Unsaturated Hydrocarbons: Types, Uses & Examples

Unsaturated hydrocarbons are organic compounds consisting of at least one double or triple covalent bond between two adjacent carbon atoms. Their structural formula is C_nH_2n−2, where n is the number of carbons.

• These types of hydrocarbons are entirely made up of hydrogen and carbon.
• The IUPAC nomenclature represents the position of the double or triple bond by writing a number before the compound's name or before the suffix,—ene or—yne.
• They contain fewer hydrogen atoms bonded with the chain of carbon atoms.
• The two important categories of unsaturated hydrocarbon include alkenes and alkynes.
• You can see their uses from fuel that we use to run our car to the plastic products available in the market.
• In this article, we will explore the various types of unsaturated hydrocarbons, their properties, uses, and sample questions.

Key Terms: Unsaturated Hydrocarbons, Alkenes, Alkynes, Hydrocarbons, Carbon, Hydrogen, Types of Unsaturated Hydrocarbon, Unsaturated Hydrocarbon Examples

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What are Unsaturated Hydrocarbons?

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Organic compounds that are made up of hydrogen and carbon are known as hydrocarbons. One of the categories of hydrocarbon is unsaturated hydrocarbons, which is formed by either a double or triple bond with two adjacent carbon atoms. 

• The presence of double and triple bonds makes these compounds highly unreactive.
• The term ‘unsaturated’ means that a greater number of hydrogen atoms can be easily added to the hydrocarbon in order to make it saturated.
• Unsaturated hydrocarbons are converted into saturated hydrocarbons by the process of hydrogenation, halogenation, and hydration.
• These inorganic compounds are used for the production of pesticides, fuel, paints, lacquers, etc.
• This category of hydrocarbon is obtained with the use of petroleum through a process called cracking. 
• The degree of unsaturation can be determined using the following formula:

DU = 2C + N - F - H + 22

where,

• C, N, F, and H represent the number of carbon, nitrogen, halogen, and hydrogen atoms in the compound, respectively. 

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Properties of Unsaturated Hydrocarbons

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The physical and chemical properties of unsaturated hydrocarbons include:

Physical Properties of Unsaturated Hydrocarbons

The physical properties of unsaturated hydrocarbons are as follows:

• In the case of alkenes, both cis and trans isomers are present.
• In the former, the functional groups are present on the same side of the bond.
• The functional groups are present on either side of the covalent bond in trans isomers. 
• The unsaturated hydrocarbons are nonpolar, i.e., they have a weak intermolecular force.
• As a result, they have a low solubility in water but completely dissolve in nonpolar organic solvents like Benzene.
• The presence of high-density electrons in double and triple bonds of unsaturated hydrocarbons results in electrophilic addition reactions.

Chemical Properties of Unsaturated Hydrocarbons

The chemical properties of unsaturated hydrocarbons are as follows:

Combustion

The unsaturated hydrocarbons produce water and carbon dioxide upon combustion and burn with a yellow flame. It can be represented using the following formula:

C_xH_y+\(\frac{y+4x}{4}\)O₂ → \(\frac{y}{2}\)H₂O+xCO₂ 

For example ethene,

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Hydrogenation

It involves the addition of hydrogen gas to an unsaturated hydrocarbon in the presence of a catalyst – Pd, Pt – and heat or pressure. For example, hydrogenation of ethene results in the formation of ethane. Therefore, 

H₂C=CH₂ + H₂ →    H₃C-CH₃

Halogenation

Halogenation involves the addition of at least one halogen (like F₂, Cl₂, Br₂, I₂) into the compound, resulting in the formation of a haloalkane or alkyl halide.

• For example, halogenation of ethene results in the formation of ethane which is given as:

H₂C=CH₂ + Br₂  →    H₃CBr-CH₂Br

Hydration

The process involves the addition of water in the presence of a strong acid (catalyst), thus resulting in the formation of alcohol. This reaction follows Markonikov’s rule. For example, the hydration of ethyne results in the formation of acetaldehyde. 

