Title 17. Hydrocarbons.pdf ✅

Hydrocarbons 293 17 Hydrocarbons QUICK LOOK Compounds containing only C and H are known as hydrocarbons. The main sources of hydrocarbons are Natural gas → Methane (90%), Ethane, Butanes and Hexanes in decreasing quantities. Petroleum → Aliphatic hydrocarbons Coal → Aromatic hydrocarbons Petroleum, Crude oil or Mineral Oil: It is a dark coloured viscous oil (with unpleasant smell due to sulphur compounds) found deep in earth’s crust. It may be of three types: Paraffinic base type → It contains mainly of saturated hydrocarbons. Asphaltic base type → It contains mainly cycloparaffins Mixed base type → It contains both paraffinic and Asphaltic hydrocarbons. Crude oil as such is of little importance. However, it can be separated into a number of useful fractions by fractional distillation. Table 17.1: Temperature Range of Condensation Name of the fraction Temperature range of condensation Approximate composition Uses Uncondensed gases Upto 30 C° C C 1 5 − Domestic fuel, synthesis of organic chemicals, production of carbon black. Gasoline (Petrol). On refractionation gives: (i) Petroleum ether (ii) Gasoline or petrol 30 200 C − ° 30 80 C − ° 80 200 C − ° C C 5 10 − C C 5 6 − C C 6 10 − As a solvent for fat, oil, varnish and rubber. Fuel for the internal combustion engines of automobiles and aeroplanes, solvent and dry-cleaning. Kerosene 200 300 C − ° C C 10 16 − Illuminant, fuel for stoves, for making oil gas. Heavy oil. On re- fractionation gives: (i) Gas oil, (ii)Fuel oil, Diesel oil. 300 350 C − ° C C 16 18 − Fuel for diesel engines, for conversion to gasoline by cracking. (5) Residual oil. On refractionation gives: (i) Lubricating oil (ii) Paraffin wax (iii) Petroleum jelly 350 400 C − ° C C 18 40 − C C 18 20 − C C 20 30 − Lubrication Candles, boot polishes, wax paper, taprolin cloth and electrical insulation. (Vaseline) C30 – C40 In medicines, cosmetics, toilets and lubricants. (6) Residue, which may be either pitch (asphalt) or petroleum coke. > ° 400 C Pitch is used in water proofing of roofs, road making, stabiliser for wood and metal. Petroleum coke is used as fuel. Thus petroleum is a source of infinite number of useful compounds, it is said to be more precious than gold and hence also named as liquid gold or black gold. Octane Number: Octane number of a gasoline is defined as the percentage of iso-octane present in a mixture of iso-octane and n-heptane which matches the fuel (gasoline) in knocking; higher the octane number of a gasoline better is its quality. 3 2 5 3 n-Heptane (Octane number 0) CH (CH ) CH = ⋅ ⋅ 3 3 3 2 3 3 Iso-octane (2, 2, 4-Trimethylpentane) (Octane number = 100) CH CH | | CH C CH CH CH | CH − − − − Ordinarily gasolines having an octane number of 74 are used in motor cars, scooters and motorcycles. These gasolines are known as regular gasolines. Gasolines having octane number lower than this value are called third grade gasolines, while gasolines having an octane number of 84 and above are known as premium gasolines. Compounds having more than 100 and less than 0 octane numbers are also known. For example, triptane (2, 2, 3- trimethylbutane) has an octane number of 125, while n-nonane has an octane number of 45. Aviation gasoline (gasoline used as fuel in aeroplanes) has an octane number of 100 or more. Presence of following types of organic compounds increases the octane number of gasoline. In case of straight chain hydrocarbons, octane number decreases with increase in the length of the chain. Branching of chain increases the value of octane number. Introduction of double or triple bond in an alkane increases the value of octane number. Cyclic alkanes have relatively higher value of octane number than the corresponding n-alkanes. The octane number of aromatic hydrocarbons are exceptionally high.
