Hydrocarbons: Sources, Classification and Test for Saturated and Unsaturated Hydrocarbons

Hydrocarbons

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A hydrocarbon is an organic compound made up of only carbon and hydrogen atoms. Hydrocarbons form the basis of organic chemistry, and are the main components of fossil fuels (petroleum, natural gas, and coal-derived products).

Hydrocarbons occur naturally mainly as fossil fuels, formed over millions of years from the decayed remains of dead plants and animals (organic matter) that had been buried under layers of sediment, and subjected to heat and pressure in the absence of air.

Please read on carbon and its compounds here.

 

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Natural sources of Hydrocarbons 

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1. Petroleum (crude oil)

Petroleum refers to a thick (viscous), dark, oily liquid mixture found trapped in porous rock layers deep underground or under the seabed, usually beneath an impermeable rock layer that traps it. Petroleum is also called crude oil.

Petroleum (or crude oil) is formed from the remains of marine microorganisms (plankton, algae) that settled on ancient seabeds, (that is, the place they were buried), and converted by heat and pressure over geological time into a complex mixture of hydrocarbons.

Note: Natural gas is formed in a similar manner as petroleum.

Petroleum is a mixture of hundreds of different hydrocarbons (mostly alkanes, some cycloalkanes and aromatics), plus small amounts of sulfur, nitrogen, and oxygen compounds.

 

2. Natural gas

Natural gas is found trapped above (or with) petroleum deposits (or independently). It consists mainly of methane (CH₄), with smaller amounts of ethane, propane, and butane.

Natural gas is formed in a similar way as petroleum (by the same decomposition processes as explained above), often from the same organic source material.

 

3. Coal

Coal is a solid fossil fuel formed from ancient plant materials (mainly from swamp forests) under heat and pressure.

It is noteworthy to state that although coal itself is mostly carbon, its destructive distillation yields coal gas and coal tar, which are rich in hydrocarbons.

Please read more on the amorphous allotrope of carbon (coal and coke) here.

 

4. Other/minor natural sources

They include the following:

• Bitumen and asphalt deposits. They are found in naturally occuring tar pits. A tar pit (or an asphalt seep) is a natural surface pool of heavy, sticky petroleum residue where crude oil leaks upward from underground deposits.

• Biogenic methane from decomposition of organic matter in swamps ("marsh gas"), and from the digestive processes of some organisms.

• Trace natural hydrocarbons released from plants. For example, some essential oils/terpenes are hydrocarbons or hydrocarbon derivatives.

You can read on trioxocarbonates(IV), physical and chemical properties, tests and uses here.

 

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Classification of Hydrocarbons

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Hydrocarbons are broadly classified as aliphatic (open-chain or non-aromatic ring compounds) or aromatic (benzene-ring-based).

Within aliphatic hydrocarbons, the key classification at this level is by the type of carbon-carbon bonding. In this regard, aliphatic hydrocarbon can both be saturated and unsaturated.

 

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Aliphatic Hydrocarbons

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Aliphatic hydrocarbons are open-chain or non-aromatic ring compounds. They can be saturated or unsaturated depending on the type of carbon-carbon bonding. Saturated and unsaturated aliphatic hydrocarbons are explained below:

 

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Saturated Hydrocarbons

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Saturated hydrocarbons contain only single covalent bonds (C–C) between carbon atoms. Each carbon atom is bonded to the maximum possible number of hydrogen atoms. Also, each carbon atom always have four single covalent bonds around it. Examples of saturated hydrocarbons include the following:

• Alkanes

They have a general formula of CₙH₂ₙ₊₂. The first member has only one carbon atom, the second member, two, third member, three, and so on. The members of the alkane family include methane (CH₄), ethane (C₂H₆), propane (C₃H₈), butane (C₄H₁₀), pentane (C₅H₁₂), hexane (C₆H₁₄) and so on.

Alkanes are relatively unreactive, (a reason they are called "paraffins," meaning "little affinity"). Below are the reactions alkane undergoes (or chemical properties of alkanes).

A) Substitution reaction: Alkanes undergo substitution reaction with atoms like halogens in the presence of UV light). They do not undergo addition reactions since there is no double bond to add across.

The equation below shows substitution reaction in alkanes (through words).

Alkanes + Halogens -UV Light or Heat -> HaloAlkane + Hydrogen Halide

The equations below shows the halogenation of alkanes in the presence of UV light (as catalyst) to form chloromethane, dichloromethane, trichloromethane and tetrachloromethane.

• CH₄ + Cl₂ -UV Light -> CH₃Cl_(g) + HCl_(g)

• CH₃Cl_(g) + Cl_2(g) -UV Light -> CH₂Cl_2(g) + HCl_(g)

• CH₂Cl₂ + Cl₂ -UV Light -> CHCl_3(g) + HCl_(g)

• CH₃Cl + Cl₂ -UV Light -> CCl_4(g) + HCl_(g)

You can read on various questions and answers in chemistry here.

 

B) Combustion reaction: Alkanes burn in excess oxygen to give CO₂ and H₂O (complete combustion) or CO/soot with limited oxygen (incomplete combustion). 

