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 skeleto

n 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 C_nH_2n+2) wherein the carbon atoms are joined in a snake-like structure, branched (general formulaC_nH_2n+2, n>3) wherein the carbon backbone splits off in one or more directions, or cyclic (general formula C_nH_2n, 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:

• General Methods of Preparation of Alkanes

• Properties of Alkane

• Cycloalkanes

• Petroleum

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 C_nH_2n+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 109^o 28' or 109.5^o.

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

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

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, C_nH_2n+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, 109.5^o; 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. 

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