Aliphatic Hydrocarbon
Category : JEE Main & Advanced
Organic compounds composed of only carbon and hydrogen are called hydrocarbons. Hydrocarbons are two types
(1) Aliphatic Hydrocarbon (Alkanes, Alkenes and Alkynes).
(2) Aromatic Hydrocarbon (Arenes)
(1) Sources of aliphatic hydrocarbon
Mineral oil or crude oil, petroleum [Petra ® rock; oleum ® oil] is the dark colour oily liquid with offensive odour found at various depths in many regions below the surface of the earth. It is generally found under the rocks of earth’s crust and often floats over salted water.
(2) Composition
(i) Alkanes : found 30 to 70% contain upto 40 carbon atom. Alkanes are mostly straight chain but some are branched chain isomers.
(ii) Cycloalkanes : Found 16 to 64% cycloalkanes present in petroleum are; cyclohexane, methyl cyclopentane etc. cycloalkanes rich oil is called asphaltic oil.
(iii) Aromatic hydrocarbon : found 8 to 15% compound present in petroleum are; Benzene, Toluene, Xylene, Naphthalene etc.
(iv) Sulphur, nitrogen and oxygen compound : Sulphur compound present to the extent of 6% include mercaptans [R-SH] and sulphides [R-S-R]. The unpleasant smell of petroleum is due to sulphur compounds. Nitrogenous compounds are pyridines, quinolines and pyrroles. Oxygen compounds present in petroleum are. Alcohols, Phenols and resins. Compounds like chlorophyll, haemin are also present in it.
(v) Natural gas : It is a mixture of Methane (80%), Ethane (13%), Propane (3%), Butane (1%), Vapours of low boiling pentanes and hexanes (0.5%) and Nitrogen (1.3%). L.P.G. Contain butanes and pentanes and used as cooking gas. It is highly inflammable. This contain, methane, nitrogen and ethane.
(vi) C.N.G. : When natural gas compressed at very high pressure is called compressed natural gas (CNG). Natural gas has octane rating of 130 it consists, mainly of methane and may contain, small amount of ethane and propane.
(3) Theories of origin of petroleum : Theories must explain the following characteristics associated with petroleum,
Its association with brine (sodium chloride solution). The presence of nitrogen and sulphur compounds in it. The presence of chlorophyll and haemin in it. Its optically active nature. Three important theories are as follows.
(i) Mendeleeff’s carbide theory or inorganic theory
(ii) Engler’s theory or organic theory
(iii) Modern theory
(4) Mining of petroleum : Petroleum deposits occurs at varying depth at different places ranging from 500 to 15000 feet. This is brought to the surface by artificial drilling.
(5) Petroleum refining : Separation of useful fractions by fractional distillation is called petroleum refining.
| Fraction | Boiling range \[{{(}^{o}}C)\] | Approximate composition | Uses |
| Uncondensed gases | Upto room temperature | \[{{C}_{1}}\text{ }{{C}_{4}}\] | Fuel gases: refrigerants; production of carbon black, hydrogen; synthesis of organic chemicals. |
| Crude naphtha on refractionation yields, | \[30\text{ }\text{ }{{150}^{o}}\] | \[{{C}_{5}}\text{ }{{C}_{10}}\] | |
| i) Petroleum ether | \[30\text{ }\text{ }{{70}^{o}}\] | \[{{C}_{5}}\text{ }{{C}_{6}}\] | Solvent |
| (ii) Petrol or gasoline | \[70\text{ }\text{ }{{120}^{o}}\] | \[{{C}_{6}}\text{ }{{C}_{8}}\] | Motor fuel; drycleaning; petrol gas. |
| (iii) Benzene derivatives | \[120\text{ }\text{ }{{150}^{o}}\] | \[{{C}_{8}}\text{ }{{C}_{10}}\] | Solvent; drycleaning |
