Alkane

• Alkanes


  Aliphatic saturated hydrocarbons that have the general formula CnH2n+2
• Alkanes


  • The carbon atoms are surrounded by the highest no of H-atoms
  • Can be unbranched or branched
  • The C-atoms are sp³ hybridized and the compounds have a general tetrahedral orientation
• Alkanes


  • Pentane (CH₂CH₂CH₂CH₂CH3)
• Alkanes are found on earth and on other planets
• Petroleum


  A complex mixture of organic compounds formed from the anaerobic decomposition of plant and animal remains over time, most of which are alkanes and aromatic compounds
• Natural gas


  Consists mainly of methane with smaller quantities of ethane, propane, butane and isobutane
• Refining of petroleum (crude oil)

  1. Fractional distillation to give gas (C3, C4), gasoline (C5-C11), kerosene (C9-C16) and gas oil or diesel (C15-C25)

  2. Distillation under reduced pressure to further give lubricating oil and waxes, leaving asphalt behind
• Octane number


  The measure by which a fuel's antiknock properties are judged
• Straight chain hydrocarbons are far more prone to induce engine knock than branched ones
• 2,2,4-Trimethylpentane


  Assigned an octane number of 100
• Heptane


  Assigned an octane number of 0
• A mixture of 87% 2,2,4-trimethylpentane (isooctane) and 13% heptane would be rated as 87 octane gasoline
• Catalytic cracking


  1. The method used to obtain good fuel from high-boiling kerosene fraction (C12-C14)

  2. Involves the breaking apart of the molecules, which then rearrange to smaller, highly branched alkanes containing 5-10 C-atoms
• Catalytic reforming


  The process by which straight-chain alkanes from straight-run gasoline are converted into aromatic molecules such as toluene
• Aromatic compounds also have high octane ratings and are desirable components of gasoline
• Ways organic chemists represent structural formula of compounds


  • Dash structural formula
  • Condensed structural formula
  • Bond-Line formula
  • Three Dimensional formula
• Dash structural formula


  Involves drawing the chains of atoms in a straight line and often the lone pair of electrons are omitted. This shows the connectivity of atoms in a molecule represented in two dimensions and is a rather convenient way to draw. It does not represent the actual shape of the molecule drawn.
• Condensed structural formula


  Involves writing a formula without showing all the individual bonds. In a condensed structure, each central atom is shown together with the atoms that are bonded to it. The atoms bonded to a central atom are often listed after the central atom (as in CH3CH, rather than H3C-CH3) even if that is not their actual bonding order. In many cases, if there are two or more identical groups, parentheses and a subscript may be used to represent all the identical groups. When a condensed structural formula is written for a compound containing double or triple bonds, the multiple bonds are often drawn as they would be in a Lewis structure.
• Condensed structural formula


  • E.g. 2-propanol can be written as CH3CH(OH)CH, CH3CHOHCH3, (CH₃)₂CHOH. 2-butene will be written as CH3CH=CHCH3.
• Bond-Line formula


  This is the quickest of all because it only shows the carbon skeleton. In a line-angle formula (as it is also called), bonds are represented by lines, and carbon atoms are assumed to be present wherever two lines meet or a line begins or ends. Nitrogen, oxygen, and halogen atoms are shown, but hydrogen atoms are not usually drawn unless they are bonded to an atom that is drawn. The number of H atoms necessary to fulfill the carbon atom's valency is assumed to be present.
• Three Dimensional formula


  This type of formula shows the arrangement in space of atoms in a molecule. In this representation, bonds that project upward out of the plane of the paper are indicated by a wedge (), while those that lie behind the plane are indicated with a hashed wedge (,), and those that lie in the plane of the paper are indicated by a line.
• Reduction of alkyl halides

  Alkyl halide + Zn + HX → Alkane + ZnX2
• From Grignard reagents


  Alkyl magnesium halide + H2O → Alkane + Mg(OH)X
• Hydrogenation of alkenes


  Alkene + H2 (over Raney Ni, Pt or Pd) → Alkane
• Wurtz reaction


  Alkyl halide + 2Na + Alkyl halide → Alkane + 2NaBr
• From alcohols


  Alcohol + HI + P → Alkane
• Alkanes


  • C1-C4 unbranched are gases
  • C5-C17 unbranched are liquids
  • C18+ unbranched are solids
• Increasing molecular weight


  Boiling point increases
• Branching of alkanes


  Boiling point decreases
• Introduction of other atoms/functional groups

  Boiling point increases as polarity increases
• Density of alkanes


  Least dense of organic groups, considerably less than 1 g/mL
• Solubility of alkanes


  • Almost totally insoluble in water due to low polarity and inability to form hydrogen bonds
  • Liquid alkanes and cycloalkanes are soluble in one another and in low polarity solvents like benzene, chloroform, carbon tetrachloride
• Solubility of alkanes


  • Alkanes are almost totally insoluble in water because of their very low polarity and their inability to form hydrogen bonds
  • Liquid alkanes and cycloalkanes are soluble in one another, and they generally dissolve in solvents of low polarity e.g. benzene, chloroform, carbon tetrachloride
• Melting and boiling points of some n-alkanes
• Alkanes


  • Only have strong sigma (6) bonds
  • C- and H-atoms have approximately the same electronegativity resulting in a roughly equal sharing of the electrons by the bonding atoms
  • Atoms in alkanes are not partially charged and generally neither nucleophiles nor electrophiles are attracted to them
  • Alkanes are very unreactive compounds and are referred to as paraffins (which means "little affinity" for other compounds)
• Combustion of alkanes


  Complete combustion of alkanes (like other hydrocarbons) gives water and carbon(IV)oxide
• Pyrolysis of alkanes


  Heating of alkanes in the absence of air, involves generation of radicals which then react with each other
• Halogenation of alkanes


  1. Chlorination and bromination of alkanes may be brought about by light, heat or catalysts

  2. Iodides are prepared by treating the chloro- or bromoderivative with sodium iodide in methanol or acetone
• Radical substitution reaction


  Radicals are involved as intermediates and the end result is the substitution of a halogen atom for one or more of the hydrogen atoms of the alkane
• Stages of radical substitution reaction


  1. Chain initiating step: homolytic cleavage of halogen molecule into radicals

  2. Chain propagating steps: a radical reacts to produce another radical leading to propagation of radicals

  3. Chain terminating steps: combination of any two radicals to form a molecule brings the reaction to an end by reducing the number of radicals available for propagation