Reactivity of Aliphatic and Aromatic Hydrocarbons
Rannilo D. Rocas Jr.
Department of Biochemistry, College of Humanities and Sciences,
De La Salle Medical and Health Sciences Institute, City of Dasmariňas, Cavite
17 Mar 2020
ABSTRACT
Hydrocarbons are compounds composed only of carbon and hydrogen atoms. The first type of hydrocarbon is aliphatic
hydrocarbon, which is classified as alkane, alkene, and alkyne. The second type is aromatic hydrocarbon, a circularly
structured organic compounds that contain sigma bonds along with delocalized pi electrons. Saturated hydrocarbons contain
only single bonds between carbon atom, while unsaturated hydrocarbons contain double bonds or triple bonds. Unsaturated
hydrocarbon is more reactive since it contains pi electrons which are nucleophilic in nature and thus more reactive. In this
experiment, the reactivity of following compounds we’re tested: petroleum ether, cyclohexane, cyclohexene, xylene, hexane,
and petrolatum. Amongst these compounds, the cyclohexene is the most reactive, while the other compounds didn’t react
with the reagents. It reacted with KMnO4, with a color change from purple to brown, due to the presence of MnO2. In bromine
test, the color of the solution became clear, indicating that the reaction proceeded. Cyclohexene when added in sulfuric acid,
resulted in a single layered solution, since the product of the reaction was soluble in sulfuric acid. All the hydrocarbons are
nonpolar, hence became soluble only in nonpolar solvent (hexene). Hydrocarbons reacted with oxygen and underwent
combustion reaction.
Keywords: Hydrocarbons, Reactivity, Saturated, Unsaturated, Aliphatic, Aromatic, Solubility, Combustion.
Introduction:
Hydrocarbons are the simplest organic compounds,
composed only of carbon and hydrogen. It is also
separated into two types, namely aliphatic hydrocarbons
and aromatic hydrocarbons. There are three types of
aliphatic hydrocarbons, wherein each type was
characterized by the number of covalent bonds being
formed. Alkanes are aliphatic hydrocarbon with single
covalent bond, alkene having double covalent bond, and
alkyne having triple covalent bond. Aliphatic hydrocarbons
also form rings which is called cycloalkane, cycloalkene,
and cycloalkyne. Aromatic hydrocarbons are ring systems
which contains overlapping p orbitals. The condition of the
electrons in the aromatic compound gives a physical and
chemical properties that are different from alkanes. The
aromatic word was coined to the compound since these
compounds were particularly fragrant. However, the term
aromatic in modern chemistry denotes that these
compounds has a very stable ring (Ball, 2014).
Figure 1: Classification of hydrocarbons
Unsaturated compounds contain a double or triple
bond, which enables these compounds to be more
reactive. A saturated organic compound has only single or
bonds between carbon atoms and are less reactive than
unsaturated
compounds
(Smith,
2007).
Some reactions that unsaturated hydrocarbons
partake in are oxidation by the use of KMnO4, bromination
by Br2, and addition reaction by H2SO4 Aromatic
compounds and alkanes are not as reactive as alkenes and
alkynes.
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Alkenes react with potassium permanganate solution
undergo oxidation reaction with alkenes. The dilute KMnO4
solution has a deep purple color; no reaction would result in
retention of the color. When it reacts with unsaturated
aliphatics it produces MnO2, a brown precipitate.
carbon dioxide and water.
Figure 5: Combustion of hydrocarbons
Objectives:
1. Differentiate the two types of hydrocarbons by
their properties.
Figure 2: Reaction with KMnO4
Unsaturated hydrocarbons react rapidly with
bromine. The reaction is the addition of the elements of
bromine to the carbons-carbon double bond. Alkenes, but
not alkanes or aromatic hydrocarbons will react with Br2 in
solution to produce the corresponding alkyl bromide (or
dibromoalkane). The decoloration of the bromine indicates
that the reaction proceeds. (Clark, 2019).
2. Classify the hydrocarbons according to their
functional group.
3. Predict the reactivity of hydrocarbons.
Methodology:
A. Solubility of Hydrocarbons
On a clean separate test tubes, two drops of each of
the following were transferred; petroleum ether,
cyclohexane, cyclohexene, and xylene. Three drops of
deionized water were then added, the test tubes were
shaken up and the observed color was recorded. The same
procedure was repeated using a different solvent (hexane).
The miscibility of the compounds were recorded.
Figure 3: Reaction with Br2
Alkenes react with concentrated sulfuric acid in the
cold to produce alkyl hydrogensulphates. Saturated
hydrocarbons are unreactive and aromatic compounds also
are unreactive because addition reactions are difficult due
to resonance stabilization.
B. Baeyer’s test
On a clean separate test tubes, two drops of each of
the following were transferred; petroleum ether,
cyclohexane, cyclohexene, and xylene and deionized water
(blank). One drop of Baeyer’s Reagent was added to each
test tube. The test tubes were shaken up and the colors
observed were recorded.
C. Reaction with Bromine
Figure 4: Reaction of Cycloalkene with sulfuric acid
Almost all hydrocarbons will undergo combustion, this
occurs when hydrocarbon reacts with oxygen to produce
On a clean separate test tubes, two drops of each of
the following were transferred; petroleum ether,
cyclohexane, cyclohexene, and xylene and deionized water
(blank). One drop of Bromine water was added to each test
tube. The test tubes were shaken up and the colors of each
were recorded.
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D. Reaction with Sulfuric Acid
On a clean separate test tubes, two drops of each of
the following were transferred: petroleum ether,
cyclohexane, cyclohexene, and xylene and deionized water
(blank). One drop of conc. Sulfuric acid was added to each
test tube. The test tubes were shaken up. The number of
layers and the colors of each were recorded.