HC≡CH + H₂O  →    H₂C=CH-OH

Hydrohalogenation

Hydrohalogenation involves the addition of hydrohalic acids like hydrochloric acid (HCl) and hydrogen bromide (HBr) to unsaturated hydrocarbons, resulting in the formation of haloalkanes (in case of alkenes) or vinyl halide (in case of alkynes). 

• For example, hydrohalogenation of ethene results in the formation of bromoethane.

H₂C=CH₂ + HBr  →    H₃C-CH₂Br

Oxidation

It involves the removal of hydrogen or the addition of oxygen from a molecule. Some of the common agents of oxidation are ozone (O3), potassium permanganate (KMnO₄), oxygen (O₂), and potassium dichromate (K₂Cr₂O₇). 

• For example, the reaction of ethylene with dilute potassium permanganate produces dihydroxy alcohol. 

C₂H₄ + KMnO₄ →    CH₂(OH)CH₂OH

Polymerization

Polymerization involves conducting reactions at a certain temperature under pressure and in the presence of a catalyst, thus, resulting in the formation of polymers. For example, polymerization of ethene produces polyethylene. 

nCH₂=CH₂+ heat/pressure/catalyst→  (-CH₂-CH₂-)_n

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Types of Unsaturated Hydrocarbons

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Unsaturated hydrocarbons can be primarily classified into the following types:

Alkenes

Alkenes are unsaturated hydrocarbons that contain at least one double covalent bond with adjacent carbon atoms. They can be generally represented as C_nH_2n.

• The carbon-carbon double bond forms at an angle of 120°.
• Alkenes have fairly low boiling and melting points.
• Ethene or ethylene (C₂H₄) is a good example of an alkene. 

Alkynes

Alkynes unsaturated hydrocarbons contain at least one triple covalent bond with adjacent carbon atoms. It can be represented as C_nH_2n-2. The carbon-carbon triple bond forms at an angle of 180°.

• As compared to alkenes, alkynes have higher melting and boiling points.
• Ethyne or acetylene (C₂H₂) is a good example of alkynes.

Aromatic Hydrocarbons

Aromatic Hydrocarbons unsaturated hydrocarbons are ring-shaped hydrocarbons containing delocalized pi () electrons. They are relatively stable and do not share any common properties with the other unsaturated hydrocarbons.

• Benzene (C₆H₆) is the most common example of aromatic hydrocarbons.
• The carbon-carbon bond forms an angle of 120°.

Unsaturated Hydrocarbons 

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Uses of Unsaturated Hydrocarbons

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Unsaturated hydrocarbons are utilized for several purposes. Some of them are as follows:

• Alkenes are used to artificially ripen several fruits.
• The unsaturated hydrocarbons are generally useful in the production of plastics.
• Alkenes are used to prepare mustard gas – a poisonous gas primarily used in chemical warfare.
• LDPE (a low-density variation of polyethylene) is widely used for the production of grocery bags.
• Egg cartons, disposable cups, and several other products are usually made with the help of polystyrene. 
• Alkenes are used for the production of detergent and several industrial chemicals, including alcohol.
• Unsaturated hydrocarbons are also utilized as anaesthetics.
• Several products of high industrial importance are manufactured with the help of alkynes.

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Things To Remember

• The hydrocarbons that have multiple bonds with adjacent carbon atoms are known as unsaturated hydrocarbons.
• The unsaturated hydrocarbons that form at least one double covalent bond with adjacent carbon atoms are known as alkenes. 
• Aromatic hydrocarbons are a type of unsaturated hydrocarbons that have properties that are quite different from other unsaturated hydrocarbons.
• The unsaturated hydrocarbons enter into several chemical reactions - halogenation, hydration, oxidation, hydrogenation, and polymerization - that result in the production of different compounds.
• This category of hydrocarbon plays an important role in many industrial reaction and process.