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Aliphatic compounds are a family of organic compounds formed by hydrocarbons, both cyclic and open chain, that do not have aromatic rings in their structure. They therefore represent one of two classes of hydrocarbons, the other being precisely the family of aromatic hydrocarbons.
Aliphatic compounds are made up of many of the compounds that are present in crude oil or petroleum. This includes alkanes, cycloalkanes, alkenes, dienes, polyenes, alkynes, diynes, and more. They also include some of the most important plastic polymers, such as polyethylene, polypropylene, and others.
The word aliphatic comes from the Greek word aleiphar , which means fat, alluding to the fact that many of the aliphatic compounds are liquid mineral oils or form fatty solids with different uses.
General characteristics of aliphatic compounds
In appearance, all aliphatic compounds are either colorless gases or liquids, or they form opaque white solids. In the latter case, some solids such as high molecular weight paraffins usually acquire a yellowish coloration over time. However, this color is not due to the aliphatic compound itself, but to the product of oxidation reactions with oxygen in the air or decomposition by UV rays from sunlight.
In addition to these characteristics, aliphatic compounds have the following properties:
physical properties
They have a low melting and boiling point
The intermolecular forces or cohesive forces between the molecules of aliphatic compounds are very weak, so not much energy is required to separate their molecules from each other. In view of this, these compounds generally have considerably low melting and boiling points. The only exception is high molecular weight aliphatic hydrocarbons, but even in these cases the melting points are usually not very high.
They can exist in solid, liquid and gaseous states.
By the very fact of having low melting and boiling points, many of these compounds are gases at normal atmospheric temperature and pressure. Compounds like methane, ethane, ethylene, and acetylene are just a few examples of gaseous aliphatic hydrocarbons, and there are many more.
On the other hand, compounds such as hexane, heptane, and cyclohexane are all liquid at room temperature, while alkanes from octadecane have melting points of more than 25 °C, which makes them solid at room temperature.
They usually form amorphous solids.
Long-chain aliphatic hydrocarbons have a tendency to solidify in the form of disordered structures without the periodic regularity of crystalline structures. For this reason, instead of forming faceted crystals with well-defined shapes, they tend to form amorphous solids opaque to visible light.
They are insoluble in water, but soluble in most organic solvents.
Aliphatic compounds are nonpolar, so they are not soluble in any polar solvent such as water or alcohols. Instead, they are very soluble in organic solvents such as cyclohexane, benzene, petroleum ether, etc.
Chemical properties
They are nonpolar compounds
As mentioned above, aliphatic compounds are nonpolar compounds. This is because the elements that make them up, carbon and hydrogen, have similar electronegativities (2.55 and 2.2 respectively). This makes the CH bonds nonpolar covalent bonds. In addition, the other bonds that are formed between carbon atoms are completely nonpolar (pure covalent bonds) because both atoms are the same.
Finally, any small dipole moments that are generated in one part of the molecule as a consequence of the small difference between the electronegativities of carbon and hydrogen are generally offset or canceled out by equivalent dipole moments that are formed in another part of the molecule. molecule and pointing in the opposite direction. In this way, the very structure and molecular geometry of aliphatic hydrocarbons contributes to their being nonpolar molecules.
are combustible
This means that they burn and burn when reacting with molecular oxygen. This combustion reaction releases enough energy in the form of heat to keep going until the aliphatic compound or oxygen is completely consumed.
Being combustible is actually a characteristic common to most organic compounds, but it is especially relevant to aliphatic compounds. In fact, with the exception of a few other families of organic compounds, most of the chemicals we burn as fuels are aliphatic compounds. For example, propane and butane are the most common fuels in gas stoves, while acetylene (ethyne) is used in flame cutting and metal welding equipment due to the large amount of heat released by its combustion.
are molecular compounds
All chemical bonds that form between the atoms of an aliphatic hydrocarbon are covalent bonds. For this reason, these compounds form discrete units that we call molecules, making them molecular compounds.
Some are very inert chemically
Of the different types of aliphatic compounds, alkanes or paraffins are very stable substances with very little reactivity. Apart from the combustion reaction, there are very few reactions that take part without the help of high temperatures, pressure, or the presence of ultraviolet radiation.
Some suffer addition reactions
In the case of alkenes or olefins and alkynes, which are provided with carbon-carbon multiple bonds, these substances can undergo addition reactions such as hydration to give alcohols and enols, hydrohalogenation to give alkyl halides, and hydrogenation, among others.
Classification of Aliphatic Compounds
Aliphatic compounds are classified into two large groups that are saturated and unsaturated hydrocarbons. Each of these is further divided into different types of chemical compounds. A brief description of the different types of aliphatic compounds is given below.
