Question

Which is the incorrect IUPAC name?
(A) \[\;3 - Pentyne\]
(B) \[3 - Methyl - 2 - Butanone\]
(C) \[2 - Ethyl - 3 - Methyl - 1 - Butene\]
(D) \[3 - Ethyl - 2 - Methyl{\text{ }}Pentane\]

Answer 1

Hint: The naming of a chemical compound is done according to the guidelines set by the International Union of Pure Applied Chemistry (IUPAC). IUPAC develops recommendations for the development of distinct, constant and consistent nomenclature of the compounds and their respective classes.

Complete answer:
(A) Pentyne is the ${4^{th}}$ member of the homologous series of the alkyne family. Now, according to IUPAC while naming an alkyne chain the numbering is done in such order so as the triple bond gets the minimum number. The triple bond on the third carbon atom is second from the opposite side so it should be named \[2 - Pentyne\]. So, \[\;3 - Pentyne\] does not exist.
(B) \[3 - Methyl - 2 - Butanone\] is a four membered carbon ring with a methyl group as substitute and ketone as the functional group. While naming a ketone the numbering of the chain is done in such a manner so that the carbonyl carbon gets the lowest number and then the preference is given to the substituted group. Hence, \[3 - Methyl - 2 - Butanone\] exists.
(C) \[2 - Ethyl - 3 - Methyl - 1 - Butene\] is a four membered carbon ring with two alkyl groups as substitutes and alkene as the main group. In the given compound the longest carbon chain that can be formed is of four carbons hence the parent alkene is butane. The substituted alkyl groups are named alphabetically according to their positions. So, \[2 - Ethyl - 3 - Methyl - 1 - Butene\] exists.
(D) \[3 - Ethyl - 2 - Methyl{\text{ }}Pentane\] is a five membered carbon ring with two alkyl groups as substitutes and alkane as the main group. In the given compound the longest carbon chain that can be formed is of five carbons hence the parent alkane is pentane. The substituted alkyl groups are named alphabetically according to their positions. So, \[3 - Ethyl - 2 - Methyl{\text{ }}Pentane\] exists.

Note:While doing the nomenclature of any organic compound two things are must to be taken care of. One is a substituted group and the other is the main/functional group. The positions and alphabetically arrangement is a must to check, in order to name the compound correctly according to IUPAC guidelines.