Alkanes:

Introduction:

Alkanes are saturated aliphatic hydrocarbons. We have already learnt in our earlier classes that the saturated aliphatic hydrocarbons contain the ordinary formula, C_nH_2n+2, since of their zig-zag  samples due to the tetrahedral geometry of  sp³ hybridized carbon, carbon atoms that are close mutually often join up by expulsion of 2 hydrogen atoms to form a ring. These ring compounds are referred to as cyclic aliphatic hydrocarbons, also termed alicyclic hydrocarbons or cycloalkanes that contain the general formula, C_nH_2n.

Alkanes are as well recognized as paraffins. The name paraffin comes from 2 Latins words, 'Paraum and affinis', that signify 'little affinity'. This name was suggested as such hydrocarbons were apparently unreactive. It is noticed that, under ordinary conditions, alkanes are inert toward reagents these as acids, alkanes, oxidizing and reducing agents, halogenation, pyrolysis, aromatization, and so on. Numerous of such reactions proceed throughout the formation of highly reactive free radicals.

In this chapter, first we shall converse composition and fractionation of petroleum, as it is the main source of alkanes and preparation, physical properties and spectral properties of alkenes. To end with, we shall learn several chemical reactions of alkanes and cycloalkanes.

Petroleum: A Source of Alkanes

Petroleum is the chief of source of many acyclic as well as cyclic alkanes. This complex mixture of alkanes takes places abundantly in diverse natural deposits in the earth.

Petroleum is an oily, thick inflammable and generally dark coloured liquid. The origin of the word petroleum is from the Latin words Petra (rock) and oleum (oil), as it is originate in abundance close to the surface of the earth trapped via rock structure.

The major oil-producing country of the world is the USA. The other major oil producing countries are Russia, Gulf countries, Romania, Nyammar, Venezucla, Iran, Pakistan, India and Nigeria.

Composition:

The composition of petroleum varies through the locality of its occurrence but all illustrations contain mixtures of alkanes ranging in size from methane by only one carbon atom to long chain hydrocarbon containing up to 40 carbon atoms. Cyclohexane, nephthalenes and other aromatic hydrocarbons are as well present in little amounts. In addition to hydrocarbons, oxygen, nitrogen and sulphur enclosing compounds also as metallic constituents might as well be present. Actually, as many as 500 compounds contain sometimes been detected in a single of petroleum.

Liquefied petroleum gas (LPG) encloses a mixture of propane and butane. Natural gas is originated along by petroleum whose chief components are methane (80%) and ethane (10%). The remaining 10% being a mixture of higher hydrocarbons. In addition hydrocarbons, natural gas as well includes carbon dioxide and nitrogen.

Fig: Fractionation of Petroleum

The petroleum attained directly from the ground is not readily usable, since it is a mixture of many compounds. Separating the crude petroleum into valuable components is termed refining.  The 1^st step in refining is fractional distillation.

Fractionation of Petroleum:

The first step in the refining of petroleum involves its separation into fractions of different boiling ranges via fractional distillation. Crude petroleum is heated in a furnace at 650 K and the hot liquid is then exceeded through a flash chamber wherever the low boiling fractions are volatilized via lowering the pressure. The vapours are then passed through a tall bubble tower. This tower is filled by horizontal stainless steel trays. Each and every tray is afforded through chimneys covered by a loose cap termed bell cap in Fig. As the vapours ascend, they become steadily cooler and, thus, diverse fractions condense at different heights. The higher boiling fractions condense in the lower portion of the tower. This allows the division of crude petroleum vapours into a number of fractions, each condensing inside a definite temperature range. Each and every fraction is a mixture of different hydrocarbons. Consequently, it has to be purified prior to employ.

The significant petroleum a fraction along through their boiling ranges and chief utilizes is following in Table.

Table: Fractionation of Petroleum

 Synthetic petroleum:

Through the growth of civilization and the growth of industry, the demand of gasoline and petroleum products is rising day by day. The natural resources are limited and it is feared that they will quickly be exhausted. Keeping this in mind, the chemists contain tried different methods of manufacturing synthetic fuels. Given processes contain shown some promise.

Bergius process: In this process, finely powdered coal is hydrogenated in existence of catalysts, these as tin and leads to provide a mixture of liquid hydrocarbons. Throughout this procedure, the carbon rings in coal undergo fission to provide smaller fragments which are then hydrogenated to open chain and cyclic hydrocarbons. Gasoline (bp up to 473) and kerosene (bp up to 573 K) are attained on fractional distillation of hydrogenation products.

Fischer-tropsch process: This process was developed in the year 1923 via 2 German chemists, Franz Fischer and Has Tropsch. Water gas, that is a mixture of carbon monoxide and hydrogen, is attained via the reaction of steam with red hot coke. The water gas is mixed with half its volume of hydrogen and the catalyst utilized in the process is a mixture of cobalt (100 parts), thoria (5 parts), magnesia (8 parts) and kieselguar (100 parts). This water gas when hydrogenated and passed over a catalyst at 470-870 K under 1-10 atm pressure yields crude petroleum.

 nCO +  mH₂  → Mixture of hydrocarbons +  H₂O

The crude oil obtained is refined by the fractional distillation process as described earlier.

Octane number:

The most generally utilized fuel for automobiles is gasoline. Not all fuel is uniformly good. Let us see how we can distinguish between good quality fuels. This can be finished via comparing their octane numbers. Octane number is a measure of the quality of gasoline: the higher the octane number, the better the fuel.

