Alkanes, Alkenes, Alkynes

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Hydrocarbon is any of a class of organic chemical compounds composed only of the elements carbon (C) and hydrogen (H). The carbon atoms join together to form the framework of the compound; the hydrogen atoms attach to them in many different configurations. Hydrocarbons are the principal constituents of petroleum and natural gas (Carey, 2016). They serve as fuels and lubricants as well as raw materials for the production of plastics, fibres, rubbers, solvents, explosives, and industrial chemicals. Hydrocarbons are classified into aliphatic and aromatic, based on its sources and properties. For the purposes of this paper, the focus will be on aliphatic hydrocarbons. Aliphatic hydrocarbons are those derived by chemical degradation of fats or oils. This group is further divided into alkanes, alkenes and alkynes, divided according to the types of bonds they contain. Alkanes have single bonds, alkenes contain a carbon-carbon double bond, and alkynes have a carbon-carbon triple bond. Also, alkanes are labelled as saturated hydrocarbons, while alkenes, alkynes, and aromatic hydrocarbons are said to be unsaturated.

There are many properties of hydrocarbons, one of which is combustibility. Most hydrocarbons will burn over a flame- the hydrocarbon reacts with oxygen to produce carbon dioxide and water. In the former, the test compound viable for combustibility is butane and acetylene. In the first set-up, butane was transferred in a test tube by submerging it underwater through water displacement. Butane lighters contain iso-butane isomer, whose formula is C4H10. In the lighter the gas is compressed, so that this equilibrium:

C4H10 (l) + energy [pic 1]C4H10 (g)

is forced to the left. From le Châtelier's principle, the shift on equilibrium can be predicted. Take note that the effect of temperature is disregarded. If there is an increase in pressure on the reactants, the equilibrium will be attained by decreasing the pressure on the products side. Since there are no molecules of gas on the reactants and there is one molecule of gas on the product, the equilibrium shift will be to the left. There are times when the equilibrium is shifted far to the left and in this cases, iso-butane condenses into liquid state (recall that condensation is the change of the physical state of matter from gas phase into liquid phase). When the valve is opened, pressure is reduced, thus iso-butane will increase pressure to attain equilibrium, so more of it evaporates and is changed into its vapor state, in which this gaseous from of iso-butane can be burned. The combustion of butane:

2C4H10 + 13O2 8CO2+ 10H2O[pic 2]

It was observed that the combustion of butane produced a blue flame with no soot (transparent to whitish smoke was formed). This signifies that a complete combustion took place. It means that there is a sufficient oxygen supply that was reacted with butane, to fully convert the carbon atoms into carbon dioxide and hydrogen atoms to water. the complete combustion of butane provided enough energy to excite and ionize the gas molecules in the flame. From the emission of the excited molecular radicals, which emit most of their light below 565 nanometers and it can be seen from the visible light spectrum below that 565 nanometers fall in the green to blue regions of the spectrum.

[pic 3]

image retrieved from http://www.astronomersgroup.org/EMspectrum.html

Acetylene was collected from a hole of a piece of calcium carbide (CaC2) wrapped in aluminum foil and submerged in water. The gas was collected same as through with that of butane, through water displacement.

CaC2 (s) + 2H2O (l) C2H2 (g) + Ca(OH)2 (aq)[pic 4]

The reaction yields acetylene (C2H2) and calcium hydroxide (Ca(OH)2). Furthermore, the combustion of acetylene:

2C2H2 (g) + 5O2 2H2O + 4CO2[pic 5]

It was observed that the combustion of acetylene produced a reddish to orange flame with a black soot (black smoke was formed). This signifies that an incomplete combustion took place. The amount of oxygen is not enough to convert the carbon atoms into carbon dioxide and hydrogen atoms to water. since there is insufficiency in oxygen, hydrogen reacts with oxygen first, and whatever is left of oxygen is reacted with carbon producing a black solid compound (carbon) and a colorless, odorless and very toxic gas (carbon monoxide). The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke (Clark, 2015). Carbon monoxide on the other hand binds to the iron in hemoglobin. Once it is attached, it is very difficult to release and takes up all the binding sites for oxygen. in due course, the blood will be unable to transport oxygen, thus leading to suffocation and death.

In the latter, the reaction with bromine, this involved the exposure of butane with bromine solution in the presence and absence of light, the exposure of ethylene to the bromine solution and the exposure of acetylene to the bromine solution.

After the collection of butane, it was subjected with 1 mL bromine and solution and two set-ups were made; one of which is in the presence of light and the other is the absence of light. In the set-up with the sun’s exposure, it was observed that there was a decolorization of the solution from a yellow-orange color to a lighter shade (light yellow). It can be accounted for that the bromine, which causes the brown color of the solution is being consumed by the reaction with the hydrocarbon. This reaction is called halogenation wherein there is a replacement of one or more hydrogen atoms in an organic compound by a halogen (fluorine, chlorine, bromine or iodine). Unlike the complex transformations of combustion, the halogenation of an alkane appears to be a simple substitution reaction in which a C-H bond is broken and a new C-X bond is formed (Reusch, 2013). Furthermore, halogenation or radical reaction involves three steps, namely initiation, propagation, and termination.

In initiation, heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and start the chain process. Light in this process served as a catalyst. The reaction of Br2 with butane does not normally takes place but due to the ultraviolet light from the solution’s exposure to sunlight, radicals, which are very reactive are yielded. Two bromine radicals (Br ) were formed.

[pic 6][pic 7]

[pic 8][pic 9]

Br