Question Video: Identifying a Hydrocarbon That Is Not a Structural Isomer of Hexane Given a Set of Skeletal Formulas | Nagwa Question Video: Identifying a Hydrocarbon That Is Not a Structural Isomer of Hexane Given a Set of Skeletal Formulas | Nagwa

Question Video: Identifying a Hydrocarbon That Is Not a Structural Isomer of Hexane Given a Set of Skeletal Formulas Chemistry • Third Year of Secondary School

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Which of the following hydrocarbons is not a structural isomer of hexane? [A] Hydrocarbons A [B] Hydrocarbons B [C] Hydrocarbons C [D] Hydrocarbons D [E] Hydrocarbons E

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Video Transcript

Which of the following hydrocarbons is not a structural isomer of hexane?

Hexane and all of the molecules shown in the answer choices are hydrocarbons. Hydrocarbons are organic molecules composed of only carbon and hydrogen atoms. Let’s begin by examining the structural formula provided for hexane. Let’s use pink circles to represent carbon atoms. Each line segment in this type of structural formula represents a single covalent bond between atoms. The points at which two line segments meet and at which line segments end are the locations of the carbon atoms. The hexane molecule contains six carbon atoms. The hydrogen atoms are not shown. So, let’s draw a displayed formula of hexane to help us.

Because each carbon atom must be surrounded by a total of four bonds, the hexane molecule contains a total of 14 hydrogen atoms. This means that the molecular formula of hexane is C6H14. Hexane is a type of hydrocarbon, known as an alkane. Alkanes are saturated hydrocarbons, which means that there are only single bonds between the carbon atoms. When looking at the answer choices, we notice that they are all alkanes. Molecules (A) through (D) are branched-chain alkanes, and molecule (E) is a cycloalkane.

We can also see that, like hexane, all of the molecules contain six carbon atoms. So then which of these molecules is not a structural isomer of hexane? The key to solving this problem is to remember that structural isomers differ by how their atoms are connected, but they must have the same molecular formula. To help us find the molecule that has a different molecular formula than hexane, let’s get started by drawing the displayed formulas for the first three molecules. We’ve condensed any side chains in these displayed formulas to save some space.

We can see that molecules (A) through (C) each contain 14 hydrogen atoms. Therefore, they all have the molecular formula C6H14 and are structural isomers of hexane. Molecule (D) contains a total of 14 hydrogen atoms, so it has the same molecular formula as hexane and is a structural isomer. Finally, we can see that molecule (E) only contains 12 hydrogen atoms and its molecular formula is therefore C6H12. Because molecule (E) has a different molecular formula than hexane, it cannot be a structural isomer of hexane. In conclusion, the hydrocarbon which is not a structural isomer of hexane is answer choice (E).

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