Animated Solution for Chemistry - Organic Chemistry: Choose the correct name for compound given below

Welcome to a fascinating journey into the world of organic nomenclature! Today, we are tackling a molecule that features both a double bond and a triple bond.

Our primary objective is to determine its exact IUPAC name, which includes figuring out its stereochemistry.

When a molecule has multiple functional groups or bonds, the rules of nomenclature act as our guiding compass. Let's break this down step by step.

Numbering the Chain

The first critical step in naming any organic compound is identifying and numbering the longest continuous carbon chain.

The golden rule here is to assign the lowest possible locants (numbers) to the multiple bonds. If we number our $6$-carbon chain from right to left, the double bond lands at position $2$ and the triple bond at position $4$.

Interestingly, if we number from left to right, we also get locants $2$ and $4$. So, how do we break the tie?

According to IUPAC rules, when there is a tie in locants, the double bond gets priority over the triple bond. Therefore, numbering from the right is the correct path.

This gives us a base name of $2\text{-bromohex-2-en-4-yne}$.

Assigning E/Z Configuration

Now comes the exciting part—determining the stereochemistry across the double bond using the Cahn-Ingold-Prelog (CIP) priority rules.

Let's focus on carbon $2$. It is bonded to a bromine atom and a methyl group. Since the atomic number of bromine ($35$) is significantly higher than that of carbon ($6$), the bromine atom takes the higher priority.

Next, we examine carbon $3$. It is bonded to a hydrogen atom and the rest of the alkyne carbon chain. Carbon has a higher atomic number than hydrogen, so the carbon chain claims the higher priority here.

Finally, we draw an imaginary axis straight through the double bond. We observe that our two high-priority groups—the bromine atom and the carbon chain—are situated on opposite sides of this axis.

In stereochemistry, when high-priority groups are on opposite sides, we assign the E configuration (from the German word Entgegen, meaning opposite).

Putting it all together, the complete and correct IUPAC name is $(2\text{E})\text{-2-bromohex-2-en-4-yne}$.