Question

What is the functional group present in a molecule having the formula ${C_{12}}{O_9}$ ?
A) Carboxylic acid
B) Anhydride
C) Aldehyde
D) Alcohol

Answer 1

In the given molecular formula, it is clear that there are no Hydrogen atoms. Hence we can write down the general molecular formula for every functional group and use the method of elimination to find the correct functional group present in ${C_{12}}{O_9}$

Complete answer: Let us write the general molecular formulas for every functional group given.
For Carboxylic acid, the general molecular formula is $RCOOH$
Similarly, we know that the general molecular formula for Anhydride is ${(RC = {\rm O})_2}O$
For Aldehyde, we write the general molecular formula as $RCHO$
Finally for Alcohol, we can write the general molecular formula as $ROH$
We already know that in the given molecule there are no Hydrogen atoms. Hence by using the method of elimination, we can see that Carboxylic acid, Aldehyde and Alcohol require at least one Hydrogen atom to form their respective functional groups.
Therefore, Anhydride is the only group that doesn’t require any Hydrogen atoms and is the functional group present in ${C_{12}}{O_9}$
The compound ${C_{12}}{O_9}$ is known as Mellitic anhydride and is formed by heating Mellic acid which is benzene substituted with six carboxylic acid groups. On heating Mellitic acid, water is eliminated forming three anhydride groups having the formula ${C_{12}}{O_9}$.

Hence, the functional group present in the given molecule is Anhydride i.e. option (B).

Note: The given compound having the molecular formula ${C_{12}}{O_9}$ is Mellitic anhydride. This compound consists of three anhydride groups formed around benzene. This compound is also known as Benzenehexacarboxylic tri anhydride.