Quick Revision NCERT - CHEMISTRY 294 The Octane number of gasolines can be improved by following methods: By cracking, By alkylation, By isomerisation, By adding gasoline additives (e.g. TEL), By adding BTX (benzene, toluene and xylene) Knocking: The objectionable metallic sound produced during the working of an internal combustion engine is known as knocking. It leads to wastage of fuel. Knocking depends upon the composition of the fuel, it falls off in the following order: Straight chain paraffins > Branched chain paraffins > Olefins > Aromatic hydrocarbons, Knocking may also be prevented or minimised by adding small amounts of certain compounds known as anti-knock compounds, e.g. tetraethyl lead (TEL) [(C2H5)4Pb]. Such petrol is known as leaded petrol or ethyl petrol. Tetraethyl lead is used as a mixture of TEL (63%), ethylene bromide (26%), ethylene chloride (9%) and methylene blue (2%). About 0.5-0.6 ml of this mixture is added to 1 litre of petrol. In the cylinder of conbustion engine, TEL decomposes to produce radicals which combine with the radicals produced due to irregular combustion. As a result, the reaction chains are broken and smooth burning of fuel occurs. This prevents knocking, heat Pb(C H ) Pb 4C H 2 5 4 2 5 → + This lead is deposited in the cylinder which is removed by ethylene bromide to form lead bromide volatile at the ignition temperature, BrCH CH Br 2 2 − → CH2 = CH Br , 2 2 + Pb Br PbBr (volatite) + → 2 2 However, use of TEL in petrol is facing a serious problem of lead pollution. To avoid this, a new compound cyclopentadieny -lmanganese carbonyl (called as AK-33-X) is used, in developed countries, as anti-knocking compound. Cetane Number It is used for measuring the knocking character of fuel used in diesel engines. Cetane (n-hexadecane, n–C16H34) ignites very readily and is given a cetane number of 100 while α- methylnaphthalene ignites very sluggishly in a diesel engine and hence is given zero cetane number. Centane number of a fuel is defined as the percentage of cetane in a mixture of cetane and α-methylnaphthalene that will have the same ignition characteristics as the fuel under examination. Synthetic Petrol Bergius process: Finely powdered coal (coal is a mixture of high molecular complex organic compounds deficient in hydrogen) is heated with a catalyst (organic compound of tin) in presence of hydrogen. 400 C - 500 C 2 250 atm Coal H Hydrocarbons(l) ° ° + → Fischer-tropsch process: Hard coke is the raw material. Steam is passed over red hot coke to give water gas (a mixture of CO and hydrogen). Carbon monoxide is removed from a part of the water gas. Hydrogen so obtained is mixed with rest of the water gas in a ratio of synthesis gas (i.e. H2 and CO in a ratio of 2:1) which is heated in presence of a catalyst (cobalt-thorium oxide). Co,Cu 2 n 2n 2 Zn Oxide n CO n H C H nH O + → + ; 2 n 2n 2 2 n CO (2n 1)H C H nH O + + → + + The overall yield of Fischer – Tropsch process (66%) is higher than that obtained from obtained from Bergius process (60%) and thus, unlike Bergius process, it can be operated commercially on a small scale. Petroleum products (e.g. ethylene, propylene, isobutene, acetylene, etc.) are used in preparing thousands of organic chemicals and products like dyes, drugs, explosive, plastics, fibers, rubber, etc. Hence petroleum is literally called as liquid gold or black gold. Petrochemicals Organic compounds obtained directly or indirectly from natural gas or petroleum are called petrochemicals. Some petrochemicals are: Saturated hydrocarbons–Methane, ethane, propane, butanes, pentanes, hexanes, cyclohexanes etc. Unsaturated hydrocarbons – Ethylene, propylene, butylenes, acetylene etc. Aromatic hydrocarbons – Benzene, toluene, etc. Halogenderivatives– CH Cl,CH Cl ,CHCl ,CCl ,CH 3 2 2 3 4 = CHCl (Vinyl chloride). Alcohols – Methyl alcohol, ethyl alcohol, allyl alcohol, butyl alcohol, glycol etc. Aldehydes and Ketones–Formaldehyde, acetadehyde, acetone etc. Alkenes and alkadienes – Styrene, 1, 3-butadiene etc. Acids – acetic acid. Nitriles – acrylonitrile. α-Methylnaphthalene (cetane number = 0) Slow ignition CH (CH ) CH 3 2 14 3 ⋅ ⋅ β β β β α α α CH 4 Hexadecane or Cetane (cetane number = 100) Spontaneous ignition
