Video Transcript
In this video, we will learn how to
define substitution reactions and write and interpret equations for substitutions of
alkanes with halogens.
This video is about reactions
involving alkanes. Alkanes are a type of hydrocarbon,
in other words, their molecules made up of hydrogen atoms and carbon atoms. Specifically, alkanes are a
hydrocarbon made up of only single bonds. The simplest alkanes are methane,
ethane, and propane, although many other alkanes exist with longer, branched, or
cyclic carbon chains. In the structures drawn here, we
can confirm there are hydrogen atoms and carbon atoms connected by single bonds. Sometimes, we refer to alkanes as
being saturated. Because they contain only single
bonds, it allows the carbon atoms in the chain to bond to the maximum number of
hydrogen atoms. The carbon atoms are saturated with
hydrogen atoms.
If we bring in a similar molecule
that is not an alkane for comparison, we can see that replacing the carbon-carbon
single bond with a double bond reduces the number of hydrogens in the compound. Doing so turns our saturated alkane
into an unsaturated alkene.
This video is also about
substitution reactions. In a substitution reaction, two
atoms or groups of atoms swap places. In the reaction we’ve written here,
a bromine atom can knock off one of the hydrogen atoms of the methane molecule. That hydrogen atom can then bond
with the other bromine atom that was left behind. As a result of this substitution or
swap, bromomethane and hydrogen bromide are produced. We can also call this specific
reaction a halogenation reaction because we are adding a halogen to the alkane. The substitution reactions of
alkanes that we’ll look at in this video involve adding atoms and functional groups,
such as halogens, to the carbon chain of the alkane.
It’s worth noting that alkanes in
general are unreactive. This reaction requires the added
energy from ultraviolet light to proceed. On the other hand, haloalkanes are
more reactive than alkanes. For this reason, they’re often used
as reactants in these kinds of substitution reactions. If we want to attach a new atom or
functional group to the carbon chain, it’s easier to knock off a halogen atom than
to knock off a hydrogen atom.
One example of such a reaction is
this one here that takes bromoethane and water to produce ethanol and hydrogen
bromide. In this equation, one of the things
being substituted isn’t an atom, but rather a group of two atoms. We call groups of atoms like this
functional groups. A functional group is a group of
atoms that shows similar chemical properties when it occurs in different
compounds. The functional group in this
reaction, with an oxygen atom and a hydrogen atom, is called a hydroxy group.
If we add a hydroxy group to a
two-carbon alkane, we get the compound ethanol. Ethanol is an example of an
alcohol. Alcohols are a group of compounds
that all have hydroxy functional groups. Alcohols share many similar
chemical properties because they share this hydroxy functional group.
Let’s take a look at what
functional groups might be attached to alkanes. We just mentioned that a hydroxy
group contains an oxygen atom and a hydrogen atom. There’s also the amino group, which
consists of a nitrogen atom and two hydrogen atoms. A nitro group consists of a
nitrogen atom and two oxygen atoms. A carboxyl group has this branched
structure of a carbon atom, two oxygen atoms, and a hydrogen atom.
Often, a compound’s name will have
prefixes or suffixes that indicate what functional groups are present. For example, compounds that contain
hydroxy groups use the suffix -ol, for example, methanol. Amino groups use the prefix
amino-. If we attach that group to a
methane molecule, we get aminomethane. Similarly, the nitro group uses the
nitro- prefix to make compounds like nitromethane. For carboxyl groups, the suffix is
-oic acid, such as the compound methanoic acid.
When discussing these functional
groups in general, you may see them attached to the letter R. The R represents a carbon
chain. Since these functional groups can
attach to a wide variety of carbon chains, we use the letter R as a placeholder to
represent any carbon chain.
In addition to using prefixes and
suffixes for halogens and functional groups to name compounds, there are other
compounds we need to name more specifically. In the reaction we’ve drawn here,
we can add fluorine to propane to create fluoropropane. This is a substitution reaction
where one of the fluorine atoms from the fluorine molecule attaches to the carbon
chain of the propane. However, there are two different
possible arrangements of this product. These two versions of fluoropropane
have the same chemical formula but different arrangements. So we call them isomers. The key difference is the location
of the fluorine. Either the fluorine atom can attach
to one of the carbons on the end of the carbon chain or it can attach to one of the
carbons in the middle of the carbon chain.
