JEE Chemistry Hydrocarbons Notes Set 03

Sources and Composition of Petroleum

Crude oil (petroleum) is a dark, foul-smelling oil found in the earth's crust, usually floating over brine. It is a mixture, not a compound.

Natural gas is mainly methane (about 80%) with ethane, propane, butane and traces of nitrogen. It is compressed to make CNG, which has a very high octane rating. LPG is mostly butane (with propane) and is sold as cooking gas.

Three theories try to explain the origin of petroleum: Mendeleeff's carbide (inorganic) theory, Engler's organic theory and the modern theory. The presence of brine, nitrogen and sulphur compounds, chlorophyll derivatives and optical activity supports an organic origin.

Refining by Fractional Distillation

Refining means separating crude oil by fractional distillation into cuts of different boiling range. The lighter the fraction, the higher it condenses in the tower.

Different books quote slightly different boiling limits, so the order of the fractions matters more than exact numbers.

Purification of Fractions

1. Shaking with cold conc. \( \ce{H2SO4} \) removes unsaturated and sulphur compounds such as thiophene, which would give a bad smell and corrosion.
2. Doctor sweetening converts foul-smelling thiols into disulphides with sodium plumbite and sulphur: \( \ce{2 RSH + Na2PbO2 + S -> RSSR + PbS + 2 NaOH} \).
3. Passing the fraction over adsorbents (alumina, silica, clay) removes unwanted compounds.

Making More Petrol

1. Cracking: heavy fractions are heated so that large molecules split into smaller alkanes and alkenes. Liquid phase cracking uses 475 to 530 degrees C at 7 to 70 atm and gives petrol of octane number about 65 to 70. Vapour phase cracking of kerosene or gas oil uses 600 to 800 degrees C at a few atmospheres, and catalysts such as silica-alumina raise the yield.
2. Bergius process: coal and hydrogen with an iron oxide catalyst at 450 to 500 degrees C and about 250 atm give a mixture of hydrocarbons.
3. Fischer-Tropsch process: steam on red-hot coke gives water gas, \( \ce{C + H2O -> CO + H2} \). This mixture over a cobalt catalyst (with thoria, magnesia and kieselguhr) is converted to a mixture of hydrocarbons, \( \ce{xCO + yH2} \) gives a mixture of hydrocarbons and \( \ce{H2O} \). The product is called synthetic petrol.

Knocking and Octane Number

1. Knocking is the metallic pinging sound when fuel in an engine ignites unevenly before the spark. A fuel that knocks less allows a higher compression ratio and a more efficient engine, so low-knocking fuels are preferred.
2. Tendency to knock decreases in the order: straight chain alkanes > branched alkanes > olefins > cycloalkanes > aromatics.
3. Octane number is the percentage by volume of iso-octane in an iso-octane and n-heptane mixture that knocks like the fuel being tested. Iso-octane (2,2,4-trimethylpentane) is defined as 100 and n-heptane as 0.
4. A sample that behaves like 60% iso-octane and 40% heptane has octane number 60.

Octane number falls as a straight chain gets longer, rises with branching, rises when a double or triple bond or a ring is present, and is very high for aromatic hydrocarbons. Some compounds fall outside the scale: n-nonane has a value near -45 and 2,2,3-trimethylbutane about 124.

Antiknock Compounds and Other Ways to Raise the Octane Number

1. Tetraethyl lead, \( \ce{Pb(C2H5)4} \), is added with 1,2-dibromoethane (and some dichloroethane). The halides turn the lead formed in the engine into volatile \( \ce{PbBr2} \), which leaves with the exhaust, but this causes lead pollution. Cyclopentadienyl manganese tricarbonyl is used in some countries as a lead-free substitute.
2. Isomerisation: n-pentane with \( \ce{AlCl3} \) at about 200 degrees C gives isopentane, and n-hexane gives branched hexanes.
3. Alkylation: isobutane adds to isobutene in the presence of conc. \( \ce{H2SO4} \) to give iso-octane.
4. Aromatisation (reforming): n-heptane over Pt/\( \ce{Al2O3} \) at about 500 degrees C gives toluene and hydrogen.
5. Blending with benzene, toluene, xylenes or with methanol or ethanol also raises the octane number.

Cetane Number and Flash Point

Diesel oil is graded by its cetane number, the percentage of cetane (n-hexadecane, number 100) in a mixture with alpha-methylnaphthalene (number 0) that has the same ignition quality. A good diesel needs a high cetane number, which is the reverse of what a good petrol needs. The flash point is the lowest temperature at which an oil gives enough vapour to form an explosive mixture with air; it is measured with Abel's apparatus.

Petrochemicals