Versatile Nature of Carbon: Saturated Hydrocarbons (Alkanes)

Introduction:

Carbon is one of the most important non-metallic elements. This is because carbon atoms make up the backbone of many important molecules in the human body, including proteins, DNA and RNA, sugars, and fats. The special significance of carbon lies in forming two types of compounds—organic and inorganic compounds. The entire field of organic chemistry is based on carbon and the bonds it forms. All compounds, such as proteins, carbohydrates, and fats, contain carbon, and all plant and animal cells consist of carbon compounds, classified as organic compounds. Studying the chemistry of carbon and its compounds is undoubtedly essential for understanding the elements available around us.

In the previous session, we studied the bonding in carbon as covalent bonding, which is responsible for carbon forming many compounds. In this session, let us learn some characteristic properties of carbon responsible for forming many such compounds and the organic compounds formed by carbon.

Explanation:

Factors Responsible for Making Many Compounds of Carbon:

The nature of the covalent bond allows carbon to form many compounds.

Two factors that are responsible for this are –

1. Catenation
2. Tetravalency

1. Catenation:

Carbon has the exclusive ability to form bonds with other carbon atoms, giving rise to large molecules. This property is known as catenation.

2. Tetravalency

Since a carbon atom has a valency of four, it can bond with four other carbon atoms or atoms of other mono-valent elements. Compounds of Carbon are formed with hydrogen, nitrogen, oxygen, chlorine, and several other elements giving rise to compounds with specific properties.

These properties depend on elements other than carbon present in the molecule.

The two characteristic features seen in carbon, tetravalency, and catenation, give many compounds. These compounds are known as organic compounds.

Hydrocarbons

The term hydrocarbon means the organic compounds that contain only carbon and hydrogens atoms. Hydrocarbons are of different types. Hydrocarbons are generally colorless, hydrophobic, and have only weak odors.

Based on carbon-carbon bonding, they can be classified into two main categories.

• Open chain hydrocarbons (Aliphatic compounds)
• Closed chain hydrocarbons (Cyclic compounds)

Open-chain Hydrocarbons (Aliphatic compounds)

The hydrocarbons that contain atoms connected to the open-chain are called open-chain compounds. Open-chain compounds are also called aliphatic compounds and contain straight or branched chain compounds.

for example, Ethane, Isobutane

So far, we know the structure of Methanei that we studied in the last session. Another compound formed by carbon and hydrogen is ethane with a molecular formula of C₂H₆ as shown above.

In order to come to the structure of simple carbon compounds, the first step is to link up the carbon atoms together with a single bond (Fig. a) and then use the hydrogen to satisfy the remaining valencies of carbon (Fig. b).

For example, the structure of ethane can be achieved in the following steps –

Step 1:             C—C (Figure(a): Carbon atoms linked together with a single bond.)

Three valences of each carbon atom remain unsatisfied, so each is bonded to three hydrogen atoms giving:

Step 2:

Saturated Hydrocarbon (Alkanes):

The term SATURATED means each carbon is bonded to four other atoms through single covalent bonds (– C – C –).

Hydrogen atoms generally occupy all available bonding positions after the carbons have bonded. Saturated hydrocarbons are also known as alkanes.

The simplest alkane is Methane, with the molecular formula CH₄. 

Alkanes follow the general formula C_nH_2n+2, where “n’ is a positive integer.

Alkanes consist of only single bonds. When n=1 (only one carbon atom), Methane is formed with the formula CH₄.

 When n=2, four covalent bonds are made for each carbon atom, and the compound with the molecular formula C₂H₆ is formed. Similarly, C₃H₈ is formed; all these (Methane, ethane, and propane, respectively) are alkanes.

If n=1, C₁H_2×1+2 CH₄ (Methane)

If n=2, C₂H_2×2+2  C₂H₆ (Ethane)

These alkanes are arranged straight because the carbon atoms are connected in one continuous chain with no branches. 

The first six alkanes’ formulas with an unbranched chain are tabulated below.

Chains, Branches, and Ring Structures

Carbon atoms form compounds with different types of arrangement, such as long-chain, branched-chain, and cyclic-chain structures.

For example:

Butane:

The molecular formula of a hydrocarbon having four carbon atoms = C₄H₁₀

Name of this compound: Butane

Let us take another look at Butane. If we make the carbon ‘skeleton’ with four carbon atoms, we see that two different possible ‘skeletons’ are –

Filling the remaining valencies with hydrogen atoms gives the following structure –

The name of this hydrocarbon is Butane or n-butane.

The name of this hydrocarbon is Isobutane. In this one, a carbon atom is arranged as a branched chain.

These two hydrocarbons, n-butane, and isobutene, have the same molecular formula but different structural formulas and are called structural isomers.

Structural Isomers: The compounds have molecular formulas, but different structural formulas called structural isomers.

Another example is pentane: C₅H₁₂

In pentane, carbon atoms can be arranged in three ways, first forming straight-chain, second forming one side chain and third forming two side chains.

Straight chain of pentane

A hydrocarbon also has five carbon atoms. However, one carbon atom is attached as a branched chain.

Thus, the name of this hydrocarbon is Iso–pentane or simply isopentane.

This hydrocarbon also has five carbon atoms, but two carbon atoms are attached as a branched chain. The name of this hydrocarbon is neo–pentane or simply neopentane

Here, n-pentane, isopentane, and neopentane are structural isomers having similar molecular formulas C₅H₁₂

Properties of Isomers

1. Since Isomers are different compounds, they have different properties.
2. Generally, branched-chain isomers have low boiling and melting points compared to straight-chain isomers.
3. For example: For Isobutane, the boiling and melting points are -12°C and -160°C, respectively, which is very less than compared with 0°C and -138°C for the n-butane.
4. The more branching present in the molecule, the lower the boiling and melting points for that molecule.

Physical properties of Alkanes

Nature: Alkanes are colourless and odourless.

They possess weak van der Waals forces.

Due to the weak forces, the first four members, C₁ to C_4, are gases, C₅ to C₁₇ are liquids, and those containing 18 carbon atoms, or more are solids at 298 K.

Shorter chain alkanes have low melting and boiling points; however, as the number of carbon atoms in the chain increases, the melting and boiling points also increase.

Boiling point and melting point:  it increase with the increasing molecular weight as the Van Der Waals force rises with the increasing molecular weight.

Alkanes are generally non-polar due to the covalent bonds between C-C and C-H. Moreover, there is a very small difference between the electronegativities of carbon and hydrogen.

Alkanes are insoluble in water but soluble in non-polar solvents.

Summary

• Carbon occurs in all living matter, substances derived from (food and fuels), in the earth’s crust and the atmosphere.
• The two characteristic features seen in carbon, tetravalency, and catenation, give many compounds.

• Hydrocarbons are organic compounds that contain only carbon and hydrogen atoms in their structure.
• Depending upon the types of carbon-carbon bonds present, hydrocarbon can be classified into two main categories.
• Open-chain hydrocarbons (Aliphatic compounds) and Closed chain hydrocarbons (Cyclic compounds)
• The hydrocarbons, which contain atoms linked to the open chain, are called open-chain compounds.
• Open-chain compounds are further divided into saturated and unsaturated hydrocarbons
• Each carbon is bonded to four other atoms in saturated hydrocarbon through single covalent bonds (– C – C –). They are also known as alkanes.