Hydrocarbons 295 These petrochemicals are widely used in the manufacture of iso-octane (aviation fuel), plastics (polythene, polyvinyl chloride (PVC) etc), synthetic fibers (nylone terylene, dacron etc.), synthetic rubber insecticides, pesticides, detergents, dyes, perfumes, explosives etc. Coal: It is believed to be formed beneath earth’s surface by the slow decomposition of vegetable matter over the years. Coal is mainly made up of carbon. Its carbon content, however, varies with the type of coal. For example, common varieties of coal such as anthracite contains 90% carbon, bituminous contains 70% C, lignite contains 40% C and peat contains 10-15% C. Coal also contains a number of arenes (aromatic hydrocarbons) such as benzene, toluene, xylenes, naphthalene and anthracene in addition to some organic compounds of sulphur and nitrogen. Aromatic Hydrocarbons from Coal. Aromatic hydrocarbons are obtained by destructive distillation of coal. When coal is subjected to destructive distillation, i.e., heated to 1270K to 1675K in the absence of air, it decomposes to give the following main products. Coke: It is the solid residue left after the distillation. It is mainly used as a fuel and as a reducing agent in metallurgy. Coal gas: It mainly consists of low molecular mass alkanes and alkenes. It is used as a fuel. Ammonical liquor: It is a solution of ammonium in water and is removed by absorbing in a suitable mineral acid like dil. H2SO4 .The ammonium sulphate thus produced is used as a fertilizer. Coal tar: It is a thick black viscous liquid with a disagreeable smell. It is a rich source of aromatic hydrocarbons (arenes) such as benzene, toluene, 1, 2-, 1, 3- and 1, 4- dimethylbenzenes (xylenes), naphthalene, anthracene and phenanthrene. These are obtained by fractional distillation of coal tar into a number of fractions. Table: 17.2 Fractions of Coal Tar Distillation Name of the Fraction Temperature range (in K) Major components Light oil Upto 443 Benzen, Toluene, Xylenes Middle oil or Carbolic oil 443 – 503 Phenol, Naphthalene, Pyridine Heavy oil or Creosote oil 503 – 543 Methyl and higher alkyl phenols, naphthalene, naphthols, etc. Green oil or Anthracene oil 543 – 633 Anthracene, phenanthrene. Pitch (left as residue) Non - volatile 92 – 94% carbon (used for making black paints, varnish for wood and for water proofing). Classification of Hydrocarbons Hydrocarbons can be classified as follows: Alkane or Paraffins The functional group is an atom or group of atoms that causes a compound to behave in a particular way, i.e. it is the functional group that gives rise to homologous series. An alkyl group is the structure remaining after one H is removed from an alkane. Alkanes have general formula, C H . n 2n 2+ (1) Preparation 2 Reney Ni C H C H n 2n n 2n 2 → H ,200 300 C − ° + (Sabatier Senderen’s reduction) Alloy containing equal amount of Ni and Al is digested with NaOH, the Al is dissolved away and the residual very finely divided Ni, is washed and stored under water or ethanol. Reduction of alkyl halide by metal and acid or Mg/anhyd. ether, then H O2 RX Zn H RH Zn X ⊕ ++ Θ + + → + + Mechanism: Zn Zn 2e → + ++ − RX e R X R − − Θ + → + → R C H OH R H OC H 2 5 2 5 Θ Θ + → − + Pri and sec alkyl halide → LiAlH4 Alkane Sec and tert alkyl halide → NaBH4 Alkane