Yaworski 1
Michael Yaworski
Mrs. Johnson
SCH 4UI
September 26, 2014
Lab: Reactions of Saturated and Unsaturated Hydrocarbons
Introduction:
Purpose: The purpose of this lab was to compare the reactivity of saturated versus unsaturated
hydrocarbons by comparing the reactions of two hydrocarbons of the same carbon chain length,
but where one was saturated and one was not (cyclohexane and cyclcohexene), with a halogen
(bromine).
To complete this lab, two reactions were going to take place: a substitution reaction with
cyclohexane and aqueous bromine, and a halogenation addition reaction with cyclohexene and
aqueous bromine.
Cyclohexane and cyclohexene were each combined with aqueous bromine in separate test
tubes. Each test tube was observed and then shaken to induce a reaction by agitating the mixture.
The test tubes were observed again, and then the test tube containing the mixture of aqueous
bromine and cyclohexane was then put under heat and light because that was required to induce
the substitution reaction. The test tube was then finally observed.
A hydrocarbon is an organic compound consisting of only carbon and hydrogen atoms.
The carbons are bonded together and hydrogen atoms are bonded to each of the carbons. A
saturated hydrocarbon is a hydrocarbon where the bonds between each of the carbon atoms are
only single bonds so that there is the maximum possible number of hydrogen atoms in the
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compound. A multiple bond between carbon atoms would mean there is less room for hydrogen
atoms to bond to the carbon atoms. That is what an unsaturated hydrocarbon is. It is a
hydrocarbon with at least one carbon-carbon multiple bond (double or triple bond).
A substitution reaction occurs for aromatic compounds (such as cyclohexane) where a
hydrogen atom in the aromatic compound will be substituted with a halogen atom from the
halogen reactant. A halogenation addition reaction occurs when an alkene (a hydrocarbon with a
carbon-carbon double bond, like cyclohexene) reacts with a halogen molecule. The double bond
between carbons will break, leaving two spots for atoms to bond to, and the halogen molecule
will add to the hydrocarbon, making it an alkyl halide.
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Observations:
Substance
Cyclohexane
Cyclohexene
Aqueous
Bromine
Physical
Description
-
liquid
clear
colourless
choking
odour
-
liquid
clear
colourless
choking
odour
-
orange
clear
smells
unpleasant
Before Shaken
Added Bromine
After Shaken
-
-
two separated
substances
(heterogeneous
mixture)
- top layer is
colourless and
clear
- bottom layer is
yellow and
clear
- meniscus is
orange
- heterogeneous
mixture
- top layer is
clear and
colourless
- bottom layer is
bright yellow
and clear
- meniscus is
yellow and
clear
N/A
-
-
heterogeneous
mixture
top layer is
orange
bottom layer is
yellow
yellow
meniscus
heterogeneous
mixture
both layers
and meniscus
are colourless
N/A
After Heat and
Light
- heterogeneous
mixture
- top layer is
cloudy
- bottom layer is
clear
- both substances
are colourless
N/A
N/A
Analysis:
The purpose of this lab was to compare the reactivity of saturated versus unsaturated
hydrocarbons. To do that, two reactions took place: cyclohexane and aqueous bromine in a
substitution reaction, and cyclohexene and aqueous bromine in a halogenation addition reaction.
The reaction conditions for each of the two hydrocarbons were important to compare because
that was what determined which hydrocarbon was more or less reactive.
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Comparing the Reactions that Occurred:
When cyclohexane was combined with aqueous bromine in a test tube (before being
shaken), it was a heterogeneous mixture where the cyclohexane (colourless liquid) remained on
the top layer and aqueous bromine remained on the bottom layer (yellow liquid). There were no
reactions occurring at that time. After the test tube was shaken for a few seconds, the top layer
was orange and the bottom layer was yellow. A reaction still had not occurred at this point. The
reason for the physical change in the mixture was because the bromine molecules were more
soluble in the cyclohexane, so they moved to the top layer. Bromine molecules and cyclohexane
are both nonpolar, so they dissolve in each other, as opposed to water, which is polar and does
not dissolve in either of them. The bromine molecules that moved to the top layer after being
shaken coloured the top layer orange because that is the colour of the bromine molecules. The
bottom layer still remained yellow because not all of the bromine molecules had moved away
from the bottom, but most of them did.
The reaction occurred when heat and light were applied to the test tube. The chemical
change was evident because there was no more orange colour in either of the two substances.
