Question Video: Naming a Bromo Substituted Alkane From Its Displayed Formula Chemistry • 10th Grade

What is the name of this molecule?

03:45

Video Transcript

What is the name of this molecule? (A) 2-Bromopentane, (B) 5-bromopentane, (C) 1-bromopentane, (D) 2-bromohexane, or (E) 5-bromohexane.

The diagram drawn here is what’s known as a skeletal formula. A skeletal formula represents carbon-carbon bonds as lines. In other words, we can assume that there is a carbon atom at the intersection or end of each of the lines in the diagram. Hydrogen atoms are also assumed to be present but not directly indicated by the skeletal formula. The benefit of a skeletal formula is that by minimizing the visual presence of carbon and hydrogen atoms, it allows us to clearly and quickly visualize which atoms and functional groups are attached to the carbon chain. In this case, we can see that a bromine atom is attached to the carbon chain.

In this question, we are being asked to name the compound that appears in this skeletal structure. The five answer choices give us five possible names that each follow a similar template: a number followed by the prefix bromo- followed by pentane or hexane. Let’s take a look at the three individual parts of the name to figure out which one is represented in the diagram here.

The part of the name that ends in -ane describes the molecule’s carbon chain. Is this alkane, pentane, or hexane? Well, the diagram shows that there are six carbon atoms in the structure. A six-carbon chain is called hexane. Hexa- means six, just like a hexagon has six sides. On the other hand, pentane would have only five carbons. So we can write in hexane as the first part of the final answer.

The prefix bromo- appears in all five of the answer choices. This prefix represents the bromine atom attached to the carbon chain. We can include it in our final answer. The last part of the name to figure out is the number that goes in front of it.

Eliminating the answer choices that use pentane instead of hexane, we’re left with two options. Is this 2-bromohexane or 5-bromohexane? The number in this name will indicate the position of the attached atom or functional group. In this case it’s the attached bromine atom. The bromine atom is attached to this carbon right here. So the number that appears in the name will be the position of this carbon atom one, two, three, four, five, or six along the chain.

We’ve already numbered the carbons right to left, but we could do it another way. We could number them left to right. How do we know which way is correct? As a tiebreaker, we can say that the carbon atoms are numbered to minimize the values that appear in the name. Numbering the carbons from right to left, like we’ve done in pink here, makes the carbon of interest carbon number two, a lower number than if we numbered the carbons from left to right.

Since the bromine atom is attached to carbon number two, the number we include in the name of this compound is two, giving us the full name 2-bromohexane. This corresponds to answer choice (D). The names of compounds like this reveal important information about the carbon chain, the attached atoms or functional groups on the carbon chain, and the location of those attachments. So what is the name of this molecule? That’s choice (D) 2-bromohexane.

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