What are aliphatic hydrocarbons?

• 26 October 2023
• 94 Views

Aliphatic hydrocarbons:

In chemistry, compounds whose structure consists of only two elements, carbon and hydrogen, are called hydrocarbons. Hydrocarbons are generally divided into two main categories, aliphatic and aromatic. These compounds can have a cyclic or chain structure. It should be noted that the structural form of most hydrocarbons depends on the structure of their constituent atomic bonds.

In other words, hydrocarbons can be divided into two groups: saturated and unsaturated. In fact, alkanes are considered saturated hydrocarbons, while alkene, alkyne and aromatic hydrocarbons are classified as unsaturated hydrocarbons.

Classification of aliphatic hydrocarbons:

Aliphatic compounds are divided into three categories: alkanes, alkenes and alkynes, and in this section we will examine each of them separately.

Alkanes:

Alkanes are hydrocarbons in which each carbon atom uses all four valencies to form bonds. In fact, there is no double or triple bond in these compounds. Hence, alkanes are called saturated hydrocarbons. This group of aliphatic hydrocarbons is also known as paraffin. Note that in Latin paraffin means low affinity. The general formula of alkanes is CnH2n+2. In these compounds, the carbon atom has sp3 hybrid, so the bond angle is about 28.109 degrees.

These types of hydrocarbons make up the most components of crude oil. Methane (the simplest alkane), ethane and propane are examples of them. Methane makes up the majority of natural gas. Other gaseous alkanes include ethane, propane and butane. The most important liquid alkanes are: pentane, hexane, heptane and octane.

Alkanes can generally be straight or branched. Straight chain alkane is an alkane whose structure has all carbon atoms connected to one or at most two other carbon atoms. Often the word normal is written at the beginning or end of the right chain alkane name. On the other hand, branched alkane is an alkane in which at least one carbon atom is connected to three or four other carbon atoms in its structure.

It should be noted that alkanes do not have functional groups, so their chemical properties are different from other organic compounds. In fact, these compounds are considered as parent compounds in organic chemistry. Therefore, they will be important in terms of synthesis and naming of other organic compounds.

Properties and characteristics of alkanes:

1. 1.Alkanes are saturated compounds, so they do not have much tendency to carry out chemical reactions. This class of hydrocarbons only participates in burning reaction, cracking reaction and hydrocracking as well as reaction with halogens.
2. 2.Alkanes are colorless gas, liquid or solid.
   Alkane molecules are connected only by non-polar covalent bonds, so they are considered non-polar molecules.
3. 3.In terms of solubility and according to the rule "like dissolves like", since alkanes are non-polar molecules, they dissolve well in non-polar solvents such as benzene, ether or chloroform. While they are insoluble in water or other polar solvents.
4. 4.If alkanes burn, a blue and yellow flame is produced without soot. As a result of burning water, carbon dioxide and energy are released.
5. 5.An increase in the number of carbon atoms in alkanes increases their melting point, boiling point and viscosity. It should be noted that branched alkanes have a lower boiling point than unbranched alkanes.

Alkene:

Alkenes are hydrocarbons in which there is at least one double bond between carbon atoms. Hence, alkanes are called unsaturated or unsaturated hydrocarbons. In fact, carbons still have empty capacity, so they can take hydrogen and become saturated. This group of aliphatic hydrocarbons are also known as olefins. The general formula of alkenes is CnH2n. In these compounds, the carbon atom has an sp2 hybrid, so the bond angle is about 120 degrees.

Classification of alkenes:

Alkenes can be classified based on the number of alkyl groups attached to the C=C unit. This characteristic is called the degree of substitution or the degree of substitution. These degrees are:

• Monosubstituted: In this case, there is only one alkyl group attached to the carbon atom with a C=C bond.
•  
• Disubstituted: In this case, there are two alkyl groups attached to the C=C bond.
•  
• Three substitutions: In this case, there are three alkyl groups attached to the C=C bond.
•  
• Four substitutions: In this case, there are four alkyl groups attached to the C=C bond.

Properties and characteristics of alkenes:

A large number of physical properties of alkanes are similar to the physical properties of alkanes. For example, the density of alkanes is about 0.6 to 0.8 grams per cubic centimeter, so they are lighter than water. These compounds are soluble in non-polar solvents such as ether, chloroform and dichloromethane or in organic solvents such as hexane, but are insoluble in water. It can be said that alkenes are slightly more polar than alkanes.

This class of hydrocarbons, up to the number of five carbon atoms, are in the form of gas at ambient temperature, but with the increase in molecular weight, they become liquid. In addition, the boiling point of alkanes is very close to alkanes, and like alkanes, the boiling point also increases with the increase of carbon number. Also, the dipole moment of alkenes is more than the dipole moment of alkanes.

Alkynes:

Alkynes are hydrocarbons in which there is at least one triple bond between carbon atoms. Therefore, alkynes are also called unsaturated or unsaturated hydrocarbons. This group of aliphatic hydrocarbons is also known as acetylene or acetylene group. The general formula of alkynes is CnH2n-2. In these compounds, the carbon atom involved in the triple bond has sp hybrid, so the bond angle is about 180 degrees. Alkynes also exist in nature like alkenes.

Classification of alkynes:

If only one alkyl group is attached to one of the carbon atoms involved in the C≡C bond, it is called monosubstituted alkyne or terminal alkyne (because the C≡C bond is at the end of the carbon chain). It will be shown as R–C≡C–H. Whereas, if alkyl groups are attached to both carbon atoms involved in the C≡C bond, it is called disubstituted alkyne or internal alkyne. It will be represented as R–C≡C–R

Properties and characteristics of alkynes:

The physical properties of alkynes are similar to carbon alkenes. These compounds have less polarity than alkanes and alkynes. So they dissolve well in solvents with lower polarity such as carbon tetrachloride, benzene and ether, but they are insoluble in water. In addition, like other aliphatic hydrocarbons, they are lighter than water. Examination of the melting and boiling points has shown that, like alkenes, the boiling point increases with the increase in the number of carbons and also decreases with branching. It should be noted that the hydrogen attached to the triple bond is acidic. Therefore, the acid strength of alkynes is higher than alkenes and alkanes.