The colour change did not guarantee that it was chemical change, but it did make sense in terms
of the predicted reaction and there actually was chemical change. The bromine molecules, Br2 ,
had split off and then bromine atoms substituted with hydrogen atoms. The substitution reaction
that occurred is as follows:
(l)
cyclohexane
+
Br2 (aq)
∆ 𝑎𝑛𝑑 𝑙𝑖𝑔ℎ𝑡
→
∆ 𝑎𝑛𝑑 𝑙𝑖𝑔ℎ𝑡
+ aqueous bromine →
(aq)
+
HBr (aq)
bromocyclohexane + hydrobromic acid
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The important fact about this reaction was that heat and light were required as a catalyst
to react cyclohexane and bromine together; room temperature and shaking were not enough.
Similarly to the cyclohexane, when cyclohexene was combined with aqueous bromine in
a test tube (before being shaken), it was a heterogeneous mixture where the cyclohexene
(colourless liquid) remained on the top layer and aqueous bromine remained on the bottom layer
(yellow liquid). There were no reactions occurring at that time. However, unlike with
cyclohexane, after the test tube was shaken for a few seconds, there was an almost immediate
reaction. The observation of the reaction of cyclohexene and bromine after being shaken was
very similar to that of cyclohexane and bromine after being exposed to heat and light. Both
layers in the mixture became colourless because the bromine molecules had split up into bromine
atoms and bonded to carbon atoms in the hydrocarbon. The reaction that occurred is as follows:
(l)
cyclohexene
+
Br2 (aq)
→
(aq)
+ aqueous bromine → 1,2-dibromocyclohexane or o-dibromocyclohexane
The reaction above was a halogenation and addition reaction. The double bond between
carbon atoms in cyclohexene had been broken and the bromine atoms (halogen atoms) had
bonded to each of the carbon atoms that were part of the double bond. In the reaction with
cyclohexene and aqueous bromine, no catalyst was required and the reaction occurred at room
temperature. However, with cyclohexane and aqueous bromine, the reaction needed heat and
light as a catalyst. Therefore, cyclohexene, an unsaturated hydrocarbon, was more reactive with
bromine, a halogen, than cyclohexane, a saturated hydrocarbon.
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Explaining the Difference in Reactivity:
According to the valence bond theory, by Linus Pauling, a covalent bond is formed when
two orbitals overlap to produce a new orbital with electrons spinning oppositely (Nelson
Chemistry 12). The orbital is a space where the probability of finding an electron in an atom is
high.
Sigma bonds are covalent bonds created by an end-to-end overlap of atomic orbitals. Pi
bonds, however, are covalent bonds created by a side-to-side overlap of atomic orbitals. A
double bond, such as the one in cyclohexene, is composed of a sigma bond and a pi bond.
The single covalent bonds present between carbon atoms in cyclohexane are examples of
sigma bonds. One of those sigma bonds would look like this:
(Nelson – Chemisty 12. Section 4.2, figure 8)
A pi bond, however, would look like this:
(Nelson – Chemisty 12. Section 4.2, figure 10)
As shown in the figures above, the electron density in pi bonds is located above and below the
bond axis because the overlap is side-to-side instead of end-to-end, like sigma bonds. This means
that the pi bonds are further away from the forces of attraction (the electrons are further away
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from the nuclei of the bonding atoms) and are not on the same plane as sigma bonds. A double
covalent bond looks like this:
(Nelson – Chemisty 12. Section 4.2, figure 13)
The double covalent bond consists of a sigma and pi bond. The sigma bond is closer to
the forces of attraction than the pi bond because the sigma bond is in the same plane as the atoms
bonding, whereas the pi bond is above and below that plane. This means that the pi bond is
further away from the forces of attraction between the nuclei and the electrons. When the
electrons are in the same plane as the nuclei, the forces of attraction are stronger because the
nuclei carry protons. Thus, the pi bond is easier to break than the sigma bond, so a compound
with a double covalent bond instead of a single covalent bond must be more reactive than a
compound with only single covalent bonds.
This proves why cyclohexene was more reactive than cyclohexane. The two compounds
are of the same carbon chain length, but the only difference is that cyclohexene has a double
covalent bond (it is unsaturated), whereas cyclohexane has only single covalent bonds (it is
saturated). Therefore, cyclohexene was more reactive than cyclohexane because of the presence
of the double bond.
In conclusion, saturated hydrocarbons are less reactive than unsaturated hydrocarbons
due to the presence of double and triple bonds.
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Explanation of Heterogeneous Mixture:
In each phase of the lab, the mixtures in both test tubes were always heterogeneous. This
was because both of the two substances were always opposite in polarity, thus not dissolving.
The term “like dissolves like” is describing that nonpolar substances will dissolve in other
nonpolar substances, and polar substances will dissolve in other polar substances. However, a
nonpolar substance will not dissolve in a polar substance. Hydrocarbons (cyclohexane and
cyclohexene) are nonpolar, and the water present in aqueous bromine is polar, so the mixtures
before any reactions did not dissolve.