Therefore 2, 2, 4-trimethylpentane (iso-octane), that is considered a good fuel, is given an octane number of 100 whereas n- heptane, a extremely poor fuel, is following an octane number of zero. Mixtures of such 2 compounds are utilized to define octane numbers between 0 and 100. Octane number is the percentage of 2, 2, 4-trimethylpentane present in a mixture of 2, 2, 4-trimethylpentane and n-heptane which has similar ignition properties as the fuel under examination. For instance, a fuel that performs also as 1: 1 mixture of 2, 2, 4-trimethylpentane and n-heptane has an octane number 50. Commercial gasoline has octane number 81, 74 and 65 for the premium, regular and third grade gasoline. Good quality motor fuels utilized in modern automobiles have octane number in the 87-95 range. It has been noticed that:

• branching of the hydrocarbon chain raises octane number
• octane number decreases as the chain length amplifies
• unsaturated hydrocarbons have higher octane number than saturated hydrocarbons
• Cycloalkanes have higher octane number than the analogous acyclic alkanes.

Various additives, such as tetraethyllead, (C₂H₅)₄ Pb and tert-butyl methyl ether, (CH₃)₃COCH₃, are used to boost the octane number of gasoline. The utilize of tetraethyllead is being curtailed for ecological reasons.

Cetane number:

The working of diesel engine diverges from that of gasoline engine. In diesel engines, fuels having a lower octane number are much more useful than those having a higher octane number. In other terms, the straight chain hydrocarbons constitute a superior fuel than the branch chain hydrocarbons. Quality of diesel fuel is expressed in terms of a number termed cetane number.

The hexadecane (cetane, C₁₆C₃₄), considered a good fuel, is given a cetane number 100 whereas α-methylnahthalene, a extremely poor fuel, is following a cetane number zero. Cetane number is illustrated as the percentage of hexadecane in a mixture of hexadecane and α-methylnaphthalene that has the same ignition properties as the fuel under examination. Good quality diesel fuel needed for modern diesel engine has cetane number greater than 45.

Physical properties:

Covalent bonds of an alkane molecule are either carbon-carbon bonds or bonds between carbon and hydrogen atoms that vary very little in electronegativity. Thus, the alkane molecule is the non polar or very weakly polar. Their physical constants as boiling points densities, and so on, increase with increase in the number of carbon atoms. Expect for the few members, the boiling point amplifies via 20 to 30 degrees for each CH₂ chapter that is added to the chain. Boiling point of a covalent substance depends upon the intermolecular forces. Intermolecular forces, in turn, based upon the number of electrons, surface area of the molecule and its dipole moment. The intermolecular forces amplify through the increase in the number of electrons or in the value of the dipole moment and surface, area. The stronger in intermolecular forces, the higher the boiling point. In a particular series, with the increase in the number of carbon atoms, the surface area increases and therefore, the intermolecular forces and boiling points as well increase. Branching in a chain.

Unlike boiling point, the melting points of alkanes do not show a regular raise. It has been found that molecules with an odd number of carbon atoms have lower melting point than those with an even number of carbon atoms. A possible explanation in following here. The carbon atoms in alkanes are sp³ hybridized state with a bond angle of 109^o28. The terminal carbon atoms in a carbon chain with an odd number of carbon atoms lay on the same side, whereas those in a carbon chain by an even number of carbon atoms lie on the opposite site. This means that the packing efficiency and the interaction between the molecules in the solid state is less in alkanes enclosing an odd number of carbon atoms as compared to those by an even number of carbon atoms. This is reflected in the lower melting points of alkanes through odd number of carbon atoms. The maximum density of alkane is about 0.8; therefore, all alkanes are lighter than water. Alkanes are soluble in non polar solvents but insoluble in polar solvents.

Spectral properties:

UV spectroscopy is not of much help in the characterization of alkanes, since the alkanes don't show any absorption band above 200 nm. In the infrared (ir) spectra of alkanes, the position of C-H stretching band depends on whether the hydrogen atom is attached to a primary, secondary or tertiary carbon atom. Thus, we have the following regions:

 -CH₃   2975-2950    AND 2885-2860 CM^-1

    |

 -CH₂   2940-2915   AND 280-2845 CM ^-1

    |

 -CH   2900-2880 CM^-1

    |

Some C - H deformation absorption frequencies are -CH₃, 1435 and 1385-1370 cm-1 and >CH₂, 14801440 cm^-1. Two useful skeletal vibrations are: (CH₃)₂CH-, 1175-1165 cm and (CH₃)₃C-, 1255-1245 cm^-l, it is therefore possible to detect the presence of these groups in a molecule.

The nmr spectra of alkanes provide characteristic signals at, δ 0.9 (CH₃), δ 1.4 (-CH₂) and | δ1.5 (-CH). Let us now observe the mass spectra of alkanes. The stability of the radical ions can as well be presumed in the order tert > sec >p, therefore, the fission of bonds in alkanes take places preferentially at the branched carbon atom. Whenever alternative fissions can take place, it is the heaviest side chain that is abolished preferentially. Because alkyl radical ions are formed, all those with ¹H and ¹²C will provide peaks of odd masses in their mass spectra. In particular, alkanes give a series of peaks separated via 14 mass chapters (CH₂). The relative abundances of their peaks are generally the greatest for C₃H₇⁺ (43), C₄H₉⁺ (57) and C₅H₁₁⁺ (71), and decreases fairly regularly for the larger masses.

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