Hydrocarbons

• Hydrocarbons are compounds of carbon and hydrogen only, playing a key role in daily life

  • LPG (liquified petroleum gas), CNG (compressed natural gas), and LNG (liquified natural gas) are common fuels
  • Petrol, diesel, and kerosene oil are obtained by fractional distillation of petroleum
  • Coal gas is obtained by the destructive distillation of coal
  • Natural gas is found in upper strata during drilling of oil wells
  • Hydrocarbons are used as sources of energy, in the manufacture of polymers, solvents for paints, dyes, and drugs
• Hydrocarbons can be classified into three main categories:

  • Saturated hydrocarbons: contain carbon-carbon and carbon-hydrogen single bonds (e.g., alkanes)
  • Unsaturated hydrocarbons: contain carbon-carbon multiple bonds (double bonds, triple bonds, or both) [ e.g., alkenes , alkynes]
  • Aromatic hydrocarbons: a special type of cyclic compounds
• Alkanes are saturated open-chain hydrocarbons containing carbon-carbon single bonds

  • Methane (CH4) is the first member of the alkane family
  • Alkanes follow the general formula CnH2n+2
  • Alkanes are inert under normal conditions and were earlier known as paraffins
• Nomenclature and Isomerism in alkanes:

  • Alkanes can have structural isomers that differ in chain of carbon atoms
  • Primary, secondary, tertiary, and quaternary carbon atoms are classified based on the number of carbon atoms attached
• Example of chain isomers of alkanes:

  • C6H14 can have isomers like n-Hexane, 2-Methylpentane, 3-Methylpentane, 2,3-Dimethylbutane, and 2,2-Dimethylbutane
• Structural isomers:

  • Isomers with the same molecular formula but different properties due to differences in their structures
  • Example: Butane and pentane have structural isomers with different properties
• Identifying carbon atoms:

  • Primary (1°), secondary (2°), tertiary (3°), or quaternary (4°) carbon atoms based on their attachments to other carbon atoms
• Alkanes can be prepared from unsaturated hydrocarbons through hydrogenation using catalysts like platinum, palladium, or nickel
• Alkanes can also be prepared from alkyl halides through reduction with zinc and dilute hydrochloric acid
• The Wurtz reaction involves treating alkyl halides with sodium metal in dry ethereal solution to produce higher alkanes with an even number of carbon atoms
• Sodium salts of carboxylic acids, when heated with soda lime, undergo decarboxylation to produce alkanes with one carbon atom less than the carboxylic acid
• Kolbe's electrolytic method involves electrolyzing an aqueous solution of sodium or potassium salt of a carboxylic acid to obtain alkanes with an even number of carbon atoms at the anode
• Alkanes are almost non-polar molecules due to the covalent nature of C-C and C-H bonds and the minimal difference in electronegativity between carbon and hydrogen atoms
• The weak van der Waals forces in alkanes result in the first four members (C1 to C4) being gases, C5 to C17 being liquids, and those with 18 or more carbon atoms being solids at 298 K
• Alkanes are colourless, odourless, and almost non-polar, leading to their low solubility in water due to their non-polar nature
• Petrol, a mixture of hydrocarbons, is used as a fuel for automobiles and for dry cleaning clothes to remove grease stains
• In relation to solubility, polar substances are soluble in polar solvents, while non-polar substances are soluble in non-polar solvents (like dissolves like)
• The boiling point of alkanes increases steadily with an increase in molecular mass due to the increase in intermolecular van der Waals forces with larger molecular size or surface area
• Isomeric pentanes show differences in boiling points, with pentane having a continuous chain of five carbon atoms having the highest boiling point compared to branched isomers
• Alkanes are generally inert towards acids, bases, oxidising, and reducing agents, but they undergo substitution reactions under certain conditions
• Substitution reactions of alkanes involve the replacement of hydrogen atoms by halogens, nitro group, and sulphonic acid group
• Halogenation of alkanes proceeds via a free radical chain mechanism involving initiation, propagation, and termination steps
• The rate of reaction of alkanes with halogens follows the order F2 > Cl2 > Br2 > I2, and the rate of replacement of hydrogens of alkanes is 3° > 2° > 1°
• Combustion of alkanes in the presence of air or dioxygen results in complete oxidation to carbon dioxide and water, releasing a large amount of heat
• Incomplete combustion of alkanes with insufficient air or dioxygen leads to the formation of carbon black, used in various applications like ink and pigments
• Controlled oxidation of alkanes with a regulated supply of dioxygen or air and suitable catalysts produces a variety of oxidation products like alcohols and acids
• Alkanes can be isomerized to branched chain alkanes in the presence of anhydrous aluminium chloride and hydrogen chloride gas
• Aromatization of n-alkanes with six or more carbon atoms at high temperature and pressure in the presence of specific catalysts leads to the formation of benzene and its homologues
• Aromatization or reforming is a reaction where substances in the presence of oxides of vanadium, molybdenum, or chromium supported over alumina get dehydrogenated and cyclised to benzene and its homologues
• Toluene (C7H8) is a methyl derivative of benzene
• For the preparation of toluene, it is suggested to use an alkane
• Methane reacts with steam at 1273 K in the presence of a nickel catalyst to form carbon monoxide and dihydrogen, which is used for the industrial preparation of dihydrogen gas
• Pyrolysis is a process where higher alkanes decompose into lower alkanes and alkenes when heated to higher temperatures
• Pyrolysis of alkanes is believed to be a free radical reaction
• The preparation of oil gas or petrol gas from kerosene oil or petrol involves the principle of pyrolysis
• Dodecane, a constituent of kerosene oil, when heated to 973K in the presence of platinum, palladium, or nickel, gives a mixture of heptane and pentene
• Alkanes contain carbon-carbon sigma (σ) bonds, allowing free rotation about the C-C single bond
• Rotation around a C-C single bond in alkanes results in different spatial arrangements of atoms, known as conformations or conformers or rotamers
• Ethane can have an infinite number of conformations by rotation around C-C single bonds
• The two extreme cases of ethane conformations are the eclipsed conformation and the staggered conformation