Alkanes: chemistry
Occurrence, Preparation and Properties
Occurrence
Alkanes has very good percentage in nature. On earth natural gas contains about 80% of methane and 10% of ethane and rest of 10% is mixture of higher alkanes. Higher alkanes are solid in nature and are obtained from the oil distillation as residue. The Pitch Lake which is situated in Trinidad and Tobago is the largest natural deposits of solid alkanes i.e. asphalt. The asphalt is popularly known as bitumen.
The atmosphere of Jupiter contains about 0.1% of methane and 2 ppm of ethane. The atmosphere of Saturn contains about 0.2% of methane and 5 ppm of ethane. The atmosphere of Uranus contains about 1.99% of methane. The Neptune contains about 1.5% of methane, Titan about 1.6% of methane. It was examined by Huygens probe that the atmosphere of Titan periodically rains liquid methane on to the surface of moon.
In the tail of comet Hyakutake the methane and ethane has been detected.
Natural oil and gases are the main commercial sources of alkanes.
General Methods of Preparation of Alkanes
The general method of Preparation of alkanes can be divided into three groups
A. Preparation of Alkanes From the Compounds Containing Same Number of Carbon Atoms.
(1) Preparation of Alkanes by the Hydrogenation of Unsaturated Hydrocarbons i.e. Alkenes and Alkynes
Alkanes are prepared by the catalytic reduction of unsaturated hydrocarbons.
When mixture of unsaturated hydrocarbons and hydrogen is passed over finely divided nickel at 200oC –300oC, alkane is obtained. The obtained alkanes have same number of carbon atom present in the reactant.
Examples:
(a) When the mixture of ethylene (Ethene) and hydrogen is passed over finely divided nickel at 200oC –300oC, the Ethane is prepared.
(b) When the mixture of Propene (Propylene) and hydrogen is passed over finely divided nickel at 200oC –300oC, the Prpane is prepared.
In these reactions the number of carbon atoms is same as in the reactant.
This reaction is known as Sabatier and Senderens Reaction.
The same result is obtained by using of palladium and platinum as catalyst, but since palladium and platinum are expensive, thus nickel is used as catalyst.
(2) Preparation of Alkanes by the reduction of Alcohol
(i) When alcohol is reacted with phosphorous triodide, it is converted into respective alkyl iodide.
The alkyl iodide obtained in this reaction can be converted into respective alkanes by different methods.
(a) Alkyl halide is reduced to respective alkane when reacted with nascent hydrogen.
(b) The alkyl halide obtained in the reaction is converted to respective alkane by the method of catalytic reduction using palladium as catalyst.
(c) The alkyl halide is reduced to respective alkane when reacted with concentrated hydriodic acid at 1500C. In this reaction the high temperature is necessary as maximum boiling point of hydriodic acid (hydrogen iodide) is 1260C.
(ii) The reduction can be performed directly also, when the phosphorus and excess of hot concentrated hydriodic acid (hydrogen iodide) are reacted with alcohol at 1500C, alcohol is reduced to respective alkane.
Example
(a) When methyl alcohol is reacted with phosphorous and excess of hydriodic acid (hydrogen iodide) at 1500C, the methane is formed.
(b) Similarly, when ethyl alcohol is reacted with phosphorous and excess of hyriodic acid (hydrogen iodide) at 1500C, the ethane is formed.
(3) Preparation of Alkanes by the reduction of Ketones
When ketone is reacted with phosphorus and hot concentrated hydriodic acid (hydrogen iodide) at 1500C, ketone is reduced to respective alkane.
Example
(a) When acetone is reacted with phosphorus and hot concentrated hydriodic acid (hydrogen iodide) at 1500C, acetone is reduced to respective propane.
(a) When ethyl methyl ketone is reacted with phosphorus and hot concentrated hydriodic acid (hydrogen iodide) at 1500C, it is reduced to respective butane.
(4) Preparation of alkane by the reduction of aldehyde
When aldehyde is reacted with phosphorus and hot concentrated hydriodic acid (hydrogen iodide) at 1500C, aldehyde is reduced to respective alkane.
Example
When acetaldehyde is reacted with phosphorus and hot concentrated hydriodic acid (hydrogen iodide) at 1500C, acetaldehyde is reduced to respective propane.
