Video Transcript
In this video, we will learn how to
describe addition reactions of alkenes and predict what products are formed. We will focus on hydrogenation,
halogenation, and hydration.
Alkenes are unsaturated
hydrocarbons that contain at least one carbon-carbon double bond. A hydrocarbon is a compound that
contains only carbon and hydrogen atoms. And an unsaturated hydrocarbon is a
compound that contains at least one double or triple bond. Alkenes participate in a variety of
reactions due to the presence of an electron-rich carbon-carbon double bond. One type of reaction which they can
undergo is an addition reaction.
An addition reaction is a type of
reaction where two or more molecules combine to make a single type of molecular
product. There are four alkene addition
reactions that we want to examine in this video: hydrogenation, halogenation,
hydrohalogenation, and hydration.
Let’s start by examining
hydrogenation. Hydrogenation is a chemical
reaction between molecular hydrogen and a compound, typically in the presence of a
catalyst. Over the course of the reaction of
an alkene with molecular hydrogen, the pi bond between the carbon atoms is broken,
as is the bond between the hydrogen atoms. This allows for two new
carbon-hydrogen bonds to be formed, and an alkane is produced. This reaction relies on a catalyst
to facilitate the interaction between molecular hydrogen and the alkene by bringing
the two molecules closer together.
Three common catalysts are used to
hydrogenate an alkene: platinum in the form of platinum oxide, palladium supported
on carbon, or Raney nickel, a finely powdered nickel aluminum alloy. Hydrogenation is commonly used to
solidify liquid oils, like vegetable oil. Vegetable oil consists of a variety
of triglyceride molecules, three ester groups with long hydrocarbon chains. Each hydrocarbon chain typically
contains one or more alkenes. By reacting the oil with molecular
hydrogen in the presence of Raney nickel, some or all of the carbon-carbon double
bonds react and become saturated. The partially or fully hydrogenated
oil has a higher melting point and can exist as a solid at room temperature. Products like margarine and
shortening can be produced via this process.
Now let’s take a look at another
addition reaction. Halogenation is a chemical reaction
that involves the addition of one or more halogens to a compound. Over the course of the reaction
between an alkene and diatomic chlorine, the pi bond between the carbon atoms is
broken, as is the bond between the chlorine atoms. This allows for two new
carbon-chlorine bonds to be formed. Overall, the process is very
similar to hydrogenation. But the product of this reaction is
a vicinal dichloride. Vicinal is the term used to
describe two atoms or groups bonded to adjacent carbon atoms.
At room temperature and atmosphere
pressure, alkenes can undergo halogenation with diatomic chlorine, bromine, or
iodine. Although alkenes do react with
diatomic fluorine, this reaction is much more complex and is not considered to be a
simple addition reaction. So we will not cover it here.
A halogenation reaction with
bromine, or bromination, is often used to test whether a hydrocarbon is an alkane or
an alkene. 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 UV 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 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 dibromide, which is colorless. Thus, if bromine water is added to
an unknown hydrocarbon and is decolorized, the hydrocarbon likely contains an
alkene.
We should be aware that bromine
also reacts with alkynes, phenols, and anilines and can indicate the presence of
these functional groups as well. Another common test for alkenes is
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,
an oxidation reaction occurs, producing a diol, which is colorless in solution, and
manganese dioxide, a brown precipitate. As this reaction is an oxidation
reaction and not an addition reaction, we won’t look into how these products are
formed in this video.
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.
Now let’s return to addition
reactions. We’ve looked at how to add two
hydrogen atoms to an alkene or two halogen atoms to an alkene. But there is an addition reaction
that allows us to add both a hydrogen atom and a halogen atom. Hydrohalogenation is a reaction
where an alkene is reacted with a hydrogen halide, like hydrogen bromide. Over the course of this reaction,
the pi bond between the carbon atoms is broken, as is the bond between the hydrogen
and bromine atoms. This allows for two new bonds to be
formed: a carbon-hydrogen bond and a carbon-bromine bond. The product of this reaction is a
haloalkane, also called an alkyl halide.
But unlike hydrogenation and
halogenation, the reagent in this reaction, a hydrogen halide, is not
symmetrical. This means that the bromine atom
could have formed a bond with the first carbon atom of the chain instead of the
second, producing a different haloalkane. We can predict which of these two
products is the major product by considering Markownikoff’s rule.
Markownikoff’s rule is best
understood by examining reaction mechanisms and carbocations but can be simplified
to state that the acidic hydrogen atom will add to the carbon of the double bond
with the greatest number of hydrogen substituents. The acidic hydrogen atom is the
hydrogen of the hydrogen bromide. And the carbon of the double bond
with the greatest number of hydrogen substituents is the first carbon atom of the
chain. This means that the hydrogen atom
will form a bond with the first carbon atom and the bromine will form a bond with
the second carbon atom. Following Markownikoff’s rule, the
major product of the reaction of propene with hydrogen bromide will be
2-bromopropane, not 1-bromopropane.
The hydrogen halide used for
hydrohalogenation may be hydrogen chloride, hydrogen bromide, or hydrogen
iodide. Hydrogen fluoride can also be used
for this reaction, although it is much less common due to the slow rate of reaction,
the toxicity of hydrogen fluoride, and the reactivity of hydrogen fluoride with
standard glassware.
