Video Transcript
In this video, we will learn how to
write and interpret the names and formulas of alkenes and describe trends and
properties, such as melting points. Alkenes are unsaturated
hydrocarbons that contain at least one carbon–carbon double bond. A hydrocarbon is a compound that
only contains carbon and hydrogen atoms. And an unsaturated hydrocarbon is a
compound that contains at least one double or triple bond. The general formula for an alkene
that contains only one carbon–carbon double bond is C𝑛H2𝑛, where 𝑛 represents the
number of carbon atoms in the molecule. If an alkene has three carbon atoms
and only contains one double bond, then it must contain two times three hydrogen
atoms and have the molecular formula C3H6.
Now let’s take a look at the
simplest alkenes, one containing two carbon atoms and one containing three carbon
atoms. We can give each alkene a name to
differentiate between the two molecules, following the rules established by the
International Union of Pure and Applied Chemistry, or IUPAC for short. To name an alkene, we begin by
naming the longest continuous chain of carbon atoms that contains the carbon–carbon
double bond. This name consists of a prefix that
indicates the number of carbon atoms that are in the continuous chain, followed by
the suffix -ene to indicate that the molecule is an alkene. The first 10 prefixes are shown in
the table. Then, we name any substituents that
are attached to the base chain by placing the substituent name in front of the base
chain name.
Following these rules, the
two-carbon alkene is given the name ethene, eth- meaning two carbon atoms and -ene
for alkene. And the three-carbon alkene is
given the name propene, prop- meaning three carbon atoms and -ene for alkene. Naming alkenes becomes slightly
more complicated when there are four or more carbon atoms in the continuous
chain. Let’s consider these displayed
formulas. Following the naming rules for
naming alkenes, we get the name butene for both of these structures, although they
are clearly different. To differentiate between the two
butene molecules, we will need to indicate the location of the double bond in the
name. To do this, we number the carbon
atoms of the chain so that the double bond has the lowest possible position
number.
Looking at the top butene molecule,
we could number the carbon atoms of the chain from left to right or from right to
left. If we number the chain from left to
right, the double bond starts with carbon atom number one. And if we number the chain from
right to left, the double bond starts with carbon atom number three. Therefore, we should number this
carbon chain from left to right to give the double bond the lowest possible position
number. Looking at the bottom butene
molecule, the double bond starts with carbon atom number two, regardless of the
direction in which we count the carbon chain. Next, we will add the position
number to the name in between the prefix and suffix, separated by dashes. This means that we will name the
two structures of butene but-1-ene and but-2-ene.
But-1-ene and but-2-ene are
positional isomers, molecules with the same molecular formula and functional group,
in this case an alkene, but the functional group has a different position on the
carbon chain. As the number of carbon atoms in a
molecule increases, so too does the number of possible straight-chain positional
isomers. As such, it is important to
distinguish between these molecules by using displayed formulas, skeletal formulas,
structural formulas, condensed formulas, or the IUPAC name.
Now that we can recognize and name
alkenes, let’s take a look at how alkenes are produced. Alkenes are often obtained through
the process of cracking. Cracking is a type of decomposition
reaction where larger organic molecules are broken down into smaller molecules. The large molecules used in this
process are often long, undesirable hydrocarbons distilled from crude oil. In catalytic cracking, the
hydrocarbons are vaporized and heated with a catalyst. This causes the hydrocarbon to
break apart or crack into smaller alkanes and alkenes, which are then separated. In this example, decane was shown
to crack into ethene and octane, although many other products are possible depending
on where the carbon chain breaks.
Cracking is an industrial method
for producing alkenes. But alkenes can also be prepared in
the laboratory by dehydrating an alcohol with sulfuric acid. Dehydration is a chemical reaction
in which a water molecule is eliminated from the reactant. In the dehydration of an alcohol,
the hydroxy group and a hydrogen atom bonded adjacent to the position of the hydroxy
group will be eliminated. And a new double bond will form
between the two carbon atoms that each lost a substituent. Depending on the hydroxy group
position, this reaction may be in equilibrium with the hydration of an alkene and
may require the temperature of the reaction to be closely monitored.
Now that we’ve examined how to
produce alkenes via cracking or dehydration, let’s take a look at some of the
properties of alkenes. Many of the properties that alkenes
exhibit can be explained by examining the electrostatic attractions between alkene
molecules. Let’s consider propene and
hex-1-ene. There exists a week electrostatic
force of attraction between two propene molecules, called dispersion force. Dispersion forces are due to the
random motion of electrons in the molecules and are covered in more detail in
another video.
There are also dispersion forces
between hex-1-ene molecules. The dispersion forces between
hex-1-ene molecules are greater than the dispersion forces between propene
molecules. This is because hex-1-ene molecules
have more electrons than propene molecules. And there is more surface area
contact between adjacent hex-1-ene molecules than adjacent propene molecules. In general, as the length of the
alkene carbon chain increases, so too does the strength of the dispersion force
between the molecules.
