?
A. ${C}_{6}{H}_{6}$
B. $C{ H }_{ 3 }-C\equiv CH$
C. $C{H}_{2}=C{H}_{2}$
D. $C{H}_{3}-C\equiv C-C{H}_{3}$

Answer 1

Hint: The acidity of a compound is the measure or a tendency of the compounds to act as the donor of the electrons. The acidity of the compounds depends upon the s-character of the compound. And also the +I effect of the group.

Complete step by step answer:
- The acidity of a compound is a tendency to give ${H}^{+}$ ions in a solution.
- The easier a compound gives the ${H}^{+}$ ions, the more acidic the compound is.
- The compounds can be bonded with single bond, double bond or triple bond.
- The single bonds and exactly one bond in the double and triple bond is a sigma bond and is known to have the s-character.
- The s-character refers to the contribution of the sigma bond in a compound.
- The amount of s-character in a compound also affects the acidity of a compound.
- The more the s-character, the more acidic will be the compound.
- The acidity also depends upon the groups that show +I effect like the methyl group.
- When a group that has the tendency to release or donate electrons, is attached to the carbon chain, then in this case the charge is replayed through the chain. This effect is known as +I effect or positive inductive effect.
- The more no. of groups that show +I effect, the less is the acidity of that compound.
Now, keeping these points in mind, let us look at the question.
- The alkyne groups are the most acidic as they have a carbon atom which has a $sp$ hybridization.
- Therefore, looking at the options, option (b) and (d) are the most acidic compounds as they are alkynes and have the greatest s-character.
Now, amongst ${C}_{6}{H}_{6}$ and $C{H}_{2}=C{H}_{2}$, ${C}_{6}{H}_{6}$ has more no. of C-H bonds and hence can dissociate ${H}^{+}$ ions more easily than $C{H}_{2}=C{H}_{2}$. Hence, ${C}_{6}{H}_{6}$ is more acidic than $C{H}_{2}=C{H}_{2}$. Therefore, the least or the weak acid is $C{H}_{2}=C{H}_{2}$. Hence the correct option is option (c).

Note: Among $C{ H }_{ 3 }-C\equiv CH$ and $C{H}_{3}-C\equiv C-C{H}_{3}$, $C{ H }_{ 3 }-C\equiv CH$ is more acidic in nature. This is due to the fact that in $C{H}_{3}-C\equiv C-C{H}_{3}$, no hydrogen atom is attached to $sp$ hybridized carbon.