Saturated Hydrocarbons

  • What are saturated Hydrocarbons?

Alkanes, also known as paraffins, are saturated hydrocarbons. These are chemical compounds that consist only of the elements carbon (C) and hydrogen (H) (i.e.,hydrocarbons), wherein these atoms are linked together exclusively by single bonds (i.e., they are saturated compounds) without any cyclic structure (i.e. loops). Alkanes belong to a homologous series of organic compounds in which the members differ by a constant relative atomic mass of 14.

Each carbon atom must have 4 bonds (either C-H or C-C bonds), and each hydrogen atom must be joined to a carbon atom (H-C bonds). A series of linked carbon atoms is known as the carbon skeleton or carbon backbone. In general, the number of carbon atoms is often used to define the size of the alkane (e.g., C2-alkane).

An alkyl group is a functional group or side-chain that, like an alkane, consists solely of singly-bonded carbon and hydrogen atoms, for example a methyl or ethyl groups.

Saturated hydrocarbons can be linear (general formula CnH2n+2) wherein the carbon atoms are joined in a snake-like structure, branched (general formulaCnH2n+2, n>3) wherein the carbon backbone splits off in one or more directions, or cyclic (general formula CnH2n, n>2) wherein the carbon backbone is linked so as to form a loop. According to the definition by IUPAC, the former two are alkanes, whereas the third group is called cycloalkanes. Saturated hydrocarbons can also combine any of the linear, cyclic (e.g., polycyclic) and branching structures, and they are still alkanes (no general formula) as long as they are acyclic (i.e., having no loops)

 We would study about saturated hydrocarbons in more detail under following topics:

The simplest possible alkane (the parent molecule) is methane, CH4. There is no limit to the number of carbon atoms that can be linked together, the only limitation being that the molecule is acyclic, is saturated, and is a hydrocarbon. Saturated oils and waxes are examples of larger alkanes where the number of carbons in the carbon backbone tends to be greater than 10.

Alkanes are not very reactive and have little biological activity. Alkanes can be viewed as a molecular scaffold upon which can be hung the interesting biologically-active/reactive portions (functional groups) of the molecule.

Alkanes have general formula CnH2n+2 where n ranges from 1 to n,

              e.g.,         CH4,              C2H6,            C3H8 ...........

                          methane        ethane           propane

Alkanes are also known as paraffins due to their less reactive nature. (latin : Parum = little; affins = affinity)

Each carbon atom in alkane is sp3 hybridized and its four bonding orbitals are directed towards the four corners of regular tetrahedron at an angle of 109o 28' or 109.5o.

The tetrahedral of alkane is described below, e.g., C2H6

                                          tetrahedral-of-alkane

All the C-C & C-H bonds are strong sigma bonds.

                                      sigma-bonds

      Bond                     C-C (sp3-sp3)                                    C-H (sp3-s)

     Bond length              1.54 A                                               1.112 A

     Bond energy             82.76 k cal mol-1                          98.67 k cal mol-1

Carbon-carbon chain in alkanes consisting of more than two carbon atoms is not linear but zig-zag as usually written for the sake of convenience of representation.

The simplest alkane CH4

is also known as marsh gas or fire damp.

 constitutes 40% of coal gas.

is mean source for the preparation of synthesis gas.

 synthesis-gas

The main source of alkanes is petroleum and natural gas.

Natural gas contains about 80% methane, 10% ethane and 10% rest mixture of higher members. It also contains small amounts of H2, N2 and CO2 etc.

Rafer to the follwoing video for naming of alkanes:  

                                       

  • Isomerism in alkanes :

Alkanes are open-chain (acyclic) hydrocarbons comprising the homologous series with the general formula, CnH2n+2, where n is an integer. They have only single bonds and therefore are said to be saturated.

1. Structural Isomerism in Alkanes:

The number of isomers of alkanes increases with increase in carbon atoms.

Alkanes                   C4H10    C5H12      C6H14       C7H16     C8H18      C10H22

Possible Isomers          2          3            5              9            18           75

Monosubstituted alkanes with at least 3 carbon atoms show isomerism

CH3CH2CH2X and CH3CHXCH3 : Position isomerism

Disubstituted alkanes with at least two carbon atoms show isomerism e.g.

CH3CHX2 and CH2XCH2X : Position isomerism

2. Sterioisomerism In Alkanes:  

Free Rotation about the Carbon-Carbon Single Bond: Conformations

           

Electron diffraction and spectroscopic studies have verified this structure in all respects, giving the following measurements for the molecule : bond angles, 109.5o; C-H length, 1.10Å;  C-C length, 1.53 Å. Similar studies have shown that with only slight variations, these values are quite characteristic of C-H and C-C bonds and of carbon bond angles in alkanes.

This set of bond angles and bond lengths still does not limit the molecule of ethane to a single arrangement of atoms, since the relationship between the hydrogens of one carbon and the hydrogens of the other carbon is not specified. Different arrangement of atoms that can be converted into  one another by rotation about single bonds are called Conformations. Arrangement I is called the eclipsed conformation and arrangement II is called its staggered conformation.

You might be interested in referring IIT JEE Organic Chemistry Syllabus, Books and Revision Notes on Organic Chemistry

To know more about the study material of engineering and medical exams, please fill up the form given below:

Name
Email Id
Mobile

Exam
Target Year

Related Resources
Cycloalkanes

Cycloalkanes This class of hydrocarbons are...

Properties of Alkane

Properties of Alkanes Physical Properties of...

General Methods of Preparation of Alkanes

General Methods of Preparation of Alkanes...

Petroleum

Petroleum Petroleum is an essential source of...