Hydrocarbons II

04 Carbon and its compound

EXISTENCE OF CARBON COMPOUNDS

  • Straight chains: One carbon atom is bonded to another carbon forming a straight line (without any branches) as in low molecular weight hydrocarbons. For example ethane.
extCarbComp
  • Branches:One of the carbon atoms is bonded to more than two carbon atoms as in most carbon compounds with higher molecular weight. For example- Isobutane.
extCarbComp01
  • Rings: Three or more carbon atoms are linked together so that they form closed cycles known as cyclic compounds. For example: Cycloheptane.
extCarbComp02

CYCLIC HYDROCARBONS

  • A saturated cyclic hydrocarbon is ‘cyclohexane’. The formula of cyclohexane is C6H12.
cyclichydro
  • An unsaturated cyclic hydrocarbon is ‘benzene’.  The formula for benene is C6H5.
cyclichydro02

NAMING OF HYDROCARBONS

Number of Carbon atoms Name
1 Meth
2 Eth
3

Prop

4 But
5 Pent
6 Hex
7 Hept
8 Oct
9 Non
10 Dec

IUPAC NOMENCLATURE FOR BRANCHED-CHAIN SATURATED HYDROCARBONS

  • Name the first found longest chain of carbon atoms in the structure of the compound.
  • Name separately the alkyl groups present as side chains (branches) that are considered substituents as methyl
    (CH3)

    or ethyl

    (C2H5)

         groups.

  • Number the carbon atoms of the longest carbon chain in such a way that the alkyl groups (substituents) get the lowest possible number.
  • Write the position of alkyl group indicated by the number of the carbon atom to which it is attached.
  • Write the ‘position and name of alkyl group’ just before the name of ‘parent hydrocarbon’ to obtain the IUPAC name of the compound.
  • For example:-
iupac

ISOMERS

  • Isomers are the organic compounds having the same molecular formula but different structures.
  • For Example, the normal-butane has a straight chain structure whereas iso-butane has a branched chain structure.

HOMOLOGOUS SERIES

  • A homologous series is a group of organic compounds having similar structures and similar chemical properties in which the successive compounds differ by CH2 group.
  • Consider CH4 and C2H6. The difference is -CH2 unit and the difference between C2H6 and C3H8   is also -CH2 unit. So CH4, C2H6 and C3H8 are homologues.
  • Similarly, in case of alkenes the first member is ethene and the successive members are C3H6, C4H8, and C5H10, i.e they differ from each other by a –CH2 unit.
  • The most basic homologous series  in organic chemistry is that of alkanes with general formula CnH2n+2 , alkenes with general formula CnH2n and alkynes with general formula CnH2n-2 .

CHARACTERISTICS OF A HOMOLOGOUS SERIES

  • All the members of a homologous series are represented by the same general formula.
  • Any two adjacent homologues differ 1 carbon atom and 2 hydrogen atoms in their molecular formulae.
  • The difference in the molecular masses of any two adjacent homologues is 14 u.
  • All the compounds of a homologous series show similar chemical properties because they all have the same functional groups in them.
  • The members of a homologous series gradually change their physical properties with increase in molecular mass.
  • Physical properties like melting and boiling point, solubility, etc. change because depend on the mass and the total number of bonds in a compound.
  • This happens because as molecular mass increases the number of bonds also increases.

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