Video: Products of Heptane Cracking

The cracking of a nonane molecule can produce a range of products. a) If the cracking of a nonane molecule produces two molecules including propene, what is the name of the second product? b) Which of the following pairs of molecules can be produced by the cracking of a nonane molecule, assuming there are no other products? [A] Propane + heptene [B] Hexene + propene [C] Octane + methane [D] Ethene + hexene [E] Butane + pentene. c) Which of the following groups of molecules can be produced by the cracking of a nonane molecule, assuming there are no other products? [A] Ethene + ethane + butene [B] Hexene + propane + ethene [C] Propane + ethene + butene [D] Butene + propane + methane [E] Propene + propene + propene.

12:25

Video Transcript

The cracking of a nonane molecule can produce a range of products. If the cracking of nonane molecule produces two molecules including propene, what is the name of the second product?

Cracking is the process of breaking down hydrocarbons into smaller hydrocarbons. Broadly speaking, there are two types of cracking: thermal cracking and catalytic cracking. But for the application here, it doesn’t matter which one we’re using. Let’s take a quick look at our starting material, nonane.

The word nonane can be broken into two parts: “non” and “ane.” The prefix “non” means nine and “ane” indicates the chemical we’re dealing with is an alkane. So in the context, the prefix “non” means that we have an alkane with nine carbon atoms. The question tells us that in this particular scenario a molecule of nonane is being broken down into propene and one other chemical.

The simple one-step cracking in an alkane will produce an alkene plus an alkane. Propene is an alkene. So at least, we know that the mystery molecule will be an alkane. We can look at a sample alkane, like hexane, to show why we always produce an alkene plus an alkane. In cracking, the first step is breaking carbon–carbon bonds. For this example, let’s break the bond in the middle. Now, we’ve got two fragments. But in our two fragments, we have carbon atoms with only three bonds. This is not stable.

The next step in cracking is to rearrange the hydrogens. For instance, this hydrogen here might move to join the other fragment. But this again leaves carbon atoms with only three bonds. So the last step of cracking is the formation of carbon-carbon double bonds. In reality, the process isn’t necessarily this clean. But this is the overall process. The important thing is that atoms are conserved. So we know that all the atoms in nonane will end up somewhere in propene and the other mystery product.

The easiest way to answer this question is to consider the number of carbon atoms in the chemicals. There are nine atoms of carbon in nonane and three in propene. Remember the prefix “prop” means three. So all we need to do is take away three from nine to work out the number of carbon atoms in our mystery alkane. This equals six. The prefix indicating six is “hex.” So the name of our mystery alkane is hexane.

Now, the other way of doing this is to work with the overall molecular formula of the chemicals. A simple alkane has the general formula C_(𝑛)H_(2𝑛+2), where 𝑛 is the number of carbon atoms. This gives nonane the chemical formula C₉H₂₀. On the other hand, a simple monoalkene has the general formula C_(𝑛)H_(2𝑛), again where 𝑛 is the number of carbon atoms. This gives propene the chemical formula C₃H₆. If we take C₃H₆ from C₉H₂₀, we get C₆H₁₄. This falls into the general formula of an alkane C_(𝑛)H_(2𝑛+2), where 𝑛 equals six. This is the chemical formula for hexane. So this is a valid alternate route. Either way, we need to present the name of the second product. And the name of the second product is hexane.

Which of the following pairs of molecules can be produced by the cracking of a nonane molecule, assuming there are no other products? A) Propane plus heptene, B) hexene plus propene, C) octane plus methane, D) ethene plus hexene, or E) butane plus pentene.

As in part a), we’re looking at the cracking of nonane. Since we’ve already identified that nonane is an alkane, a simple one-step cracking process of nonane should produce an alkane plus an alkene. Secondly, we know that a molecule of nonane contains nine carbon atoms. Since atoms are conserved in a cracking process, we know that the alkane and the alkene produced from cracking should have nine carbon atoms in total. We know this because we’ve been told in the question that there are no other products.

To answer the question, what we can do is look at each pair and see whether it contains an alkane and an alkene and check that the number of carbon atoms totals to nine. If a pair fails either of these tests, it’s an incorrect answer. Let’s start with propane and heptene. Propane ends in ane. So it’s an alkane. Heptene ends in ene. So it’s an alkene. So the first pair passes the first test.

