# 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.
• 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

 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
$\left({\mathrm{CH}}_{3}–\right)$

or ethyl

$\left({\mathrm{C}}_{2}{\mathrm{H}}_{5}–\right)$

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 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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