Hydrocarbons II

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.

• 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.

• Rings: Three or more carbon atoms are linked together so that they form closed cycles known as cyclic compounds. For example: Cycloheptane.

CYCLIC HYDROCARBONS

• A saturated cyclic hydrocarbon is ‘cyclohexane’. The formula of cyclohexane is C₆H₁₂.

• An unsaturated cyclic hydrocarbon is ‘benzene’.  The formula for benene is C₆H₅.

NAMING OF HYDROCARBONS

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
  $({\mathrm{CH}}_3–)$

  or ethyl

  $({\mathrm{C}}_2{\mathrm{H}}_5–)$

       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:-

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 C_nH_2n+2 , alkenes with general formula C_nH_2n and alkynes with general formula C_nH_2n-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.