ALKENES: STRUCTURE, PROPERTIES, REACTIVITY, USES - CHEMISTRY - 2025

The alkenes or olefins are unsaturated hydrocarbons having at the least one double bond in its structure. They were called olefins because of the ability of ethene or ethylene to react with halogens to produce oil or oil. Currently this term has fallen into disuse and these compounds are usually referred to as alkenes.

The physical properties of alkenes are influenced by their molecular weight, as well as their carbon skeleton. For example, alkenes with 2 to 4 carbons (ethene to butene) are gases. With 5 to 18 carbon atoms in the long chain they are liquid. Meanwhile, alkenes with more than 18 carbon atoms are solid.

Ethene By User: Bryan Derksen, from Wikimedia Commons

The presence of the double bond provides them with great reactivity, thus undergoing many chemical reactions, such as addition, elimination, hydrogenation, hydration and polymerization, which allow it to generate numerous uses and applications.

Alkenes are produced industrially by thermal cracking of high molecular weight alkanes (paraffin waxes); catalytic dehydrogenation and chlorination-dehydrochlorination.

Chemical structure

Own source

Alkenes are characterized by having one or more double bonds in their structure. This is represented as C = C, both carbon atoms having an sp ² hybridization.

Therefore, the region of the chain where the double bond, or unsaturation, is flat. It is also worth noting that the two carbons can be linked to two other substituents (or groups).

What substituents? Anyone who replaces one of the simplest alkene hydrogens of all: ethylene (or ethene). Starting from it (A, upper image) R, which is an alkyl substituent, takes the place of one of the four hydrogens to give rise to a mono-substituted alkene (B).

So far, regardless of which hydrogen is substituted, the identity of B. is not altered. This means that it lacks stereoisomers, compounds with the same chemical formulas but with a different spatial arrangement of its atoms.

Stereoisomers

When a second hydrogen is substituted for another R, as in C, now the stereoisomers C, D and E do arise. This is because the spatial orientations with respect to both R can vary, and to distinguish one from the other one resorts to cis-trans or EZ assignments.

In C, di-substituted alkene, the two R's can be chains of any length or some heteroatom. One is in a frontal position in relation to the other. If the two Rs consist of the same substituent, F, for example, then C is the cis-stereoisomer.

In D, both R groups are even closer, since they are attached to the same carbon atom. This is the geminal stereoisomer, although more than a stereoisomer, it is actually a terminal double bond, that is, it is at the end or beginning of a chain (for that reason the other carbon has two hydrogens).

And in E, the most stable of the stereoisomers (or geometric isomers), the two R groups are separated by a greater distance, which crosses the diagonal of the double bond. Why is it the most stable? It is because as the spatial separation between them is greater, then there is no steric tension between them.

On the other hand F and G are tri- and tetra-substituted alkenes respectively. Again, they are not capable of generating any stereoisomers.

Physical and chemical properties

Solubility

They are immiscible with water due to their low polarity. But they dissolve in organic solvents.

Melting points in ºC

Ethene -169, Propene -185, 1-Pentene -165, 1-Heptene -119, 3-Octene

-101.9, 3-nonane -81.4 and 5-decene -66.3.

Boiling point in ° C

Ethene -104, propene -47, trans2butene 0.9, cis2butene 3.7, 1-pentene 30, 1-heptene 115, 3-octene 122, 3-nonene 147 and 5-decene 170.

The boiling point increases in direct relation to the carbon number of the alkene. On the other hand, the more branched its structure, the weaker its intermolecular interactions, which is reflected in the decrease in the boiling or melting point.

Density

Ethene 0.6128 mg / ml, propene 0.6142 mg / ml and 1-butene 0.6356 mg / ml, 1-pentene 0.64 mg / ml and 1-hexene 0.673.

In alkenes the maximum density is 0.80 mg / ml. That is, they are less dense than water.

Polarity

It depends on the chemical structure, substitutions and the presence of other functional groups. Alkenes have a low dipole moment, thus the cis isomer of 2-butene has a dipole moment of 0.33, while its trans isomer has a dipole moment of zero.

Reactivity

Alkenes have a great capacity to react due to the double bonds they possess. Among the reactions in which they take part are: addition, elimination, substitution, hydrogenation, hydration and polymerization.

Addition Reaction

H ₂ C = CH ₂ + Cl ₂ => ClCH ₂ -CClH ₂ (ethylene dichloride)

Hydrogenation Reaction

It occurs at elevated temperatures and in the presence of suitable catalysts (finely divided Pt, Pd, Ni)

CH ₂ = CH ₂ + H ₂ => CH ₃ -CH ₃ (ethane)

Hydration Reaction

Reaction that is the source of the generation of alcohols from petroleum derivatives:

H ₂ C = CH ₂ + H ₂ O => H ₃ C-CH ₂ OH (ethyl alcohol)

Polymerization Reactions

Ethylene in the presence of catalysts such as aluminum trialkyl and titanium tetrachloride polymerizes in polyethylene, which contains about 800 carbon atoms. This type of polymer is called an addition polymer.

Uses and applications

Polymers

-Low density polyethylene is used in the manufacture of bags, greenhouse plastics, containers, plates, glasses, etc. While the high density is more rigid and mechanically resistant, being used in the manufacture of boxes, furniture, protectors such as helmets and knee pads, toys and pallets.

-Polypropylene, a propylene polymer, is used in the manufacture of containers, sheets, laboratory equipment, toys, packaging films, filaments for ropes, upholstery and carpets.

-Polyvinyl chloride (PVC) is a polymer of vinyl chloride that is used in the manufacture of pipes, floor tiles, channels, etc.

-Polybutadiene, 1,3-butadiene polymer, is used for the manufacture of vehicle treads, hoses and belts, as well as for the coating of metal cans.

-The ethylene propylene copolymers are used in the manufacture of hoses, body and chassis parts for cars, textile coating, etc.

Alkenes

-They are used in obtaining solvents such as ethylene glycol and dioxane. Ethylene glycol is used as an antifreeze in car radiators.

-Ethylene is a plant hormone that controls growth, seed germination and fruit development. Therefore, it is used to induce the ripening of bananas when they reach their destination.

-They are used as raw material for the manufacture of many compounds such as alkyl halides, ethylene oxide and especially ethanol. They are also used in industry, personal care, and medicine.

-They are used to obtain and manufacture lacquers, detergents, aldehydes and fuels. 1,3-butadiene is used as a raw material in the manufacture of synthetic rubbers.

References

1. Chemistry Assignment. Physical Properties of Alkenes. Taken from: chemistry-assignment.com
2. Wikipedia. (2018). Alkene. Taken from: en.wikipedia.org
3. Chemistry LibreTexts. Physical Properties of Alkenes. Taken from: chem.libretexts.org
4. Whitten, Davis, Peck & Stanley. Chemistry. (8th ed.). CENGAGE Learning.
5. Francis A. Carey. Organic Chemistry. (Sixth ed., P. 194). Mc Graw Hill.
6. Houghton Mifflin Harcourt. (2016). Alkenes: Molecular and Structural Formulas. Taken from: cliffsnotes.com
7. Chipera, Angie. (April 25, 2017). What Is an Olefin in Chemistry? Sciencing. Taken from: sciencing.com