The complete combustion of alkanes is governed by the formula below:

C_nH_2n+2 + (3n+1/2)O₂ -> nCO₂ + (n+1)H₂O

The equation below shows the complete combustion of methane and propane.

• CH_4(g) + 2O_2(g) -> CO_2(g) + 2H₂O_(l)

• C₃H_8(g) + 5O_2(g) -> 3CO_2(g) + 4H₂O_(l)

 

• Cycloalkanes

These are also saturated hydrocarbons but arranged in a ring. They are generally considered as a  class of saturated hydrocarbon.

Cycloalkanes have the general molecular formula - C_nH_2n. Example of cycloalkane is cyclopropane C₃H_6, cyclobutane C₄H₈, cyclopentane C₅H₁₀, cyclohexane, C₆H₁₂, and so on.

The image below shows the cycloalkanes and their ring-like structures.

You can read on salts in chemistry, their classification, types and preparation here.

 

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Unsaturated Hydrocarbons

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Unsaturated hydrocarbon contain at least one carbon-carbon double bond (C=C) or triple bond (C≡C). In this regard, carbon atoms are not bonded to the maximum number of hydrogens possible. However, each carbon atom still always have four covalent bonds surrounding it. Examples of unsaturated hydrocarbons include the alkenes and alkynes.

• Alkenes

They contain a C=C double bond, and have a general formula CₙH₂ₙ.The first members of this family has two carbon atoms, the second member three carbon atoms, and so on. Examples of alkenes include ethene (C₂H₄), propene (C₃H₆), butene (C₄H₈), etc.

Alkenes are more reactive than alkanes. They readily undergo addition reactions(e.g. with H₂, Br₂, H₂O) because the double bond can "open up."

 

• Alkynes

They contain a C≡C triple bond with a general formula - CₙH₂ₙ₋₂. The first member of the alkyne family has two carbon atoms, the second member three, and so on.

Examples of alkynes include ethyne/acetylene (C₂H₂), propyne (C₃H₄), etc. They are more reactive than alkenes; and also undergo addition reactions.

The equations below shows the addition reaction of alkenes and alkynes.

C₂H₄ + H₂ -> C₂H₆ 

Hydrogenation reaction of alkenes. The double bond was broken into single bond.

C₂H₂ + HCl -> C₂H₄Cl₂ 

Hydrohalogenation of alkynes. The triple bond was eventually broken into single bonds.

You can read on bases and alkalis, their preparation, test and uses here.

 

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Test to distinguish Saturated from Unsaturated Hydrocarbons

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Test: Add the hydrocarbon to bromine water (orange/brown in color) or acidified/dilute potassium manganate(VII), KMnO₄ (purple in color).

Observation for Unsaturated Hydrocarbons (Alkenes and Alkynes): Unsaturated hydrocarbons, that is; alkenes and alkynes decolorizes bromine water from orange to colorless (orange → colorless). They also decolorize KMnO₄ into a brown precipitate of manganese dioxide, MnO₂ or a colorless solution depending on the reaction conditions, as a result of the addition reaction across the double/triple bond.

Observation for Saturated Hydrocarbons: Alkanes do not decolorize bromine water in the cold/dark (no reaction under normal conditions, since they can only undergo slow substitution under UV light). They also do not decolorize KMnO₄.

Please read on acids, basicity and types of acid here.

 

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Summary Table of Saturated and Unsaturated Hydrocarbons

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The table below show the features (and differences) between saturated and unsaturated hydrocarbons.

Feature	Saturated (Alkanes)	Unsaturated (Alkenes/Alkynes)
Bonding	Only single C–C bonds	Contains C=C or C≡C
General Formula	CₙH₂ₙ₊₂ (alkanes)	CₙH₂ₙ (alkenes), CₙH₂ₙ₋₂ (alkynes)
Reactivity	Less reactive	More reactive
Typical Reaction	Substitution (needs UV light for halogenation)	Addition (readily reacts)
Bromine Water Test	No color change	stays orange) | Decolorizes (orange → colorless)
KMnO₄ Test	No color change	Decolorizes (purple → colorless)
Example	Methane, ethane	Ethene, ethyne

 

You can read on pH indicators, types and importance here.

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Aromatic Hydrocarbons

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Aromatic hydrocarbons contain a benzene ring (C₆H₆ and its derivatives). They are a special, highly stable ring of alternating and delocalized double bonds.

Examples of aromatic hydrocarbons include benzene (C₆H₆), methylbenzene/toluene (C₇H₈), naphthalene (C₁₀H₈), etc.

 

Aromatic hydrocarbons undergo mainly substitution reactions (despite containing "double bonds" in the classical structure). This reason they undergo substitution reaction is due to the stability of the delocalized ring system, unlike alkenes and alkynes.

The equation below shows the substitution reaction (halogenation) of benzene in the presence of FeCl₃ or AlCl₃ as catalyst.

• C₆H₆ + Cl_{2 -} (FeCl₃ or AlCl₃ as catalyst) -> C₆H₅Cl + HCl

Please read the continuation of this topic -> fractional distillation of crude oil, fractions obtained cracking of petroleum and uses of petroleum fractions) here.