What about the number of carbon atoms in total? The prefix prop means three. So propane contains three carbon atoms. Heptene contains the prefix hept, which means seven. So heptene contains seven carbon atoms. Three plus seven equals 10. There is more carbon atoms than in a molecule of nonane. Therefore, these products could not be produced through cracking of nonane. Therefore, this is not a correct answer.

The next pair is hexene and propene. Hexene and propene both end in ene. So they are both alkenes. Therefore, they fail the first test. We need one to be an alkene and one to be an alkane and this is not a correct answer. Just for practice, we can count the number of carbon atoms anyway. Hexene has six carbon atoms, while propene has three. While six plus three does equal nine, hexene and propene could not be produced from nonane because we need one alkane and one alkene.

Now, let’s have a look at set three. Octane and methane are both alkanes. So this set fails the first test and cannot be a correct answer. Octane and methane contain eight carbon atoms and one carbon atom, respectively. And even though these sum to nine, this set fails the first test and therefore cannot be a correct answer.

Ethene and hexene fail the first test as well being both alkenes. But they fail the second test as well, having eight carbon atoms in total. Since this is also an incorrect answer, we can move on to the last option. Butane is an alkane because it ends in ane. Pentene is an alkene because it ends in ene. Therefore, this set passes the first test. The but in butane means that a molecule of butane contains four carbon atoms, while the pent in pentene means they contains five. This means that butane plus pentene passes the second test, having nine carbon atoms in total.

Therefore, of the pairs given, the only one that can be produced by the cracking of a nonane molecule assuming no other products is butane plus pentene.

Which of the following groups of molecules can be produced by the cracking of a nonane molecule, assuming there are no other products? A) Ethene plus ethane plus butene, B) hexene plus propane plus ethene, C) propane plus ethene plus butene, D) butene plus propane plus methane, or E) propene plus propene plus propene.

As we’ve covered in part a) and part b), when we crack nonane in a single step, we produce an alkane plus an alkene. But in this question, we have three products, not two. So we have one further step of cracking. This means in the second step, we either crack the alkane to produce an alkane plus an alkene plus another alkene or we can crack the alkene to produce two alkanes and a diene. Depending on which bonds break when we crack an alkene, we could also produce two alkenes. So cracking an alkane plus an alkene will produce either of these two options.

Again, since we’re starting from nonane, we know that we’ll have nine carbon atoms to start with. So we’ll need nine carbon atoms in our product. In both of the cases, we’re going to end up with two carbon–carbon double bonds, either as two monoalkenes or a diene. The di in diene indicates that it contains two carbon–carbon double bonds. So like in part b), we can perform two tests on each group and find which one will be the group that can be produced by cracking nonane.

The two tests are whether there are two carbon–carbon double bonds and whether the number of carbon atoms totals to nine. Let’s start off looking at the first group. In ethene, ethane, and butane butene, we have two monoalkenes and one alkane. Therefore, we have two carbon–carbon double bonds. In ethene, ethane, and butene, we have two, two, and four carbon atoms, respectively. This means the total number of carbon atoms produced is eight. Eight is not equal to nine and we know that our products must contain nine carbon atoms in total. Therefore, this is not a correct answer.

The second set passes the first test. It contains two monoalkenes and an alkane. However, this set contains two too many carbon atoms, 11 rather than nine. So this is not the correct answer either. The third set propane plus ethene plus butene passes the first test as well. And crucially, the third set also passes the second test, having nine carbon atoms in total. Since this group passes all the tests, it’s our correct answer. But let’s have a look at the other answers just in case.

Our fourth set doesn’t pass the first test because it only contains one alkene. It also fails the second test, having only eight carbon atoms in total. As we’ve ruled this out, we can move on to the final option. The last set fails the first test. Three equivalents of propene will contain three carbon–carbon double bonds, not the two that we require. Even though propene plus propene plus propene contains the right number of carbon atoms since it fails the first test, it’s not a correct answer.

Therefore, of the five groups given, the only one that could be produced by the cracking of a nonane molecule assuming no other products is propane plus ethene plus butene.

Nagwa uses cookies to ensure you get the best experience on our website. Learn more about our Privacy Policy.