Nuggets: Alkanes

last update 3rd Oct
  1. Organic chemistry is the study of the properties of compounds containing carbon. (carbon compounds)
  2. hydrocarbon is a compound containing carbon and hydrogen only.
  3. An organic compound contains the element carbon. (except CO, CO2, and carbonates)
  4. Organic compounds may also contain other elements like oxygen and nitrogen.
  5. saturated compound contains only single covalent bonds
  6. An unsaturated compound contains double covalent bonds.
  7. homologous series is a group of compounds with a general chemical formula, which follows a regular structural pattern in which each successive member differs from the other by a -CH2- group.
  8. Most chemical properties of organic compound (except for alkanes) are due to the presence of the function group. (e.g. alkenes have the C = C functional group)
  9. General formula for alkanes: CnH2n+2
  10. Name Formula State
    Methane CH4 gas
    Ethane C2H6 gas
    Propane C3H8 gas
    Butane C4H10 gas
    Pentane C5H12 liquid
    Hexane C6H14 liquid
    Heptane C7H16 liquid
    Octane C8H18 liquid
    Nonane C9H20 liquid
    Decane C10H22 liquid
  11. C1-4 (gas), C5-17(liquid), C18 onwards (solid: e.g bitumen)
  12. Characteristics of an alkane homologous series:
  13. (a) all members share a general chemical formula,
  14. (b) each successive member differs in its molecular formula by the addition of a -CH2- gropup,
  15. (c) its relative molecular mass differs from each group by 14,
  16. (d) all members have similar chemical properties ( (i) undergoes combustion to form CO2 and H20, (ii) undergoes substitution reaction),
  17. (e) physical properties of each member changes gradually as the number of carbon atoms increases. (as the carbon atom increases, the boiling point also increases, the compound becomes less flammable and become less volatile, the compound gets more viscous as the longer molecules gets entangled more easily)
  18. Physical Properties of Alkanes
  19. (i) insoluble in water
  20. (ii) does not conduct electricity
  21. (iii) low melting and boiling point for C1-4.
  22. Chemical Properties of Alkanes
  23. (i) All alkanes are generally unreactive because the C-C and C-H covalent bonds are strong.
  24. However they can undergo
  25. (ii) combustion: easily flammable when in gaseous and liquid states. less flammable in solid state. When solid alkane(alkanes with larger molecule size, e.g. candle wax) is burnt, it produces a smoky flame due to incomplete combustion of carbon atoms in the molecules. The smoky flame is caused by soot (carbon). This combustion is an exothermic reaction (energy released). Incomplete combustion also produces CO instead of CO2. In an incomplete combustion, the products formed are CO and H2O. In some other cases, incomplete combustion can also lead to the formation of carbon. (e.g. CH4 + O2 -> C + 2H20)
  26. (iii) (Halogenation) Substitution reactions with halogens (chlorine/bromine)
  27. Alkanes react with halogens in the presence of ultraviolet light(UV)
  28. The UV light acts as a catalyst. The UV light breaks up the Cl2 bonds.
  29. CH4 + Cl2 –> CH3Cl + HCl (hydrogen atom is replaced by chlorine atom)
  30. Substitution reaction can be repeated to form CH2Cl2, CHCl3, CCl4
  31. Isomerism (only for Pure Chem)
  32. Isomerism is where two or more organic molecules have the same molecular formula but different structural formulae. These different molecules are called isomers.
  33. Isomers have different melting and boiling points.
  34. The branched-chain or side-chain of an alkane is called alkyl groups. The formula of the alkyl groups is CnH2n+1

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