Saturated aliphatic compounds
They are hydrocarbons in which there are only single covalent bonds and all carbon atoms have sp 3 hybridization with four atoms directly bonded to it in a tetrahedral arrangement. These are divided into two subgroups, the open-chain alkanes (or just alkanes) and the cycloalkanes or cyclic alkanes.
alkanes
Alkanes are the simplest of all organic compounds. They have the formula generate C n H 2n+2 and can be linear or branched chain compounds. Linear alkanes represent the structural basis of all organic compounds, as well as the basis of the entire system of systematic organic nomenclature.
Cycloalkanes
They can be visualized as linear alkanes in which the terminal carbons lose one hydrogen each and bond together. They have the molecular formula C n H 2n (without the +2 of alkanes due to the loss of two hydrogens) and the simplest of all is cyclopropane, which forms a three-membered cycle, that is, a triangle.
unsaturated aliphatic compounds
They are aliphatic hydrocarbons that have one or more multiple covalent bonds. They can be visualized as alkanes that have lost one or two pairs of neighboring carbon hydrogen atoms to form a double or triple bond, respectively.
They are called unsaturated because they have fewer hydrogen atoms than the maximum possible amount, which corresponds to open-chain alkanes.
Unsaturated aliphatic compounds can be alkenes or alkynes.
Alkenes
They are unsaturated aliphatic hydrocarbons in which two carbon atoms are linked by a double bond. These two carbon atoms are sp 2 hybridized and are bonded to a total of three atoms each (including the other carbon) distributed around the central carbon in a trigonal planar fashion. The part of the molecule that has a double bond including the two carbons and the other 4 groups attached to them all lie in the same plane.
Some unsaturated aliphatic hydrocarbons of this type have more than one double bond, forming families of polyenes. Those with 2 double bonds are called dienes, those with three are called trienes, etc.
alkynes
Alkynes are unsaturated hydrocarbons that have a carbon-carbon triple bond, in which both carbons are sp hybridized. The general formula of these compounds is C n H 2n-2 and those that have the triple bond in the terminal position (at the end or beginning of the chain) have a slightly acidic character (that is, they behave like weak acids that can lose the last hydrogen).
Sources of aliphatic compounds
- The vast majority of aliphatic compounds come from oil and natural gas. In fact, a significant fraction of natural gas is a mixture of low molecular weight alkanes, alkenes, and alkynes. During refining, most of the light liquid fractions also contain high proportions of various aliphatic compounds, while the heavier ones usually contain, in addition to these, appreciable amounts of aromatic hydrocarbons as well as other classes of organic compounds.
- On the other hand, some aliphatic compounds, particularly methane, are produced by the action of bacterial degradation of other more complex organic substances.
- In the chemical industry, some important alkenes and alkynes are synthesized from alcohols and alkyl halides by dehydration and dehydrohalogenation reactions, respectively.
Uses and applications of aliphatic compounds
Some of the most common applications of aliphatic hydrocarbons are:
As fuels both in gaseous, liquid and solid state.
We already mentioned cooking gas and acetylene before, but there are also octane isomers and other liquids that are part of gasoline and other fuels for internal combustion engines. We also find solid paraffins that have been used for hundreds and hundreds of years to make candles.
They serve as nonpolar organic solvents.
Most liquid hydrocarbons are frequently used as non-polar organic solvents in organic synthesis or in the cleaning industry to remove mineral oils and greases. Some of these solvents are pure substances, such as cyclohexane, which is a very common solvent in the organic chemistry laboratory, while other solvents are mixtures of various liquid hydrocarbons.
They are used as lubricants.
Either as solid fats or as liquid oils, the heavier fractions from petroleum refining are used as lubricants for different types of mechanical parts, including combustion engines and other types.
They are the basis for the production of synthetic paints and related products.
Its ability to act as an apolar solvent means that these compounds are used in the manufacture of oil-based paints, inks, glues and even in the manufacture of adhesives.
Starting reagents in organic synthesis
Alkanes can be converted into other types of more reactive compounds, which makes them useful as raw materials for some organic synthesis processes. However, alkenes and alkynes are much more useful in this regard. Alkenes are frequently used as starting material for the industrial synthesis of some very important alcohols that later form the basis of many complex synthetic routes.
On the other hand, alkenes can be easily polymerized, which is why they are used in large quantities as starting material for the production of plastics. In fact, polyethylene, which is by far the most produced and consumed plastic in the world, is synthesized by polymerizing ethylene, which is the simplest alkene in the family.
Examples of Aliphatic Compounds
Below are some examples of the four main types of aliphatic hydrocarbons along with their molecular structure and molecular formula.
Examples of Alkanes and Cycloalkanes
Examples of alkenes
Examples of alkynes
References
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