While the fluorine atom could
replace any of the eight hydrogen atoms on the initial propane molecule, each of
those eight possible substitutions will produce a product that is a rotation of one
of these two options. We can distinguish these two
isomers of fluoropropane by numbering the carbons one through three. We can give the isomer on the left
the name 1-fluoropropane: one signifying that the fluorine atom is attached to
carbon number one. Similarly, if the fluorine atom is
attached to carbon number two, we can give that compound the name
2-fluoropropane.
In this situation, we numbered the
carbons from left to right, one, two, three. But why didn’t we number them the
other way around? By convention, we number the
carbons in a way that minimizes the numbers that appear in the name of the
compound. For 1-fluoropropane, that means
counting the carbons starting at the attachment instead of starting at the other
end. In this particular reaction, both
isomers are produced. However, depending on the reaction,
some isomers may appear more or less frequently than others.
This naming system works the same
way for functional groups as it did for the halogen atoms. Attaching an amino group to a
propane chain can give us 1-aminopropane or 2-aminopropane. With alcohols, we can put the
number in the middle. With a hydroxy group on carbon
number one, we get propan-1-ol, with a hydroxy group on carbon number two,
propan-2-ol. Sometimes, we will need to name an
alkane that has multiple substitutions. In this case, we can simply include
multiple prefixes or suffixes and multiple numbers.
As we name these compounds, we
should keep in mind that in chemistry there’s rarely just one correct name for a
compound. Different naming conventions might
use different ways of representing a functional group or a different order for the
components of the name, resulting in different names for the same compound.
Now let’s practice naming this
polysubstituted alkane. First, we can note that this is a
chain of four carbon atoms, so it must be some form of butane. With an attached bromine atom, we
can include the prefix bromo-. And since that bromine atom is
attached to carbon number one, we can include the number one. The last part to include is this
hydroxy group on carbon number two. To include it in the name, we can
chop off the e from butane and add the number two as well as the suffix -ol. This is the full name of the
compound, 1-bromobutan-2-ol. This name indicates that this is a
compound with a four-carbon chain and a bromine atom attached to carbon number one
and a hydroxy group attached to carbon number two.
Another thing we might see in names
like this is the prefix di-. We’ve altered this compound a bit
to make it 1,4-dibromobutan-2-ol. Dibromo- means there’s two bromine
atoms attached to the carbon chain. We indicate their location simply
by including two numbers: one comma four, one bromine atom on carbon number one and
one bromine atom on carbon number four.
It can also be useful practice to
do this process in the other direction. Based on this name, can we draw the
compound it represents? First, pentane implies a
five-carbon chain at the center of the molecule. 2-Amino means there’s an amino
group attached to carbon number two. Dichloro- means that there’s two
chlorine atoms attached to the carbon chain. The 1,1- in front of dichloro-
means that both these chlorine atoms are attached to carbon number one. As a final step, we can fill in the
rest of the possible single bonds with hydrogen atoms. Here, we have
1,1-dichloro-2-aminopentane. Although, as we mentioned before,
this is just one way to name this molecule.
While these long names might
initially seem intimidating, breaking them down into their constituent parts makes
them easier to understand. Each name contains information
about the carbon chain, the atoms or functional groups attached to that chain, and
the location of those attachments along the chain.
Haloalkanes have a variety of
everyday uses. The chemical structure of PVC pipes
and the Teflon coating on many nonstick pans are polymerized haloalkanes. In other words, they’re the same
haloalkane unit drawn here, repeated to make continuous chains. Chloroalkanes are used as powerful
solvents in a variety of chemical processes. Fluoroalkanes are used as important
building blocks to create pharmaceutical drugs. Chlorofluorocarbons, or in other
words alkanes where each of the hydrogen atoms has been replaced by a chlorine atom
or a fluorine atom, have been used as refrigerants in air conditioning systems as
well as propellants in aerosol cans. However, due to their harmful
effects on the ozone layer, chlorofluorocarbons are being phased out in most
countries.
