The Hydrocarbons

Let us first look at a group of organic compounds known as the hydrocarbons.

Definition: Hydrocarbon

An organic molecule which contains only carbon and hydrogen atoms with no other functional groups besides single, double or triple carbon-carbon bonds.

The hydrocarbons that we are going to look at are called aliphatic compounds. The aliphatic compounds are divided into acyclic compounds (chain structures) and cyclic compounds (ring structures). The chain structures are further divided into structures that contain only single bonds (alkanes), those that contain at least one double bond (alkenes) and those that contain at least one triple bond (alkynes).

Cyclic compounds (which will not be covered in this book) include structures such as a cyclopentane ring, which is found in insulating foam and in appliances such as fridges and freezers. The image below summarises the classification of the hydrocarbons.

The classification of the aliphatic hydrocarbons.

Fact:

An aliphatic compound is one that does not contain an aromatic ring:

The simplest aromatic compound is benzene. There are aliphatic cyclic compounds, but if a compound contains an aromatic ring it is an aromatic compound, not an aliphatic one.

We will look at each of the acyclic, aliphatic hydrocarbon groups in more detail in the next set of lessons.

Optional Experiment: Saturated vs. unsaturated compounds

Aim

To study the effect of bromine water and potassium permanganate on saturated and unsaturated compounds.

Apparatus

Warning:

Liquid bromine (required to make bromine water) is a highly volatile, corrosive and toxic compound. Please handle with care: wear the appropriate safety clothing including gloves, labcoat, safety glasses and mask. Work in a fumehood. If you do not have the apparatus to handle liquid bromine safely, use potassium permanganate only.

cyclohexane, cyclohexene, bromine water (\(\text{Br}_{2}(\text{aq})\)), potassium permanganate (\(\text{KMnO}_{4}\)) in an acidic solution
4 glass containers (test tubes/beakers/shallow basins), two A4 sheets of paper
2 plastic pipettes

Method

Label one piece of paper A and the other piece of paper B.
Place \(\text{20}\) \(\text{ml}\) of cyclohexane into a container and place the container on paper A.
Place \(\text{20}\) \(\text{ml}\) of cyclohexane into a container and place the container on paper B.
Repeat steps 2 and 3 with cyclohexene.
Take \(\text{12}\) \(\text{ml}\) of bromine water and add it to the beaker of cyclohexane on paper A. Observe any colour changes.
Repeat step 5 with the beaker of cyclohexene on paper A.
Take \(\text{12}\) \(\text{ml}\) of \(\text{KMnO}_{4}\) and add it to the beaker of cyclohexane on paper B. Observe any colour changes.
Repeat step 7 with the beaker of cyclohexene on paper B.

Results

Record your results in the table below.

Compound	Initial colour	Solution added	Final colour
cyclohexane		bromine water
cyclohexane		\(\text{KMnO}_{4}\)
cyclohexene		bromine water
cyclohexene		\(\text{KMnO}_{4}\)

Cyclohexane is an alkane, cyclohexene is an alkene.

Questions

Which of these compounds (cyclohexane, cyclohexene) is saturated and which is unsaturated?
What colour changes did you observe with the alkane compound?
What colour changes did you observe with the alkene compound?
Can you suggest a reason for the differences?

Discussion and conclusion

Bromine water and \(\text{KMnO}_{4}\) both have intense colours. Cyclohexane is a saturated, colourless liquid. When bromine water and \(\text{KMnO}_{4}\) are added to the cyclohexane there is no reaction and the solution becomes the colour of the bromine water or \(\text{KMnO}_{4}\).

Cyclohexene is also a colourless liquid, but it is unsaturated. This results in a reaction with bromine water and with \(\text{KMnO}_{4}\). Cyclohexene will form a bromoalkane with bromine water. Bromoalkanes are colourless liquids and the solution will be colourless - liquid bromine is decolourised by cyclohexene. Similarly \(\text{KMnO}_{4}\) will be decolourised by the cyclohexene.

Cyclohexane

Cyclohexane is a cycloalkane with the molecular formula C₆H₁₂. Cyclohexane is mainly used for the industrial production of adipic acid (an important dicarboxylic acid) and caprolactam (used to make fiber and plastics), which are precursors to nylon. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used).

In organic chemistry, the cycloalkanes are the monocyclic saturated hydrocarbons. In other words, a cycloalkane consists only of hydrogen and carbon atoms arranged in a structure containing a single ring, and all of the carbon-carbon bonds are single.

On an industrial scale, cyclohexane is produced by hydrogenation of benzene. Producers of cyclohexane accounts for approximately 11.4% of global demand for benzene. The reaction is highly exothermic, with ΔH(500 K) = -216.37 kJ/mol). Dehydrogenation commenced noticeably above 300 °C, reflecting the favorable entropy for dehydrogenation.

Hydrogenation – meaning, to treat with hydrogen – is a chemical reaction between molecular hydrogen (H₂) and another compound or element, usually in the presence of a catalyst such as nickel, palladium or platinum. The process is commonly employed to reduce or saturate organic compounds. Hydrogenation typically constitutes the addition of pairs of hydrogen atoms to a molecule, often an alkene.

The Hydrocarbons