Question

What is the IUPAC name of the following compound?

A.Pent-2-ene
B.Pent-1-ene
C.Pent-3-ene
D.1-methyl-but-2-ene

Answer 1

Hint: Giving the IUPAC name to any compound has some rules that need to be followed. Such as any functional group comes first, triple bond comes over double bond and double bond comes over single bond.

Complete answer:
The correct answer to this question is option A, the IUPAC name of the given compound is Pent-2-ene.
While giving IUPAC name to any compound, certain rules need to be followed. Here, one of the rules that is followed is that the end side nearest to the double is taken first. Thus, the name giving starts from the left side.
Also, the given compound can be written as follows.
\[{{H}_{3}}C-CH=CH-C{{H}_{2}}-C{{H}_{3}}\]
Thus, giving name to this compound becomes easier. One of the rules for naming a compound according to the rules is that, if it is possible, the compound should be written in a straight line.
Now, we know that the compound has 5 carbon atoms, thus the name of this hydrocarbon will have pentane in it. Starting from the left to take a double bond at the earliest, the compound is named as Pent-2-ene.
As the double bond comes at the second carbon atom first while going from left to right, the name ene comes at the second place. Thus, the name of the compound is Pent-2-ene and not Pent-3-ene.
It is clear from the given compound that the double is not at the first carbon atom even if we go from right to left, thus Pent-1-ene cannot be the name of the compound.
As we have straightened out the chain, the name of the compound cannot be 1-methyl-but-2-ene.
Therefore, only one option remains and it is option A, Pent-2-ene.

Note:
Whenever a compound has methyl or any other compound attached to the last carbon of the chain, either upwards or downwards, it has to straighten out as per the rules. All the rules has to be followed, otherwise the name of the compound could vary. For example if the chain would not have been straightened out in this question, the name of the compound would have been 1-methyl-but-2-ene.