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What are unsaturated hydrocarbons?

The unsaturated hydrocarbons are organic compounds which are formed only by carbon and hydrogen, having one or more multiple bonds between carbon atoms. This means that any compound of formula C x H and that presents a double or a triple bond is an unsaturated hydrocarbon.

Unsaturated hydrocarbons are made up of alkenes and alkynes, as well as aromatic compounds. The reason they are called unsaturated is because they have fewer hydrogens than the maximum they could have, given the number of carbon atoms in the structure.

Unsaturated hydrocarbons are recognized as having less than 2n + 2 hydrogens (which is the maximum number of hydrogens you can have per carbon), where n is the number of carbon atoms in the molecular formula.

An example of an unsaturated hydrocarbon is butene, whose molecular formula is C 4 H 8 . The maximum number of hydrogens for 4 carbons would be 2 (4) + 2 = 10 hydrogens. This corresponds to the number of hydrogens in a saturated hydrocarbon. Since butene has only 8 hydrogens, which is less than 10, then it is an unsaturated hydrocarbon.

Unsaturated hydrocarbons are characterized by the number of unsaturations. This indicates the number of fewer hydrogen pairs that an unsaturated hydrocarbon has compared to a saturated one that has the same number of carbons.

The number of unsaturations can be calculated for any unsaturated compound by means of the following formula:

Where C, N, H, and X represent the number of carbons, nitrogens, hydrogens, and halogens in the molecular formula, respectively. This number gives us an idea of ​​how many double or triple bonds a compound has, given its molecular formula, although care must be taken because cycles (which are not unsaturations) also contribute to the number of unsaturations:

Example

Benzene has the formula C 6 H 6 , so the number of unsaturations is:

In this case, the 4 unsaturations correspond to three double bonds and one ring.

Classification of unsaturated hydrocarbons

According to the type of hydrocarbon

  • Alkenes: are those that have at least one double bond in their structure.
  • Alkynes: are those that have at least one triple bond in their structure.
  • Aromatics: they are cyclic polyunsaturated compounds with special properties that make them particularly stable.

According to the carbon chain

  • Linear: unsaturated compounds in which all the carbon atoms are linked one after the other in a single chain.
  • Branched: are those that have a main chain and also several side chains that form branches.
  • Cyclic: are those in which there is a closed cycle of carbon atoms in which there is at least one double or triple bond (cycloalkenes, cycloalkines and aromatics).

According to the number of unsaturations

  • Monounsaturated: when they have only one multiple bond, that is, either a single double bond or a single triple bond.
  • Polyunsaturated: when the structure has more than one multiple bond, as in the case of alcadienes, alcadiinos, alqueninos, etc.

Properties of unsaturated hydrocarbons

The properties of unsaturated hydrocarbons strongly depend on the particular type of hydrocarbon in question:

Melting and boiling points

In general, unsaturated hydrocarbons have low melting and boiling points. However, compared to alkanes, alkenes with the same carbon number and similar main chain tend to have lower boiling points , while alkynes tend to have higher boiling points.

The same is true for melting points. In the following table it can be seen that both the melting point and the boiling point of 1-pentene are lower than those of pentane and these are lower than those of 1-pentine.

Physical state

Due to their low melting and boiling points, low molecular weight unsaturated hydrocarbons tend to be gaseous at room temperature. Many are also liquid, and those with the highest molecular weight are solid. In many cases, the solids are amorphous rather than crystalline solids.

All hydrocarbons, including unsaturated ones, are apolar compounds, because the electronegativity of carbon and hydrogen are very similar. The only exception is in the case of terminal alkynes, which are weakly polar due to the fact that sp hybridized carbon is slightly more electronegative than sp 2 and sp 3 carbons .

Solubility

Being apolar, unsaturated hydrocarbons are insoluble in water and other polar solvents, but they are highly soluble in nonpolar solvents such as carbon tetrachloride, cyclohexane, and others. In fact, many of these compounds are themselves apolar solvents.

