alkanes

• Alkanes


  Saturated hydrocarbons (only have single bonds, namely, C–C and C–H)
• Alkanes


  • Each C atom is sp3 hybridised, tetrahedrally surrounded by H and other C atoms with bond angle of 109.5º
  • Can exist as straight chains, branched chains, and rings (cycloalkanes)
• General formula for alkanes


  Straight chain and branched chain: CnH2n+2

  Cycloalkanes: CnH2n
• Ring strain


  Instability that exists when bonds in a molecule form angles that are abnormal. Strain is most discussed for small rings such as cyclopropanes and cyclobutanes, whose internal angles are substantially smaller than the idealised value of 109.5º. Because of their high strain, the enthalpy of combustion for these small rings is more exothermic.
• All alkane molecules are considered non-polar
• Boiling Point / Volatility


  Alkanes have relatively low boiling points (or high volatility or high vapour pressure) as they have weak instantaneous dipole–induced dipole interactions
• Increase in Mr

  Larger number of electrons and therefore size of electron cloud, thus greater polarisability, and stronger instantaneous dipole–induced dipole interactions
• Shape of the molecule


  Straight chain alkanes have higher boiling point (or lower volatility) than their corresponding branched chain alkanes
• Alkanes are soluble in non-polar solvents such as benzene but are insoluble in water and other (highly) polar solvents
• Density


  Generally, alkanes are less dense than water. The density of alkanes increases with size of the molecule
• Reasons for unreactivity


  • C–C bond and C–H bond are strong covalent bonds which are difficult to break under ordinary conditions
  • Alkanes do not possess any electrophilic (electron-deficient) sites to attract nucleophiles (electron pair donors/Lewis bases), or nucleophilic (electron-rich) sites to attract electrophiles (electron pair acceptors/Lewis acids)
• Free Radical Substitution (Halogenation)


  1. Alkanes react with halogens such as Cl2(g) or Br2(l) to give halogenoalkanes

  2. The hydrogen atom(s) in the alkane can be substituted by the halogen atom(s)
• Free radical


  An atom or group of atoms that has an unpaired electron in its valence shell, e.g. •Cl, •CH3
• Free Radical Substitution Mechanism

  1. Initiation

  2. Propagation

  3. Termination
• Initiation


  Under ultraviolet light or heat, the homolytic fission of Cl–Cl bond takes place and chlorine free radicals, •Cl, are formed
• Propagation


  1. The highly reactive chlorine free radical, on colliding with alkanes such as methane, abstracts a hydrogen atom to produce HCl and a methyl radical, •CH3

  2. The methyl radical reacts further with another chlorine molecule to form the product (chloromethane, CH3Cl) and a chlorine free radical is regenerated
• Termination


  1. When any two free radicals collide, the unpaired electrons pair up to form a stable product

  2. Termination reactions are exothermic as they involve bond forming only. The energy released can help speed up the reaction
• Formation of polysubstituted products


  The chloromethane formed can undergo further substitution via the same mechanism to give polysubstituted halogenoalkanes in the propagation step
• To obtain the monosubstituted product (e.g. CH3Cl) as the major product, limited amount of halogen / large excess of the alkane is used. This decreases the probability that the chlorine radical will collide with a monosubstituted alkane molecule, which will lead to formation of the polysubstituted product
• Reactivity of halogens with alkanes


  Down the group from fluorine to iodine, there is a decrease in reactivity between halogens and alkanes
• Reactivity of halogens with alkanes

  • F2(g) reacts explosively with alkanes, even in the dark
  • Cl2(g) and Br2(l) react with alkanes in the presence of uv light or heat, Br2 reacts less readily than Cl2, both have no reaction with alkanes in dark conditions or at room temperature
  • I2 has no reaction with alkanes
• The lack of reactivity involving iodine means that iodoalkanes are not prepared using free radical substitution reaction. The reason behind the lack of reactivity involves energetics – during the first step of the propagation stage, the reaction is highly endothermic
• Stability of the alkyl radicals formed

  The more highly substituted alkyl radical has greater stability, and will be formed faster, leading to more products from it. Order of stability of alkyl radicals: tertiary > secondary > primary
• Cause of emission of CO: incomplete combustion of hydrocarbons
• detrimental effects of CO: combines with haemoglobin in the blood to form stable carboxyhaemoglobin, which prevents transportation of O2 to all parts of the body; causes dizziness, fatigue and even death
• cause of emission of unburnt hydrocarbons: incomplete combustion of fuel
• detrimental effects of unburnt hydrocarbons: becomes photochemical smog in strong sunlight; causes respiratory problems
• detrimental effects of lead: lead poisoning; accumulation causes general mental depression, slowness in reaction times & brain damage
• causes of emission of SO2: combustion of traces of sulfur or sulfur compounds in petrol
• detrimental effects of SO2: acidic gas which forms acid rain that causes corrosion of buildings, destroy crops and marine life.
• cause of emission of oxides of nitrogen: reaction of N2 with O2 takes place at high temperature; and NO gas is readily oxidised in O2 to form No2
• detrimental effects of oxides of nitrogen: forms acid rain that causes corrosion of buildings, destroy crops and marine life; oxides of nitrogen can also cause respiratory problems in humans and interfere with nitrogen metabolism in plants; also contributes to photochemical smog