Now that we’ve learned about
haloalkanes, let’s do a practice problem to review.
What is the name of this
molecule? (A) 2-Bromopentane, (B)
5-bromopentane, (C) 1-bromopentane, (D) 2-bromohexane, or (E) 5-bromohexane.
The diagram drawn here is what’s
known as a skeletal formula. A skeletal formula represents
carbon-carbon bonds as lines. In other words, we can assume that
there is a carbon atom at the intersection or end of each of the lines in the
diagram. Hydrogen atoms are also assumed to
be present but not directly indicated by the skeletal formula. The benefit of a skeletal formula
is that by minimizing the visual presence of carbon and hydrogen atoms, it allows us
to clearly and quickly visualize which atoms and functional groups are attached to
the carbon chain. In this case, we can see that a
bromine atom is attached to the carbon chain.
In this question, we are being
asked to name the compound that appears in this skeletal structure. The five answer choices give us
five possible names that each follow a similar template: a number followed by the
prefix bromo- followed by pentane or hexane. Let’s take a look at the three
individual parts of the name to figure out which one is represented in the diagram
here.
The part of the name that ends in
-ane describes the molecule’s carbon chain. Is this alkane, pentane, or
hexane? Well, the diagram shows that there
are six carbon atoms in the structure. A six-carbon chain is called
hexane. Hexa- means six, just like a
hexagon has six sides. On the other hand, pentane would
have only five carbons. So we can write in hexane as the
first part of the final answer.
The prefix bromo- appears in all
five of the answer choices. This prefix represents the bromine
atom attached to the carbon chain. We can include it in our final
answer. The last part of the name to figure
out is the number that goes in front of it.
Eliminating the answer choices that
use pentane instead of hexane, we’re left with two options. Is this 2-bromohexane or
5-bromohexane? The number in this name will
indicate the position of the attached atom or functional group. In this case it’s the attached
bromine atom. The bromine atom is attached to
this carbon right here. So the number that appears in the
name will be the position of this carbon atom one, two, three, four, five, or six
along the chain.
We’ve already numbered the carbons
right to left, but we could do it another way. We could number them left to
right. How do we know which way is
correct? As a tiebreaker, we can say that
the carbon atoms are numbered to minimize the values that appear in the name. Numbering the carbons from right to
left, like we’ve done in pink here, makes the carbon of interest carbon number two,
a lower number than if we numbered the carbons from left to right.
Since the bromine atom is attached
to carbon number two, the number we include in the name of this compound is two,
giving us the full name 2-bromohexane. This corresponds to answer choice
(D). The names of compounds like this
reveal important information about the carbon chain, the attached atoms or
functional groups on the carbon chain, and the location of those attachments. So what is the name of this
molecule? That’s choice (D)
2-bromohexane.
Let’s review the key points of the
video. Alkanes are hydrocarbons made up of
only single bonds. We can create haloalkanes by
substituting a halogen atom for one of the hydrogen atoms on the alkane. For example, we can combine methane
and bromine under ultraviolet light to create bromomethane and hydrogen bromide. We call reactions like this, where
one atom or functional group swaps places with another atom or functional group,
substitution reactions.
Functional groups are groups of
atoms that show similar properties when they appear in different compounds. Examples include the hydroxy group
OH and the amino group NH2.
The name of an alkane reveals its
functional groups, their locations, and its carbon chain. For example, let’s take a look at
the name 1,4-dichloropentane. The pentane implies that the carbon
chain is five carbon atoms long. Dichloro- means that there are two
chlorine atoms attached to that chain. And the numbers one and four mean
that the chlorine atoms are attached to carbon number one and carbon number
four.
Haloalkanes have a variety of
common uses, such as in PVC pipes and nonstick coatings. They also have a variety of
scientific uses, as a powerful solvent or a building block for pharmaceuticals.