They are combustible

A characteristic of all hydrocarbons, including unsaturated ones, is that they burn in the presence of oxygen to become carbon dioxide and water, releasing energy in the form of light and heat.

They can polymerize

Through different chemical reactions, alkenes and alkynes can join with each other to form polymers as in the case of polyethylene and polypropylene, to name just a few. So unsaturated hydrocarbons are the starting material for many common plastics.

Chemical reactivity

The reactivity strongly depends on the type of functional group that a molecule possesses. In the case of alkenes and alkynes, these are usually quite reactive and can undergo hydrogenation, hydration, hydrohalogenation and other types of addition reactions.

On the other hand, aromatic compounds are usually quite inert because they are very stable. The characteristic reactions of these compounds are the aromatic electrophilic substitution reactions.

Nomenclature of unsaturated hydrocarbons

Nomenclature of alkenes

Linear alkenes

The name of linear alkenes is constructed by placing a prefix indicating the number of carbon atoms in the chain, followed by the carbon locator where the double bond begins, separated with hyphens from the rest of the name and ending with the suffix _eno:

Eno-locator-prefix

  • Example:

The but-1-ene is the name of the linear butene with a double bond at the first carbon while the but-2-ene having the double bond on the second carbon.

Branched alkenes

In the case of branched alkenes, the longest backbone containing both carbons of the double bond is selected first and numbered with priority to the double bond. The name is constructed by first placing the branches in alphabetical order preceded by their locators followed by the name of the main chain as indicated above.

In 3-methylbut-1-ene, the numbering is done from left to right to give priority to the double bond, instead of giving priority to the branching (the methyl group).

Polyenes

In the case of polyunsaturated alkenes or polyenes, which contain more than one double bond, the main chain that has the highest number of double bonds is selected as the main chain, regardless of the fact that there are other longer chains.

In the name of the main chain, a locator is added for each double bond and the prefix di, tri, tetra, etc. is added. before the ending -eno, to indicate the number of double bonds it has.

Note in this case that although there is a longer chain of 5 carbons, the one with 4 is selected, since it contains the two double bonds.

Nomenclature of alkynes

Alkynes are named very similarly to alkenes, with the only difference being that the name ends in -ino instead of -eno. In the cases in which there are more than one triple bond, the locators of all of them and the numerical prefix that indicates how many there are are placed.

Nomenclature of aromatic compounds

Aromatic hydrocarbons are named as derivatives of benzene and both the numbering and the way of naming them depends on how many substituents the ring has.

Aromatic rings with only one substituent

In this case it is not necessary to number the carbons of the ring, since it is understood that the carbon on which the branch is located is carbon 1. The name consists of the name of the branch followed by the word benzene.

Aromatic rings with two substituents

In this case, 3 numberings or substitution patterns can be given which are (1,2), (1,3) and (1,4). As it is indifferent to indicate which substituent is on carbon 1 and which is on the other, the prefixes ortho- (o-), meta- (m-) and para- (p-), respectively, are used to refer to them.

Aromatic rings with more than 2 substituents

In this case, the numbering that gives the smallest combination of locators is selected. If two numberings are equal, the substituents are given priority in alphabetical order.

Examples of unsaturated hydrocarbons

Below are additional examples of unsaturated hydrocarbons to those already mentioned.

Acetylene (C 2 H 2 )

Acetylene is the common name for ethyne, the simplest alkyne there is. This compound has 2 unsaturations due to its triple bond and is used, among other things, in oxyfuel and steel welding equipment.

Cyclohexene (C 6 H 10 )

Cyclic 6-carbon alkene that has two unsaturations: one due to the cycle and the other due to the double bond.

Styrene (C 8 H 8 )

This compound is commonly used in the preparation of polymers. It is an aromatic alkene with 5 unsaturations.

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