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CHEMISTRY CBSE CLASS X

Hydrocarbons

A compound made up of hydrogen and carbon only . Natural source is petroleum.

Two types--  Saturated ; unsaturated

1. Saturated Hydrocarbons (Alkanes)

Carbon atoms are connected with single covalent bonds and end with ‘ane’. They are unreactive.

The general formula is C_nH_2n+2

2. Unsaturated Hydrocarbons (Alkenes  and Alkynes)

Carbon atoms are connected by a ‘double bond’ or a ‘triple bond’ .These are obtained mostly from petroleum by a process called cracking.

1. Alkenes --- C_nH_2n
   Alkenes contain a double bond between two carbon atoms which is formed by sharing of two electron pairs. Simplest alkene is Ethene.
   
   
2. Alkynes --- C_nH_2n-2
   Alkynes contain a triple bond between two carbon atoms which is formed by sharing of three electron pairs. Simplest Alkyne is Ethyne whose common name is Acetylene.
   
   
3. Alkyl  Groups ---- C_nH_2n+2
   The group formed by the removal of one hydrogen atom from an alkane molecule .
   Methyl group (CH_3-) is formed by the removal of one  H  atom from Methane  (CH₄) , etc.
   
   
4. Cyclic  Hydrocarbons  -----
   Hydrocarbons in which the carbon atoms are arranged in the form of a ring. They may be saturated or unsaturated.

1. A saturated cyclic hydrocarbon is ‘cyclohexane’ –  C₆H₁₂
   
   Electron dot structure is obtained by putting two electron dots in place of every single bond in its structural formula.
   Cyclopropane  - C₃H₆
   Cyclobutane – C₄H₈
   Cyclopentane   -  C₅H₁₀
   General formula of cycloalkanes  is C_nH_2n
   
   
2. An unsaturated cyclic hydrocarbon is ‘ benzene’ ( aromatic  compounds ) whose electron dot structure is  --  2 electrons in place of every single bond and 4 electrons in place of every double bond in its structural formula.
   
   Single  bond  --  2 shared electrons
   Double bond  --  4 shared electrons

Naming  Hydrocarbons -

• Isomers :- The organic compounds having the same molecular formula but different structures.
• Normal butane has a straight line structure whereas iso-butane structure has a branched chain structure.
• Isomerism is possible only with hydrocarbons having 4 or more carbon atoms ie. Isomerim is not possible in methane, ethane and propane.
• 1^st isomer  is called n- pentane (normal)
• 2^nd isomer is called isopentane
• 3^rd isomer is called neopentane

Submitted By Mrs. Kritika Bhola
Email Id : [email protected]