JEE Chemistry Hydrocarbons

SYNOPSIS

1. Alkanes: General formula: C_nH_2n+2
   
   • Preparation of alkanes
     
     
   • Chemical properties of alkanes

   
   

   1. Aromatisation:
      
      
      
      
   2. Cracking (pyrolysis):
      
   _{Conformations of ethane:}
   
   

   Conformations →	Eclipsed	Staggered	Skew
   	Conformation in which hydrogen atoms attached to two carbon atoms are as close to each other as possible	Conformation in which hydrogen atoms are as farther away as possible	Any other intermediate conformation in between eclipsed and sawhorse
   Sawhorse projection
   Newman projection		[RKR1]

   
   
2. Geometrical isomers of alkenes
   
   Trans-isomer is non-polar and has zero dipole moment where a cis-isomer is polar.
   Cis-isomer has higher boiling points compared to trans-isomer because it is polar.
   In case of solids, cis-isomer has a lower melting point than trans-isomer. This is because the trans-isomer is symmetrical and fits well into the crystal lattice.
   . Preparation of alkenes 
   
   
   
   
   
   
   

    Chemical properties of alkenes 
   
   

   Test for Unsaturation: There are two tests to know whether a compound is unsaturated or not
   

   1. Bromine water test: When an unsaturated compound is reacted with bromine solution in carbon tetrachloride, the reddish-orange colour of bromine solution in carbon tetrachloride is discharged.
      
      
   2. Baeyer’s reagent test: When an unsaturated compound is reacted with cold, dilute and aqueous solution of potassium permanganate (Baeyer’s reagent), the pink colour of KMnO4 solution is discharged.
      
      
3. Preparation of alkynes
   
   
   
   

   Acidic nature of alkynes: Hydrogen atoms of ethyne attached to triple bonded carbon atom are acidic in nature.
   Example:
   
   

   Reason:
   
   

   	Alkyne	Alkene	Alkane
   Hybridisation of carbon to which H atom is attached	sp	sp2	sp3
   Percentage s character of carbon	50% (maximum)	66.6%	25% (minimum)
   Electronegativity of carbon atom	Highest	Less than alkyne and more than alkane	Lowest
   Extent of attraction of hydrogen atoms of C-H bonds towards C	Highest	Less than alkyne and more than alkane	Lowest
   Ease of liberation of H atoms as protons	Highest	Less than alkyne and more than alkane	Lowest
   Acidic character	Highest	Less than alkyne and more than alkane	Lowest

     

    Chemical properties of alkynes  
   
   

4. Benzene: 
   Conditions for aromaticity

1. Planarity: The ring should be planar. In simple words, all the carbons of the ring should be sp2 hybridised.
2. Complete delocalisation of the p electrons in the ring: The p electrons should be completely delocalized. In simple words, in the ring alternate single and double bonds should exist.
3. Hückel Rule: Presence of (4n + 2) p electrons in the ring where n is an integer (n = 0, 1, 2, . . .).

Chemical properties of benzene