Question

Which one is called ethanoic acid [CPMT $1997$]
A.$HCOOH$
B.$C{{H}_{3}}COOH$
C.$C{{H}_{3}}C{{H}_{2}}COOH$
D.$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}COOH$

Answer 1

Hint: carboxylic acid is an organic compound in which a carbon atom is attached to an oxygen atom by a double bond and a hydroxyl group, $-OH$group by a single bond. Here we also have carboxylic acid, ethanoic acid is given. So, we just have to find out the chemical formula of ethanoic acid.

Complete answer:Carboxylic acid is an organic compound with a functional group. $-COOH$ and general formula $R-COOH$where $R$denotes alkyl or aryl groups. The general structures of the carboxylic acid are shown below:


We can derive the IUPAC name of carboxylic acid from the total carbon atom from the longest carbon chain that contains the carboxyl group by dropping the final $\text{-e}$from the name of the parent alkane and then adding the suffix ‘oic acid’. The chain numbering is started from the carbon of the carboxyl group, $-COOH$.
For example, an acid $C{{H}_{3}}COOH$has two carbon atoms, and thereby it is called ethanoic acid [That is (ethan $+$oic acid)$=$ethanoic acid].


In this problem, we have four different carboxylic acids. Let us see their IUPAC name one by one.
In (A),$HCOOH$ has only one carbon atom, hence it is called methanoic acid.


In (B) we have already seen above that this is ethanoic acid.

$C{{H}_{3}}C{{H}_{2}}COOH$ has three carbon atoms and thereby the IUPAC name of this compound is propanoic acid.


Finally, (D)$C{{H}_{3}}C{{H}_{2}}C{{H}_{2}}COOH$, has four carbon atoms, then the IUPAC name would be butanoic acid.


Therefore ethanoic acid is acetic acid with the chemical formula $C{{H}_{3}}COOH$.
Thus, option (B) is correct.

Note: To approach these IUPAC nomenclature-related problems we must have to learn to give proper numbering to the longest carbon chain and should also remember the prefix is used based on the total carbon atom present in the longest chain. For example, for one carbon atom the prefix ‘$meth-$’, for two carbon atoms, the prefix ‘$eth-$’, for three carbon atoms, the prefix$prop-$’ is used, and so on.