Question

The IUPAC name of the compound $C{{H}_{2}}=CH-CH{{(C{{H}_{3}})}_{2}}$:
(A) 2-Methyl but-2-ene
(B) 3-Methyl but-2-ene
(C) 3-Methyl but-1-ene
(D) 2-Methyl but-3-ene

Answer 1

Hint: The longest chain of carbon atom containing a functional group is numbered in such a way that the functional group attached to the carbon atom possesses the lowest possible number in the chain. Name of alkyls group attached as branch are then prefixed to name of parent.

Complete solution step by step:
-In IUPAC system of nomenclature, name of organic compound is derived by identifying parent hydrocarbon and functional group attached to it.
-Following are rules for naming an organic compound:
-First, the longest carbon chain in molecules is identified. Longest chain is considered the parent chain.
-The longest chain of carbon atom containing a functional group is numbered in such a way that the functional group attached to the carbon atom possesses the lowest possible number in the chain.
-Numbering should be done in such a way that branched carbon atoms will get the lowest possible numbers.
-Name of alkyls group attached as branch are then prefixed to name of parent alkane.
-position of substituent is indicated by appropriate numbers.
-If two or more identical substituents are present then numbers are separated by comma.
-if two substituents are in equivalent position, lower number is given to one coming first in alphabetical listing
-For $C{{H}_{2}}=CH-CH{{(C{{H}_{3}})}_{2}}$, longest carbon atom chain has four carbon atoms, so alkane chain is butane. The functional group is alkene, so the name leads to butene.
-The position of double bond must be specified, so there are two ways. In one case, the double bond is between C3 and C4. In other cases, the double bond is between C1 and C2. So according to IUPAC rules, the appropriate name would be but-1-ene.
-In numbering the branching, methyl group gets number 3.

so The IUPAC name of the compound $C{{H}_{2}}=CH-CH{{(C{{H}_{3}})}_{2}}$ is (C) 3-Methyl but-1-ene.

Note: The IUPAC name for Isopentane is 2-methylbutane as the longest chain of carbon atoms is four atoms. For Neopentane, the IUPAC name is 2,2-Dimethylpropane as the longest chain of carbon atoms is having three atoms.