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# Ethane

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 Sub Topics All alkanes are members of a homologous series whose structural formulas can be pictured as having been "constructed" by inserting a methylene group into the formula of the preceding member. Ethane is gaseous at normal atmospheric temperature and pressure and in some respects is not a "natural gas liquid" in its full meaning, since ethane is seldom liquefied in isolation. In the past ethane usually been left in the natural gas stream to enhance or maintain the calorific value of marketed natural gas.

## Definition

The second member of the paraffin or the alkane series is ethane. It is made up of two methyl radicals (CH3-CH3) and for this reason is also called dimethyl. Ethane occurs in natural gas and the gases from oil wells.

## Ethane Formula and Ethane Molecule

Formula just shows the simplest ratio of the atoms present. The structural formula is the shorthand form of displaying formula. The formula is written without showing the bonds. Each carbon is written separately, with the atoms or groups that are attached to it.

The empirical formula of ethane as calculated from its percentage composition is CH3. Its empirical formula weight, therefore (12 + 3 $\times$ 1) = 15. Its molecuklar weight as calculated from its vapor density is 30. It is twice its empirical formula (CH3) weight. Hence the molecular formula of ethane is C2H6. The ball and stick model of ethane is shown below.

## preparation

• Decarboxylation of sodium salt of carboxylic acids
Ethane can be prepared in the similar fashion by taking sodium propanoate instead of sodium acetate.

C2H5COONa + NaOH $\xrightarrow[CaO]{\Delta}$ C2H6 + Na2CO

• Preparation by reduction of alkyl halides
Ethyl iodide is reduced to methane by nacent hydrogen produced by the action of Zn/Cu couple on ethyl alcohol.

C2H5I + 2[H] $\to$ C2H6 + HI

## Properties

Physical properties

These are colorless, odourless, tasteless gases which are insoluble in water, but soluble in non-polar solvents like alcohol, carbon tetrachloride etc.

Chemical properties
• Stability
Ethane when heated to 500oC in the absence of air, ethane decomposes mainly into ethene and hydrogen (pyrolysis).

C2H6 $\overset{500^{o}C}{\rightarrow}$ C2H4 + H2
• Reaction with chlorine and bromine

Like methane ethane also forms substitution products with chlorine such as C2H5Cl, C2H4Cl2 etc. On prolonged treatment the ultimate substitution product viz, hexachloroethane, C2Cl6 is formed.
• Nitration
Ethane reacts with nitric acid vaporat 400oC to yield nitroethane.

C2H6 + HNO3 $\overset{400^{o}C}{\rightarrow}$ C2H5NO2 +H2O

## Combustion

• Combustion
Ethane burns in air or oxygen with a feebly luminous flame producing carbon dioxide and water.

2C2H6(g) + 7O2 $\rightarrow$ 4CO2(g) + 6H2O(g)
• Slow combustion
Slow combustion of ethane is similar to that of methane. When treated with oxygen under high pressure and comparatively low temperature, it successively changes into ethanol, ethanal, ethanoic acid, and finally carbon dioxide and water.

CH3-CH3 $\overset{O}{\rightarrow}$ CH3CH2OH $\overset{O}{\rightarrow}$ CH3CHO $\overset{O}{\rightarrow}$ CH3COOH $\overset{O}{\rightarrow}$ CO2 + H2O

## Structure

Single bonds allow facile rotation around the bond, leading to an abundance of conformation for molecules with many such bonds. Ethane is the simplest molecule containing a carbon-carbon bond.

The ethane molecule is pictured by imagining the two carbon atoms bond to each other by $\sigma$ overlap of an sp3 hybrid orbital from each. The remaining three sp3 hybrid orbitals of each carbon overlap with the 1s orbitals of three hydrogens to form the six C-H bonds.

## Conformation of Ethane

Conformers constitute arrangements of atoms that are readily converted to other spatial orientations by rotation about $\sigma$ bonds. the specific orientation of a given conformer is termed its conformation. The simplest molecule that has discernable conformation is ethane, which has only one carbon-carbon bond about which rotation occurs.

The rotation of ethane about the $\sigma$ bond between two atoms occurs readily because overlap is not lost in any orientation.

Different conformations of the same molecule that are interconvertible by rotation around a single bond are called conformers or rotamers. Conformers are stereoisomers with the same atom connectivity but different spatial arrangements of atoms.

## Staggered Conformation of Ethane

In the staggered conformation of ethane, each C-H bond on one carbon bisects an H-C-H angle on the other carbon. The staggered conformation is most stable of all ethane conformations. At room temperature the staggered conformation is practically the only conformation present. In the staggered conformation, the three hydrogen atoms, on each of the carbon atoms are as far as apart as possible from one another.