After the reaction with cyclohexane and aqueous bromine, bromocyclohexane and
hydrobromic acid were produced, but still did not dissolve in each other. This was because
bromocyclohexane is nonpolar and hydrobromic acid is polar. Hydrobromic acid has an
electronegativity difference of 0.7 which makes it polar. Bromocyclohexane is nonpolar, even
though it gained a halogen, the electronegativity difference between carbon and bromine is only
0.3 and a big part of that compound is still a hydrocarbon.
Similarly, 1,2-dibromocyclohexane, the product formed in the reaction with cyclohexene
and aqueous bromine, is nonpolar. Water is polar, so part of the water did not dissolve in
aqueous 1,2-dibromocyclohexane, which would make it a heterogeneous mixture.
Application Questions:
1.
Trans-Fat vs Cis-Fat:
A trans-fat is a type of unsaturated fat (a fatty acid with at least one double bond between carbon
atoms within the fatty acid chain) which are typically artificially made. They do not usually
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appear in nature. Naturally, fats typically have cis configurations instead of trans configurations.
They are geometric isomers of each other, having the same structure formula, but a different
arrangement of atoms around the double bond. The difference is slight between cis and trans fats
(naturally occurring vs artificially made), but the properties are very different. Trans-fats
typically have much higher boiling points than cis-fats of the same length because trans-fats
stack together more densely. This also means that trans-fats are solids at SATP, whereas cis-fats
are liquids at SATP.
A trans-fat looks like this (elaidic acid, often found in partially hydrogenated vegetable oils):
(Wikimedia Commons)
Whereas a cis-fat looks like this (oleic acid):
(Wikimedia Commons)
The configuration of atoms around the carbon-carbon double bond are just different: cis
has the groups positioned on the same side of the double bond, making it bend, and trans has the
groups on opposite sides of the double bond, making it a straighter molecule.
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How Trans-fats are Produced:
Some meats and dairy products have some naturally occurring trans-fats, but most transfat foods are industrially processed by halogenating vegetable oil. Vegetable oils are
polyunsaturated fats (have more than one carbon-carbon double bond), so they are partially
hydrogenated in order to produce a trans-fat (only one carbon-carbon double bond). The partial
hydrogenation breaks the double bonds between carbons and adds hydrogens to those bonds
instead. Since it is only partially hydrogenated (controlling the amount of hydrogen available),
the vegetable oil becomes only partially saturated, but not completely, so it will still have one
carbon-carbon double bond. The product is a trans-fat instead of a cis-fat because trans bonds is
favoured over cis bonds since the trans configuration has lower energy than the natural cis one.
Trans-fats in Foods:
Trans-fats are solid at SATP, so they make foods have a longer shelf life as opposed to
vegetable oils or cis-fats, which are liquids at SATP. Here are some examples of foods that transfats are commonly used in:
-
Baked goods
-
Snacks
-
Refrigerator dough
-
Creamer and margarine
Fried foods also usually contain trans-fats because restaurants use partially hydrogenated oils
in their deep-fryers so that they do not have to change the oil as often.
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Trans-fat Health Risks and Cholesterol Effects:
LDL stands for low density lipoproteins and HDL stands for high density lipoproteins.
Lipoproteins are packages containing fat and cholesterol, and are covered in protein. Trans-fat
raises a person’s LDL (“bad”) cholesterol and lowers a person’s HDL (“good”) cholesterol. This
cholesterol imbalance increases the risk of coronary heart disease in humans, and can cause high
blood pressure, narrowing of arteries, heart attack and stroke.
2.
Chemical Comparison Between Monounsaturated and Polyunsaturated Fats:
Chemically, the difference between monounsaturated fats and polyunsaturated fats is just
the amount of carbon-carbon double bonds. A monounsaturated fat has exactly one carboncarbon double bond, whereas polyunsaturated fats have at least two. Both fats are generally
liquid at SATP.
Health Affects Comparison:
Both fats are considered “good” fats and should make up the majority of a person’s fat
consumption as opposed to saturated and trans fats. Both fats reduce LDL cholesterol levels,
which is a good thing. This helps to lower heart disease and stroke.
Monounsaturated fats are high in Vitamin E and are good in helping to maintain or
develop cells in the body. Polyunsaturated fats, however, give essential fatty acids to the body
(omega-3 and omega-6). A body needs omega-3 fatty acids and omega-6 fatty acids. Both fatty
acids contribute to brain function and omega-3 lowers the risk of cardiovascular disease, clotting
of blood, etc.
