Question

IUPAC name of $C{H_3} - CH = CH - C \equiv CH$ is:
A. Pent-2-en-4-yne
B. Pent-3-en-1-yne
C. Pent-3-yn-1-ene
D. Pent-2-yn-1-ene

Answer 1

Hint: The International Union of Pure and Applied Chemistry (IUPAC) has given some set of rules for deciding the name of any organic compound. The process of naming of any compound based on such rules is known as nomenclature. As there are only multiple bonds present in the compound given above, we must apply the multiple bond rule of nomenclature in deciding the name of the compound.

Complete step by step answer:
There are various set of rules given by the IUPAC for naming of compound consisting of only double (alkene) and triple (alkyne) bonds. Let us discuss them one by one.
Alkenes (or double bonded systems):
1. The double bond always ends with a suffix ‘ene’.
2. The longest chain chosen for the root name must include both carbon atoms of the double bond.
3. The root chain must be numbered from the end nearest a double bond carbon atom. If the double bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4. The smaller of the two numbers designating the carbon atoms of the double bond is used as the double bond locator. If more than one double bond is present the compound is named as a diene, triene or equivalent prefix indicating the number of double bonds, and each double bond is assigned a locator number.
Alkynes (or triple bond system):
1. The triple bond always ends with a suffix ‘yne’.
2. The longest chain chosen for the root name must include both carbon atoms of the triple bond.
3. The root chain must be numbered from the end nearest a triple bond carbon atom. If the triple bond is in the center of the chain, the nearest substituent rule is used to determine the end where numbering starts.
4. The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator.
5. If several multiple bonds are present, each must be assigned a locator number. Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature.
Thus, according to the above mentioned rules, the name of the compound is given as: \[Pent - 3 - en - 1 - yne\] .

The correct option is B. \[Pent - 3 - en - 1 - yne\] .

Note:
The longest parent carbon chain for the compound consists of five carbon atoms which has both the double as well as triple bonds. Now, the numbering is done from that side of the chain where the triple bond gets the lowest possible number. But, during the naming, the ‘en’ is placed before the ‘yne’ on the basis of alphabetical order.