| Kerosene oil | \[150\text{ }\text{ }{{250}^{o}}\] | \[{{C}_{11}}\text{ }{{C}_{16}}\] | Fuel; illuminant; oil gas |
| Heavy oil | \[250\text{ }\text{ }{{400}^{o}}\] | \[{{C}_{15}}\text{ }{{C}_{18}}\] | As fuel for diesel engines; converted to gasoline by cracking. |
| Refractionation gives, | |||
| (i) Gas oil, (ii) Fuel oil, (iii) Diesel oil | |||
| Residual oil on fractionation by vacuum distillation gives, | Above \[{{400}^{o}}\] | \[{{C}_{17}}\text{ }{{C}_{40}}\] | |
| (i) Lubricating oil | \[{{C}_{17}}\text{ }{{C}_{20}}\] | Lubrication | |
| (ii) Paraffin wax | \[{{C}_{20}}\text{ }{{C}_{30}}\] | Candles; boot polish; wax paper; etc | |
| (iii) Vaseline | \[{{C}_{20}}\text{ }{{C}_{30}}\] | Toilets; ointments; lubrication. | |
| (iv) Pitch | \[{{C}_{30}}\text{ }{{C}_{40}}\] | Paints, road surfacing | |
| Petroleum coke (on redistilling tar) | As fuel. |
(6) Purification
(i) Treatment with concentrated sulphuric acid : The gasoline or kerosene oil fraction is shaken with sulphuric acid to remove aromatic compounds like thiophene and other sulphur compound with impart offensive odour to gasoline and kerosene and also make them corrosive.
(ii) Doctor sweetening process :
\[\underset{\text{Mercaptan}}{\mathop{2RSH}}\,+N{{a}_{2}}Pb{{O}_{2}}+S\to \underset{\text{Disulphides}}{\mathop{RSSR}}\,+PbS+2NaOH\]
(iii) Treatment with adsorbents : Various fractions are passed over adsorbents like alumina, silica or clay etc, when the undesirable compounds get adsorbed.
(7) Artificial method for manufacture of Petrol or gasoline
(i) Cracking, (ii) Synthesis
(i) Cracking : It is a process in which high boiling fractions consisting of higher hydrocarbons are heated strongly to decompose them into lower hydrocarbons with low boiling points. Cracking is carried out in two different ways.
(a) Liquid phase cracking : In this process, the heavy oil or residual oil is cracked at a high temperature \[(475-{{530}^{o}}C)\]under high pressure (7 to 70 atmospheric pressure). The high pressure keeps the reaction product in liquid state. The conversion is approximately 70% and the resulting petrol has the octane number in the range 65 to 70.
The cracking can be done in presence of some catalysts like silica, zinc oxide, titanium oxide, ferric oxide and alumina. The yields of petrol are generally high when catalyst is used.
(b) Vapour phase cracking : In this process, kerosene oil or gas oil is cracked in vapour phase. The temperature is kept \[600-{{800}^{o}}C\] and the pressure is about 3.5 to 10.5 atmospheres. The cracking is facilitated by use of a suitable catalyst. The yields are about 70%.
(ii) Synthesis : Two methods are applicable for synthesis.
(a) Bergius process : This method was invented by Bergius in Germany during first world war.
Coal \[+{{H}_{2}}\underset{\underset{250\,\text{atm}}{\mathop{450-{{500}^{o}}C}}\,}{\mathop{\xrightarrow{F{{e}_{2}}{{O}_{3}}}}}\,\] Mix. Of hydrocarbons or crude oil
(b) Fischer- tropsch process : The overall yield of this method is slightly higher than Bergius process.
\[{{H}_{2}}O+C\xrightarrow{{{1200}^{o}}C}\underset{\text{Water}\,\text{gas}}{\mathop{CO+{{H}_{2}}}}\,\]
\[xCO+y{{H}_{2}}\xrightarrow{{}}\] Mix. Of hydrocarbon \[+{{H}_{2}}O\].
The best catalyst for this process is a mixture of cobalt (100 parts), thoria, (5 parts), magnesia (8 parts) and kieselguhr (200 parts).
You need to login to perform this action.
You will be redirected in
3 sec
