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.
