NEET Chemistry Notes Hydrocarbons – Concept of Hydrocarbons

NEET Chemistry Notes Hydrocarbons – Concept of Hydrocarbons

 Concept of Hydrocarbons

Concept of Hydrocarbons

Hydrocarbon means compounds containing carbon and hydrogen only. Single bond containing hydrocarbons is saturated hydrocarbon and which has multiple bond are called unsaturated hydrocarbon. Hydrocarbons are of different types and classified into three main categories.

•  saturated hydrocarbons (alkanes)
•  unsaturated hydrocarbons (alkenes and alkynes)
• aromatic hydrocarbons.

Alkanes

Alkanes are the simplest organic compounds formed by carbon and hydrogen only. Chemically, these are almost inert that’s why also called Paraffins (Latin, Pamm affinis = Little affinity). The general formula for this series is C_nH _n . Hence, the first member is CH₄ (methane).

Preparation of Alkanes

• Sabatier Senderns Reduction
  Unsaturated hydrocarbons (alkenes or alkynes) on reduction in ,the presence of finely divided catalyst (e.g. Pt/Pd/Ni) give alkanes.
  
•  Alkyl halides (except fluorides) on reduction give alkanes in the presence of Zn and HCl
  
• Wurtz Reaction
  Alkyl halides when treated with sodium metal in the presence of dry ether gives higher alkanes.
  
•  Decarboxylation
  Sodium salt of carboxylic acid (RCOONa) when heated with hot sodalime (CaO + NaOH) gives alkane.
  
• Kolbe’s Electrolysis Method
  Sodium or potassium salt of carboxylic acid on electrolysis gives alkane (R —R).
  
• Grignard reagent (BMgX) when reacts with H₂0 or BOH, gives alkane.
  
• Action of water on aluminium carbide or beryllium carbide gives methane.
  
  These reactions produce methane, hence called methanoids.

Physical Properties of Alkanes

Physical properties depend upon the intermolecular forces of attraction.

•  Boiling Point Higher the molecular mass, more will be the boiling point. If molecular mass is same, as the contact surface area increases, boiling point increases. More branched chain has less contact surface area and so also lesser the boiling point.
• Melting Point Larger is the molecular mass, higher will be the melting point, but symmetric molecules have higher melting point than asymmetric ones. Therefore, alkanes with even number of carbon atoms have higher melting point than the successive alkanes with odd number of carbon atoms.
• Solubility Alkanes are non-polar, therefore soluble in non-polar solvents such as petroleum, benzene, etc.
• Density The densities of alkanes increase with increase in the molecular masses till the limiting value of about 0.8 g/cm³ is reached.

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