UNSATURATED HYDROCARBONS: PROPERTIES AND EXAMPLES - CHEMISTRY - 2025

The unsaturated hydrocarbons are those containing at the least one carbon in its structure, and may contain triple bond since saturation of a chain means that this has received all hydrogen atoms possible in each carbon, and there are no pairs free electrons where more hydrogens can enter.

Unsaturated hydrocarbons are divided into two types: alkenes and alkynes. Alkenes are hydrocarbon compounds that have one or more double bonds within their molecule. Meanwhile, alkynes are hydrocarbon compounds that possess one or more triple bonds within their formula.

Alkenes and alkynes are widely used commercially. These are compounds with a higher level of reactivity than saturated hydrocarbons have, which makes them the starting point for many reactions, generated from the most common alkenes and alkynes.

Nomenclature

Unsaturated hydrocarbons are named differently depending on whether they are alkenes or alkynes, using the suffixes "-eno" and "-ino".

Alkenes have at least one carbon-carbon double bond in their structure, and have the general formula C _n H _2n, while alkynes contain at least one triple bond and are handled by the formula C _n H _2n-2.

Nomenclature of alkenes

To name alkenes, the positions of the carbon-carbon double bonds must be indicated. Names of chemical compounds containing C = C bonds end with the suffix "–eno".

As with alkanes, the name of the base compound is determined by the number of carbon atoms in the longest chain. For example, the molecule CH ₂ = CH-CH ₂ -CH ₃ will be called "1-butene", but that of H ₃ C-CH = CH-CH ₃ will be called "2-butene."

The numbers seen in the names of these compounds indicate the carbon atom with the smallest number in the chain in which the C = C bond of the alkene is located.

The number of carbons in this chain identifies the prefix of the name, similar to alkanes ("met-", "et-", "pro-", "but-", etc.), but always using the suffix "-eno ».

It must also be specified if the molecule is cis or trans, which are types of geometric isomers. This is added in the name, such as 3-ethyl-cis-2-heptane or 3-ethyl-trans-2-heptane.

Nomenclature of alkynes

To guess the names of chemical compounds that contain triple C≡C bonds, the name of the compound is determined by the number of C atoms in the longest chain.

Similarly to the case of alkenes, the names of alkynes indicate the position in which the carbon-carbon triple bond is found; for example, in the cases of HC≡C-CH ₂ -CH ₃, or "1-butyne", and H ₃ C-C≡C-CH ₃, or "2-butyne".

Properties

Unsaturated hydrocarbons comprise a huge number of different molecules, which is why they present a series of characteristics that define them, which are identified below:

Double and triple bonds

The double and triple bonds of alkenes and alkynes have special characteristics that differentiate them from single bonds: a single bond represents the weakest of the three, being formed by a sigma bond between two molecules.

The double bond is formed by one sigma bond and one pi, and the triple bond by one sigma bond and two pi. This makes the alkenes and alkynes stronger and requires more energy to break down when reactions occur.

Furthermore, the bond angles that are formed in a double bond are 120º, while those of the triple bond are 180º. This means that molecules with triple bonds have a linear angle between these two carbons.

Cis-trans isomerization

In alkenes and other compounds with double bonds, a geometric isomerization occurs, which differs on the side of the bonds in which the functional groups that are attached to the carbons involved in this double bond are found.

When the functional groups of an alkene are oriented in the same direction with respect to the double bond, this molecule is referred to as cis, but when the substituents are in different directions, it is called trans.

This isomerization is not a simple difference in location; Composites can vary greatly just because they are cis geometry or trans geometry.

Cis compounds usually involve dipole-dipole forces (which have a net value of zero in transs); Furthermore, they have higher polarity, boiling and melting points, and are of higher density than their trans counterparts. Additionally, the trans compounds are more stable and release less heat of combustion.

Acidity

Alkenes and alkynes have higher acidity compared to alkanes, due to the polarity of their double and triple bonds. They are less acidic than alcohols and carboxylic acids; and of the two, alkynes are more acidic than alkenes.

Polarity

The polarity of alkenes and alkynes is low, even more so in trans alkene compounds, which makes these compounds insoluble in water.

Still, unsaturated hydrocarbons readily dissolve in common organic solvents such as ethers, benzene, carbon tetrachloride, and other low or no polarity compounds.

Boiling and melting points

Due to their low polarity, the boiling and melting points of unsaturated hydrocarbons are low, almost equivalent to those of alkanes that have the same carbon structure.

Even so, alkenes have lower boiling and melting points than the corresponding alkanes, being able to decrease even more if they are cis isomeric, as previously mentioned.

In contrast, alkynes have higher boiling and melting points than the corresponding alkanes and alkenes, although the difference is only a few degrees.

Finally, cycloalkenes also have lower melting temperatures than the corresponding cycloalkanes, due to the stiffness of the double bond.

Examples

Ethylene (C

A powerful chemical compound due to its polymerization, oxidation and halogenation capacity, among other characteristics.

Ethyne (C

Also called acetylene, it is a flammable gas that is used as a useful source of lighting and heat.

Propylene (C

The second most widely used compound in the chemical industry worldwide, it is one of the products of petroleum thermolysis.

Cyclopentene (C

A compound of the cycloalkene type. This substance is used as a monomer for the synthesis of plastics.

Articles of interest

Saturated hydrocarbons or alkanes.

References

1. Chang, R. (2007). Chemistry, Ninth edition. Mexico: McGraw-Hill.
2. Wikipedia. (sf). Alkenes. Retrieved from en.wikipedia.org
3. Boudreaux, KA (nd). Unsaturated Hydrocarbons. Retrieved from angelo.edu
4. Tuckerman, ME (nd). Alkenes and Alkynes. Retrieved from nyu.edu
5. University, LT (sf). The Unsaturated Hydrocarbons: Alkenes and Alkynes. Retrieved from chem.latech.edu