The final addition reaction we will
consider is hydration. Hydration is a chemical reaction
where water is added to a compound. Hydration of an alkene may be
direct or indirect. In the direct hydration of an
alkene, water in the form of steam is reacted with an alkene in the presence of
phosphoric acid. This reaction follows
Markownikoff’s rule. Over the course of this reaction,
the pi bond between the two carbon atoms will be broken. And an acidic hydrogen atom from
the phosphoric acid will form a bond with the carbon of the double bond that has the
greatest number of hydrogen substituents. Then, the hydroxy group from the
water can form a bond with the carbon of the double bond that has the least number
of hydrogen substituents. This reaction produces an
alcohol.
The hydration of an alkene is an
equilibrium reaction. So to drive the reaction towards
the alcohol, the reaction is run under high pressure. And the resulting alcohol is
condensed and removed from the reaction vessel. Direct hydration is commonly used
in industry to produce propan-2-ol, commonly known as isopropyl alcohol, from
propene and ethanol from ethene.
Hydration can also occur
indirectly. In indirect hydration, an alkene is
first reacted with concentrated sulfuric acid. This reaction follows
Markownikoff’s rule. Over the course of the reaction,
the pi bond between the two carbon atoms will be broken. And an acidic hydrogen atom forms a
bond with the carbon of the double bond that has the greatest number of hydrogen
substituents. Then, the rest of the sulfuric acid
molecule bonds to the carbon of the double bond with the least number of hydrogen
substituents. This produces an alkyl hydrogen
sulfate.
The alkyl hydrogen sulfate is then
reacted with water. The hydroxy group from the water
replaces the hydrogen sulfate to produce an alcohol and regenerate the sulfuric acid
in solution. The overall reaction scheme for
indirect hydration of an alkene is often simplified into a single line. Like direct hydration, indirect
hydration is an equilibrium reaction. Specific temperature conditions
should be used to drive the reaction towards the alcohol. The temperature requirements are
dependent on the position of the alcohol being produced.
Before we summarize what we’ve
learned about addition reactions of alkenes, let’s take a look at a question.
Which of the following occurs upon
the addition of bromine water to propene? (A) The color of bromine disappears
with the formation of 1,2-dibromopropane. (B) The color of bromine does not
change and no reaction occurs. (C) The color of bromine disappears
with the formation of 1-bromopropene. (D) The color of bromine disappears
with the formation of 1-bromopropane. (E) The color of bromine disappears
with the formation of 1,3-dibromopropane.
The name propene ends in -ene. This tells us that propene is an
alkene. Alkenes are unsaturated
hydrocarbons that contain at least one carbon-carbon double bond. The prefix prop- tells us that
propene contains three carbon atoms. A three-carbon-atom hydrocarbon
with one double bond must have the structure shown here. Bromine water is a mixture of
diatomic bromine and water. The electron-rich carbon-carbon
double bond in an alkene readily reacts with the diatomic bromine at room
temperature and atmosphere pressure.
This reaction is a halogenation
reaction, a chemical reaction that involves the addition of one or more halogens to
a compound. Over the course of this reaction,
the pi bond between the carbon atoms is broken, as is the bond between the two
bromine atoms. This allows for two new
carbon-bromine bonds to be formed. And a compound containing two
bromine atoms is produced. As a reaction between propene and
bromine water does occur, we can eliminate answer choice (B).
The compound produced via the
reaction has a three-carbon base chain that only contains single bonds. We can name this base chain
propane: prop- for three carbon atoms and -ane for alkane, single-bonded carbon
atoms. This means we can eliminate answer
choice (C), as the product of this reaction does not contain any alkenes.
The product contains two bromine
atoms, one bonded to carbon atom number one and one bonded to carbon atom number
two. We can indicate this in the name by
adding 1,2-dibromo- in front of the base chain name. Thus, this molecule is
1,2-dibromopropane. This means the correct answer as to
which of the following occurs upon the addition of bromine water to propene is
answer choice (A).
We may be wondering why the color
of bromine disappears. Bromine water has a characteristic
brownish-orange color, and propene is a colorless gas. When bromine water is added to the
flask and the two molecules react, 1,2-dibromopropane, a colorless liquid, is
produced. Thus, the brownish-orange color of
bromine has disappeared. This visible color change is why
bromine water is often used to determine if an unknown hydrocarbon contains an
alkene.
Now let’s summarize what we’ve
learned with the key points. Addition reactions are chemical
reactions where two or more molecules combine to form one larger molecule. Hydrogenation is an addition
reaction where molecular hydrogen is added to an alkene in the presence of a
catalyst, like Raney nickel, to produce an alkane. Halogenation involves the addition
of a diatomic halogen, particularly chlorine, bromine, or iodine, to an alkene to
produce a dihalide.
Bromination or the Baeyer test can
be used to test for alkenes. Bromine water will be decolorized
by an alkene, while potassium permanganate will be decolorized and produce a brown
precipitate when combined with an alkene. Hydrohalogenation is an addition
reaction where a hydrogen halide is added to an alkene to produce a haloalkane. The major product of asymmetrical
addition reactions, like the example shown for hydrohalogenation, can be predicted
using Markownikoff’s rule.
The final addition reaction we
learned was hydration, the addition of water to an alkene to produce an alcohol. This reaction may be considered
direct or indirect, depending on the acid used in the reaction.
