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
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.