Longer alkenes tend to have a
higher melting point and boiling point, as more energy is necessary to disrupt the
stronger dispersion forces between molecules. The increasing strength of the
dispersion force with increasing chain length can be used to explain why at room
temperature small alkenes, such as ethene, propene, and butene, exist as gases. Alkenes containing between five and
15 carbon atoms tend to be in the liquid state. And alkenes consisting of more than
15 carbon atoms are in the solid state. Density and volatility are also
affected by the chain length. The density of an alkene tends to
increase as the chain length increases, while volatility tends to decrease with
increasing chain length.
We’ve examined how alkenes compare
with one another. But how do alkenes compare with
alkanes, single-bonded hydrocarbons? Alkanes and alkenes have similar
melting and boiling points. Both are nonpolar and insoluble in
water and other polar solvents. But alkanes and alkenes readily
dissolve in nonpolar solvents, such as hexane or benzene. Alkanes participate in very few
reactions, but alkenes readily react with a number of reagents. Alkenes are more reactive than
alkanes because alkenes contain an electron-rich carbon–carbon double bond. We can use this difference in
reactivity to determine if an unknown hydrocarbon contains an alkane or an alkene by
performing a bromination reaction or the Baeyer test.
Bromine water, a solution of
diatomic bromine in water, has a characteristic brownish-orange color. It reacts with both alkanes and
alkenes. However, the reaction with an
alkane requires ultraviolet light or heat, while the reaction with an alkene rapidly
occurs without any additional supply of energy. When bromine water is added to an
alkane, the resulting solution is orange in color, as without any additional supply
of energy no noticeable reaction between the alkane and bromine will occur. When bromine water is added to an
alkene, the resulting solution is colorless. This is because the alkene readily
reacts with the bromine to produce a dibromine, which is colorless in solution. Therefore, if the bromine water is
decolorized, the sample may contain an alkene.
It’s worth noting here that bromine
also reacts with alkynes, phenols, and anilines and can indicate the presence of
these functional groups as well. As an alternative to bromination,
we can perform the Baeyer test. To perform the Baeyer test, cold
alkaline potassium permanganate solution, which is purple in color, is added to the
sample. If the sample contains an alkane,
no reaction will occur and the resulting solution will be purple in color. If the sample contains an alkene, a
reaction occurs to produce a diol, which is colorless in solution, and manganese
dioxide, a brown precipitate. It is worth noting here that
potassium permanganate solution will be decolorized and a brown precipitate will be
formed when reacted with alkenes as well as alkynes and aldehydes.
After all of this discussion of
alkenes, we may be wondering what alkenes are used for. The most common use of an alkene is
as a reagent to produce other important organic molecules. For example, ethene is used to
manufacture ethanol, a solvent, gasoline additive, and disinfectant;
ethane-1,2-diol, also called antifreeze, used as an engine coolant in countries with
cold climates; and polyvinyl chloride, or PVC, a plastic commonly used for making
water supply pipes, waterproof clothing, and blood collection and IV bags. Ethene is also used to produce
polystyrene, another plastic that is the main component of disposable cups, packing
materials, and home insulation, as well as ethylene-propylene rubber found in many
garden hoses and automotive parts, like drive belts.
Before we summarize what we’ve
learned about alkenes, let’s take a look at a question.
Which of the following molecules is
but-2-ene?
The prefix but- means that the
molecule contains a chain of four carbon atoms. As such, we can eliminate answer
choice (B) as this molecule only contains a three-carbon-atom chain. We can also eliminate answer choice
(C) as this molecule contains a five-carbon-atom chain. The suffix -ene indicates that the
molecule is an alkene, an unsaturated hydrocarbon that contains at least one
carbon–carbon double bond. This means that we can eliminate
answer choice (D), as this molecule contains a triple bond, called an alkyne,
instead of a double bond.
The number two is the position
number of the alkene in the carbon atom chain. This means that the double bond
begins with the second carbon atom of the chain and connects carbon atoms two and
three. The remaining answer choice with a
double bond between the second and third carbon atoms of the chain is answer choice
(E). The displayed formula that
correctly represents but-2-ene is answer choice (E).
Now let’s summarize what we’ve
learned with the key points. Alkenes are unsaturated
hydrocarbons that contain at least one carbon–carbon double bond. The general formula of an alkene
with one double bond is C𝑛H2𝑛. We can name alkenes following the
IUPAC rules. The name of an alkene will end in
-ene. Alkenes can be produced by cracking
larger hydrocarbons or by dehydrating alcohols. Longer-chain alkenes compared to
shorter-chain alkenes tend to have stronger dispersion forces and have higher
melting and boiling points. They also tend to be more dense and
less volatile.
Alkenes have similar melting and
boiling points as alkanes but are more reactive. We can use this difference in
reactivity to test for alkenes. Bromine water will be decolorized
by an alkene. And cold alkaline potassium
permanganate solution will be decolorized and produce a brown precipitate when
combined with an alkene. Alkenes are used to produce larger
molecules. For example, ethene is an important
reagent for many compounds, particularly plastics like PVC.
