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Question Video: Predicting the Second Product of the Cracking of Heptane Given the Displayed Formula of the Other Product Chemistry • 7th Grade

Shown in the equation is one possible reaction in the cracking of heptane. Compound X is an unbranched hydrocarbon. What is the displayed formula of compound X?

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Video Transcript

Shown in the equation is one possible reaction in the cracking of heptane. Compound X is an unbranched hydrocarbon. What is the displayed formula of compound X?

The starting compound in the reaction scheme is the saturated hydrocarbon heptane with molecular formula C7H16. This is a relatively long alkane hydrocarbon. We are told that heptane is cracked and that this is only one possible reaction that occurs, which means there are other possible products. Two possible products are shown in this reaction scheme: an unsaturated alkene with a carbon-carbon double bond, in this case propene because there are three carbons in the chain, and an unknown compound X. But we are told that X is an unbranched hydrocarbon. Cracking is a type of decomposition reaction where larger, usually saturated organic molecules are broken down into smaller ones. These smaller molecules are usually more useful to industry.

The steps in a typical cracking reaction involve taking a large alkane molecule heating to high temperatures, sometimes with a catalyst. A carbon-carbon single bond breaks. A carbon-hydrogen bond breaks. Rearrangement occurs where a hydrogen atom bonds with a different carbon atom. This new carbon-hydrogen bond produces a shorter alkane. And in the other fragment of the original molecule, a double bond forms between two carbon atoms, forming an alkene.

The linear alkane given to us, heptane, was cracked or broken into the alkene propene and substance X. Therefore, X must be a shorter alkane and must contain the remaining carbon atoms. The alkene product propene has one, two, three of the carbon atoms from heptane. Therefore, X must contain four carbon atoms. A short alkane with four carbon atoms that is unbranched is butane. We were asked to give the displayed formula of compound X, and this is the displayed formula of butane.

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