(5) Preparation of alkane by the reduction of carboxylic acid
When carboxylic acid is reacted with hot concentrated hydriodic acid (hydrogen iodide) at 2000C – 2500C, carboxylic acid is reduced to respective alkane.
The yields are very good for higher alkanes and may even be improved by heating of fatty acids with hydrogen under pressure in the presence of nickel as catalyst.
Example
When acetic acid is reacted with hot concentrated hydriodic acid (hydrogen iodide) at 1500C, acetic acid is reduced to respective ethane.
(B) Preparation of Alkanes from compounds having large number of carbon atoms
(i) Preparation of alkanes by Decarboxylation of Carboxylic Acids
The alkane is prepared when sodium salt of carboxylic acid is heated strongly with sodalime.
Since, in this reaction carbon dioxide (CO2) is split out from a carboxylic group, thus it is called Decarboxylation Reaction.
Example
(a) When sodium acetate reacts with sodalime, methane is obtained.
(b) When sodium propionate is heated with sodalime, ethane is obtained.
Sodalime: The mixture of sodium hydroxide and calcium hydroxide is called Sodalime
(C) Preparation of Alkanes from compounds containing fewer carbon atoms
Reduction of Alkyl Halides to prepare Alkanes
(i) When alkyl halide is reacted with nascent hydrogen, the respective alkanes are formed.
Example
(a) When methyl iodide is treated with nascent hydrogen, methyl iodide is reduced to methane.
(b) When ethyl bromide is treated with nascent hydrogen, ethyl bromide is reduced to ethane.
(ii) Wurtz Reaction
When alkyl halide is heated with sodium metal in dry ether solution, higher alkane is produced.
Wurtz reaction is a good method to obtain higher alkanes using lower member of alkanes.
Example
When methyl bromide is heated with sodium metal in dry ether solution, ethane is produced.
Mechanism
During the given Wurtz reaction, two types of mechanisms are possible.
(a) Free Radical Mechanism
In first step sodium metal removes bromide form methyl bromide and made methyl as free radical.
In second step two free methyl radicals join to produce Ethane.
(b) Ionic Mechanism
In this mechanism formation of methane performed in two steps.
In first step methyl bromide reacts with two atoms of sodium and methyl sodium and sodium bromide are formed.
In second step methyl sodium reacts with another molecule of methyl bromide and gives ethane.
Reaction between mixture of alkyl halides
The reaction between mixture of alkyl halides are sodium gives a mixture of alkanes.
Example
When methyl bromide and ethyl bromide react with sodium gives propane along with ethane and n–butane. This happens because while reacting methyl and ethyl radical they react with themselves also.
Limitation of this reaction
As mixture of alkanes is formed in this reaction, so it becomes practically useless to separate them because of less difference in their boiling points. Thus this reaction has less practical implication.
Hydrolysis of Grignard Reagents
Alkyl magnesium halide is called Grignard Reagent. Grignard reagent (Alkyl magnesium halide) is obtained by the heating of alkyl halides with magnesium metal in anhydrous ether.
When obtained alkyl magnesium halide reacts with water, it gives respective alkanes.
Example
When ethyl bromide is heated with magnesium in the presence of dry ether, ethyl magnesium bromide (Grignard Reagent) is obtained.
When obtained alkyl magnesium halide reacts with water, it gives respective alkanes.
When this ethyl magnesium bromide (Grignard reagent) is treated with water, it gives ethane.
The Kolbe's Reaction : Electrolysis of Salt of Carboxylic Acid
On electrolysis of concentrated solution of sodium or potassium salt of carboxylic acid, a higher alkane is produced.
Example
When concentrated sodium acetate obtained from carboxylic acid is put under electrolysis, ethane is obtained at anode.
This method is called Kolbe's Method or reaction after discovered and used first in 1849 by German chemist Hermann Kolbe.
Fractional Distillation of Petroleum and Natural gas
The first five members of alkanes, i.e. methane, ethane, propane, butane and pentane are obtained in the pure form by the fractional distillation of petroleum and natural gas, as these alkanes have significant difference in their boiling point which is suitable for fractional distillation.
But in alkanes of higher series the differences in their boiling point is not significant, so obtaining them by the process of fractional distillation is not practical.
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