E. Combustion
Figure 6: Observed layers
Table 2. Baeyer’s Test
In an evaporating dish, two drops of petroleum ether
were placed, and then was ignited under the fume hood.
The same procedure was repeated using cyclohexane,
cyclohexene, and xylene.
F. Generation and Analysis of Acetylene
A pinch of calcium carbide was transferred in a dry
small vial. A syringe was used to add 1 mL of tap water. A
sufficient amount of gas was then collected using a syringe.
In a separate well of a spot plate, two drops each of
Baeyer’s reagent and bromine water was added. The gas
was injected on each well, and the reaction was observed
for any changes in color or bubble formation.
G. Analysis of Petrolatum
A small amount of petrolatum was placed on a
spot plate, then all the tests were conducted.
Results and Discussions:
Table 1. Solubility of Hydrocarbons
Hydrocarbon
Water
Hexane
Petroleum
ether
Cyclohexane
Immiscible
Miscible
Immiscible
Miscible
Cyclohexene
Immiscible
Miscible
Xylene
Immiscible
Miscible
Petrolatum
Immiscible
Miscible
All the compounds were observed to be insoluble in
water, since these compounds were nonpolar, and water
is a polar solvent, and like dissolves like. When added in
a nonpolar solvent which was hexane, all the compounds
were observed to be soluble.
Hydrocarbon
Color
Petroleum
ether
Cyclohexane
Purple
Cyclohexene
Brown
Xylene
Purple
Petrolatum
Purple
Acetylene
Purple
Deionized
water
Purple
Purple
In using the Baeyer’s test, cyclohexene turns into
color brown, while the rest of the compounds didn’t
exhibit a color change. Potassium permanganate can
only react with unsaturated hydrocarbons, and the color
change from purple to brown color indicates that the
reaction takes place since the brown color is the
indication of a presence of MnO2, which is the product of
the reaction along with diol.
Figure 7: Observed color change
Table 3. Reaction with Bromine
Hydrocarbon
Color Change
Petroleum
ether
Cyclohexane
Clear
Cyclohexene
Clear
Xylene
Cloudy (white)
Clear
4
Petrolatum
yellow
Cyclohexene
Orange
Acetylene
Lighter yellow
Xylene
Orange
Deionized
water
Yellow
Petrolatum
Non-combustible
Petroleum ether, cyclohexane, and cyclohexene
reacted with bromine based on the color change observed,
however, according to previous experiments, and expected
results, cyclohexane wouldn’t react with bromine. Only the
presence of available electron pair could cause reaction
with bromine, cyclohexene is the only compound that
readily has these electron pairs, the disappearance of the
yellow color indicates that the bromine was used up and
was
bonded
to
the
organic
compound.
All the compounds underwent combustion except
with the petrolatum, this might be caused by the physical
characteristic of the petrolatum, like having a gel like
property, unlike the other hydrocarbons which is in liquid
form.
Figure 9: Combustion under the fume hood
Conclusion:
Figure 8: Observed color change
Table 4. Reaction with Sulfuric Acid
Hydrocarbon
Observation
Petroleum
ether
Cyclohexane
2 layers
Cyclohexene
no layer
Xylene
2 layers
Petrolatum
2 layers
Deionized
water
2 layers
2 layers
All the compounds were observed to have 2 layers
except cyclohexene. Since alkenes and cycloalkenes are
the only hydrocarbons that can have addition reaction
when combined with sulfuric acid. The product formed
from this reaction is soluble in sulfuric acid, thus resulting
in a single layered liquid.
Table 5. Combustion
Hydrocarbon
Observation
Petroleum
ether
Cyclohexane
Orange
Orange
Hydrocarbons, being compounds that only contain
the hydrogen and the carbon elements, exhibit lesser
reactivity compared to other organic compounds, these
particular hydrocarbons are saturated and aromatic
compounds. Unsaturated hydrocarbons are more reactive
when compared to saturated hydrocarbons as in the
unsaturated compounds we have double bonds (pi bonds)
which are weak and can be easily cleaved by incoming
group therefore increasing the reactivity. In saturated
hydrocarbons only sigma bonds are present, these bonds
are very strong and stable. That principle strongly supports
and validates the results of the experiment. Although
aromatic compounds also contain pi bonds, it was still not
as reactive as alkenes due to resonance, this causes
stabilization of the organic compound. Cyclohexene, a pi
bond containing hydrocarbon was the most reactive
compound amongst the hydrocarbons tested in the
experiment.
It is recommended that proper labeling of the test
tubes must be done in order to avoid misinterpretations of
results, since all the hydrocarbons have the same
physical appearance. In testing the solubility, a few drops
of the solvent can be added for a better visualization of
the presence of double layer in the test tube.
Familiarizations of the properties of different types of
hydrocarbons could also be helpful, having this
background would allow for expected results, thus any
results that has an error could easily be identified and
addressed.
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References:
Ball, D. W., & Key, J. A. (2014, September 16).
Hydrocarbons. Retrieved March 17, 2020, from
https://opentextbc.ca/introductorychemistry/chapter/hydro
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Bonifacio, M. C. (2020). Organic Chemistry Laboratory
Experiments (2020th ed.).
Libretexts. (2019, June 5). Reactions of Alkenes with
Bromine.
Retrieved
March
15,
2020,
from
https://chem.libretexts.org/Bookshelves/Organic_Chemistry
/Supplemental_Modules_(Organic_Chemistry)/Reactions/A
ddition_Reactions/Electrophilic_Addition_Reactions/Reacti
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Properties of Hydrocarbons. (n.d.). Retrieved March 17,
2020, from
http://www.mendelset.com/articles/689/properties
_hydrocarbons
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Organic Chemistry: Reactions, Mechanisms, and
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