Properties and Uses of Alkenes

Alkenes are hydrocarbons because they are made up entirely of carbon and hydrogen atoms, and they are unsaturated because the alkene structure contains one or more double bonds. One sigma and one pi bond form a double bond between two carbon atoms. The characteristics of the sigma bond are similar to those of alkanes, whereas the pi bond is more reactive. 

The double bond’s carbon atoms are sp2 hybridised, resulting in a planar structure. Because rotation around the double bond is unfavourable, alkenes form rather stable isomers when substituents are placed on the same (cis) or opposite (trans) sides of the double bond. Diastereoisomers are a type of isomer.

Physical properties of Alkenes

The melting and boiling temperatures of alkenes are determined by the regularity of the packing, or the closeness, of these molecules. Higher melting and boiling temperatures are found in alkene isomers that can attain more regular packing than molecules with the same chemical formula but weaker dispersion forces. Nonpolar alkenes have a lower density than water and are immiscible in them. They are soluble in organic solvents in general. Furthermore, they don’t conduct electricity.

Simple alkenes with two to four carbon atoms, such as ethene and propene, are commonly found as gases. Liquid alkenes are those with five to sixteen carbon atoms. Waxy substances with seventeen or more carbon atoms, such as paraffin wax, are commonly used to produce candles.

Structure of double bond

One strong sigma (σ) bond (bond enthalpy around 397 kJ mol–1) develops from head-on overlapping of sp2 hybridised orbitals in alkenes, while one weak pi (π) bond (bond enthalpy about 284 kJ mol–1) arises from lateral or sideways overlapping of the two carbon atoms 2p orbitals. The C–C double bond has a shorter bond length (134 pm) than the single bond (154 pm). The pi (π) bond is a weaker bond due to poor sideways overlapping between the two 2p orbitals. Alkenes act as sources of loosely held mobile electrons because of the pi (π) bond. As a result, reagents or compounds needing electrons have an easy time attacking alkenes. Such reagents are termed electrophilic reagents.

Because alkenes have a weaker π-bond than alkanes, they are more unstable molecules. As a result, alkenes can be converted to single bond compounds by reacting with electrophilic reagents. 

Chemical properties of Alkenes

Alkenes are mostly used in addition reactions, which involve breaking one of the double bonds. Each of the bond’s carbon atoms can attach to another atom or group while remaining connected by a single bond. Hydrogenation, a reaction involving hydrogen (H2) and a catalyst such as nickel (Ni) or platinum (Pt)—is perhaps the most basic addition reaction (Pt).

Halogenation (adding halogens) is also a simple reaction for alkenes. Indeed, alkenes may be detected via the bromine (Br2) reaction. Bromine solutions are a brownish-red colour. The colour of a Br2 solution fades when it is added to an alkene because the alkene reacts with the bromine. 

Another major addition reaction is the formation of alcohol from alkene and water. This process, known as hydration, involves using a catalyst, which is usually a strong acid like sulfuric acid (H2SO4).

Uses of Alkenes

• Alkenes are widely employed in a variety of sectors. Alcohols, polymers, liquors, detergents, and fuels are all made with them as beginning components.
• In the polymer industry, ethene is the most important organic material. For instance, PVC and polyethylene. These polymers are used to make floor tiles, shoe soles, synthetic fibres, raincoats, and pipes, among other things.
• Propene is primarily used to make polypropylene and various oxidation products such as butanol, acrylic acid, acrolein, acrylic acid ester, glycerol, allyl chloride, and epichlorohydrin. It is the second most important hydrocarbon for organic products, with an estimated global production of about 50 million metric tonnes in 2000.
• Thermal cracking produces a large volume of ethylene from natural gas. It is a crucial raw material in the production of a variety of polymers.

Conclusion

Unsaturated hydrocarbons with at least one carbon-to-carbon double bond are known as alkenes (e.g., containing carbon and hydrogen solely). Olefin is a term used to describe alkenes. Because of the double bond, alkenes are more reactive than alkanes. The carbon-to-carbon double bond defines alkenes, and it is this bond that is responsible for many of the alkenes’ unique features. The double bond is a shorter and thus stronger bond than the corresponding single bond; breaking a double bond requires more energy than breaking a single bond.

The reactivity of double bonds is another essential feature. Because double bonds are more reactive than single bonds, they are more susceptible to addition reactions such as polymerization. Alternatively, certain varnishes and wood floor finishes contain molecules with double bonds on purpose to cause a polymerization reaction and generate the necessary hard layer on the surface.