Pri, sec and tert alkyl halide → Ph SnH 3 Alkane (i) From RMgX: Alkanes can be prepared by reaction of Grignard reagent with the compounds containing ‘active hydrogen atom’. A compound is said to contain active hydrogen atom in Hydrocarbons Aliphatic or Open chain Aromatic or Arenes Saturated Alkanes or Paraffins Alkenes or Olefins Alicyclic or Cyclic chain Unsaturated Alkynes or acetylenes Coal Coke (Solid residue), nearly 70% Hot vapours and gases (Cooled and passed through water) Heated to 1270-1675 K (Destructive distillation) Condensed liquid is allowed to stand and the two layer formed are separated Coal gas (Mainly contains low molecular mass alkanes and alkenes) Ammonical liquor (Upper layer), nearly 8-10% Coal tar (Lower layer), nearly 4-5%
Quick Revision NCERT - CHEMISTRY 296 which hydrogen is attached to electronegative element from which it can be easily removed. Such compounds are H O,ROH, NH ,RNH . 2 3 2 e.g. e.g. (ii) Kolbe’s Electrolytic Method: The reaction is only suitable for preparation of symmetrical alkane. 2 2 2 at anode RCOOK RCOOK 2H O R — R 2CO H 2KOH + + → + + + (iii) Decarboxylation of Acids RCOOH NaOH RH Na CO + → + 2 3 (iv) Wurtz Reaction: Alkyl bromide or iodide reacts with metallic Na to from a hydrocarbon containing twice the number of carbons that are present in alkyl halide. RX 2Na R X R R R R R R 2NaX + + → − + − + − + ′ ′ ′ ′ (v) Corsey-Posner Whitesides house Synthesis R N H R Li RH Li (RNH) 2 Θ ⊕ + Θ + → + . . By insertion of: CH2: CH2N2 dissociates to from: CH2 The insertion of: CH2 is random e.g. (2) Physical Properties: C C 1 4 → gas; C C 5 17 → colourless liquid; C18 → onwards: colourless solid Density α B.P. α viscosity 1 1 M.Pt. Branching ∝ ∝ ∝ No. of C atoms Solubility in water 1 mol.wt.of alkane ∝ (3) Chemical Properties: Alkanes undergo substitution reaction. (i) Halogenation Ease of substitution tert. H Sec.H PriH > > F Cl Br I 2 2 2 2 > > > Iodination is reversible hence it may be carried out in the presence of an oxidizing agents like HIO ,HNO ,HgO 3 3 Relative amt. Kinds of monochloro derivative = Kinds of H atoms Ist product = Ease of substitution Relative population = As point IV The % are obtained by dividing the relative amount of each product by the sum of relative amount (3+7.6 = 10.6) and multiplying by 100. Chlorination in lab is carried out with sulphuryl chloride SO Cl 2 2 instead of Cl2. (ii) Nitration: Kinds of mononitro derivative and Ist product is same as for halogenation. But nothing for population. Kinds of mononitro derivative = Kinds of H atoms + All possible fragments 3 2 HNO 3 2 3 3 2 2 2 3 3 400 C NO | CH CH CH CH CH CH NO CH CHCHCH → + ° + + C H NO CH NO 2 5 2 3 2 Nitration is carried out in vapour phase. (iii) Sulphonation: Hexane or onwards Oleum → + SO H H SO 3 2 4 Carbonium ions are intermediate. (iv) Oxidation: Extensive oxidation gives acids and less extensive oxidation gives mixed ketones. Oxidation in the presence of boric acid gives sec. alcohol. KMnO4 Me CH [O] Me C OH 3 3 + → − (v) Isomerisation: n-alkane AlCl3 300 C° → Isoalkane carbonium ion is intermediate. (vi) Cracking: Higher alkanes 400 600 Cwith or 2 without catalyst H → + − ° small alkane + alkane Chlorination population: Pri H×1: Sec H×3.8: H×3.8:Tert H×5 Population Bromination: Pri H×1: Sec H×82: Tert H×1600 1 2 3 2 1 CH CH CH CH CH 3 2 2 2 3 3 2 2 2 2 3 n hexane CH CH CH CH CH CH (A) − 3 2 2 3 3 2-methylpentane CH CH CH CH —CH (B) | CH 3 2 2 3 3 3-methyl pentane CH CH CHCH —CH (C) | CH 1 insertion at C 2 insertion at C 3 insertion at C R2 CuLi Gillman catalyst RCH LiX RCu 3 + + RCu LiX + + CuR LiX + + X X R R CH X3 3 2 5 2 4 2 5 Br CH MgBr C H NH CH Mg NHC H + → + 2 X RMgX R NH RH Mg NHR + → + ′ ′ 3 4 Br CH MgBr HOH CH Mg OH + → + 2 2 X RMgX HNH RH Mg NH + → +