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Examples of foods that contain omega-3 polyunsaturated fats are as follows:
-
Walnuts
-
Fatty fish such as salmon and herring
-
Kidney beans
-
Black beans
Examples of foods that contain omega-6 polyunsaturated fats are as follows:
-
Pecans
-
Eggs
-
Walnuts
-
Avocado
-
Corn oil
Those foods are just a few examples of what gives a body the fatty acid it needs.
Examples of foods that contain monounsaturated fats are as follows:
-
Olive oil
-
Peanut oil
-
Conola oil
-
Avocados
-
Nuts
-
Seeds
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3.
A saturated fat is a fatty acid where there are a maximum number of hydrogen atoms
bonded to each carbon. This means that there are no carbon-carbon multiple bonds because the
extra bond(s) between the carbon atoms would reduce the number of hydrogen atoms bonded to
both carbons. The hydrocarbon chain is fully “saturated” with hydrogen atoms. Saturated fats are
solid at SATP.
An example of a saturated fat would be propanoic acid:
Saturated fats occur naturally in food. Similarly to trans-fats, saturated fats are unhealthy
in that they raise LDL cholesterol levels. This increases the risk of heart attack and stroke. They
are slightly more healthy than trans-fats, though, because they do not lower HDL cholesterol
levels like trans-fats do.
Examples of foods that are high in saturated fat are as follows:
-
Fatty meats
-
Dairy products
-
Butter
-
Hard margarines
-
Cheese
Animals foods usually contain saturated fat.
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It is generally suggested to reduce trans-fat and saturated fat, and instead consume
monounsaturated fats and polyunsaturated fats. The trans-fats and saturated fats cause an
imbalance in cholesterol levels, leading to heart and circulatory issues.
4.
The fats ranked in order from easiest to hardest for bodies to digest are as follows: monoand polyunsaturated fats, saturated fats and trans-fats.
The first reason is because of the states of the fats. Monounsaturated and polyunsaturated
fats are both liquid at SATP, whereas trans-fats and saturated fats are solids at SATP. Therefore,
monounsaturated and polyunsaturated fats are easier to break down and digest because liquids
are easier to break down than solids.
Another reason is because of the carbon-carbon double bonds. As stated in the analysis,
unsaturated hydrocarbons are more reactive than saturated hydrocarbons because of the carboncarbon double bonds. Therefore, because of the double bonds present in unsaturated fats
(monounsaturated and polyunsaturated fats), it is easier to metabolize those fats. Although transfats are unsaturated and therefore are expected to be harder to digest than saturated fats, it is an
exception to the rule due to the formation of the compound. Trans-fats are artificially created
with hydrogenation and have no use for the body. Trans-fats are used only for convenience of
preservation of food and they are not meant to be digested by the body.
Another reason why saturated fats are harder to digest than unsaturated fats is because of
cholesterol level. The liver produces bile and bile aids the process of digesting fat. Bile works to
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break fat down into smaller pieces. Bile is store in the gall bladder until foods that contain fat are
eaten, in which time it releases into the small intestines. Bile contains cholesterol. So to digest
fat, cholesterol is needed. However, because saturated fats and trans-fats raise LDL levels, which
means there is less cholesterol because there is less dense lipoproteins. Monounsaturated and
polyunsaturated fats do the opposite, so they are more easily digested because of the more
balanced cholesterol levels.
Works Cited
Diffen. "Cis Fat vs. Trans Fat." Cis Fat vs Trans Fat. Diffen, n.d. Web. 25 Sept. 2014.
Coila, Bridget. "Monounsaturated Fat Vs. Polyunsaturated Fat." LIVESTRONG.COM.
LIVESTRONG.COM, 21 Oct. 2013. Web. 25 Sept. 2014.
Heart and Stroke Foundation. "Healthy Living - Dietary Fats, Oils and Cholesterol - Heart and
Stroke Foundation of Canada." Heartandstroke.ca. Heart and Stroke Foundation, Aug. 2012.
Web. 26 Sept. 2014.
Kessel, Hans Van. Nelson Chemistry 12. Toronto: Thomson Nelson, 2003. Print.
Lehman, Shereen. "Learn How Fat Is Digested." About.com. About.com, 14 June 2014. Web. 26
Sept. 2014.
Voss, Matt. "Fat Digestion." Fat Digestion. N.p., Mar. 2007. Web. 26 Sept. 2014.
Mayo Clinic Staff. "High Cholesterol." Trans Fat: Avoid This Cholesterol Double Whammy.
Mayo Foundation for Medical Education and Research, 06 Aug. 2014. Web. 26 Sept. 2014.
"Polyunsaturated Fats." Heart.org. American Heart Association, 5 Aug. 2014. Web. 26 Sept.
2014.