Hydrocarbons Class 11 Notes Chemistry Chapter 9

• Compounds of carbon and hydrogen.
• Classification of Hydrocarbons:
  

Alkane

• Open chain saturated hydrocarbon with general formula (C_nH_2n+2). 
• All the C atoms are single bonded i.e. sp³ hybridised.

Conformations of Alkane

• Conformations are the different arrangement of atoms that can be converted into one another by rotation about single bonds. 
• Eclipsed Conformation: H atoms on two adjacent carbon atoms are closest to each other i.e. dihedral angle is 0.
  
• Staggered Conformation: H atoms on two adjacent carbon atoms are farthest to each other i.e. dihedral angle is 60.
  

Preparation of Alkanes:

• Reduction of Alkyl Halides:
  RX + Zn: + H⁺  → RH + Zn²⁺ + X-
  4RX + LiAlH₄    → 4RH + LiX + AlX₃ (X≠ F)                    
  RX + (n - C₄H₉)₃ SnH  → R-H + (n - C₄H₉)3 SnX
• Grignard Reagent:
           
• Hydrogenation of Alkenes:
  
• Wurtz Reaction:
  2RX  + 2Na  → R-R + 2NaX
  2Na + 2CH₃CH₂CH₂Cl  →  CH₃CH₂CH₂CH₂-CH₂CH₃ + 2NaCl
• Corey House Reaction:
  
• Decarboxylation of a mixture of the sodium salt of a carboxylic acid:
  RCOONa +NaOH(CaO) → RH +  Na₂CO₃
•  Kolbe's electrolytic method:
  2 RCOOK + 2H₂O → R-R + 2CO₂ + H₂+ 2KOH

 Chemical Properties of Alkane

• Direct Halogenation
  RH + X₂→ RX + HX
  Order of Reactivity of X₂:    F₂ > Cl₂ > Br₂; I₂ does not react
  a. Initiation Step
  Cl-Cl2Cl^.
  b. Propagation Step
  H₃C-H +Cl^• → H₃C^• + H-Cl
  H₃C^• + Cl-Cl → H₃C-Cl +Cl^•
  c. Termination Step 
  Cl^• + Cl^• →Cl-Cl
  H₃C^• + H₃C^• → H₃C-CH₃
  Cl^• + H₃C^• → Cl-CH₃
• Nitration 
  Nitration of alkane is made by heating vapours of alkanes and HNO₃ at about 400^oC to give nitroalkanes.
  This is also known as vapour phase nitration.
  
• Combustion:
  Alkanes burn readily with non luminous flame in presence of air or oxygen to give CO₂ & water along with evolution of heat.
  C₂H₆ + 7O₂ → CO₂ +6H₂O + heat
• Aromatization
  Alkanes having six to 10 carbon atoms are converted into benzene and its homologues at high pressure and temperature in presence of catalyst.
  
• Oxidization of 3⁰ alkane:
  Tertiary alkanes are oxidized to tertiary alcohols by KMnO₄
   R₃CH + KMnO₄ → R₃COH

Alkene (olefins)

• Open chain, Unsaturated hydrocarbons with general formula (CnH₂n).
• At least one  >c=c<  (double bond) group i.e. sp² hybridisation, is present throughout the chain.
• Allene: alkene molecule in which at least one C has double bonds with each of the adjacent carbon i.e. -c=c=c- group. 
• Isomeric with saturated cycloalkanes.
  

Geometric Isomers:

Z is used if the higher - priority substituents on each C are on the same side of the double bond. letter E is used if they are on opposite sides
         

Heats of Hydrogenation: Heat of hydrogenation increases with increase in stability of alkene.
       
Order of heat of hydrogenation: 1-Butene> cis-2-Butene > trans-2-Butene
Order of stability: 1-Butene> cis-2-Butene > trans-2-Butene

Preparation of Alkenes:
1. Cracking of petroleum:

2. Dehydrohalogenation of alkyl halides:   RCH₂CH₂X + alc.KOH → RCH = CH₂
3. Dehydration of Alcohols:
Saytzeff Rule: In dehydration and dehydrohalogenation the preferential order for removal of an H is 3° > 2° > 1°

 4. Reduction of alkynes:

Chemical Properties:
1. Electrophilic Polar Addition Reactions

2. Addition of Hydrogen Halides to Alkenes: Markovnikov’s Addition:
R - CH = CH₂ + HBr → R – CHBr – CH₃
Mechanism:
R - CH = CH₂ + HBr → R – CH⁺ - CH₃  +Br^-
R – CH+ - CH₃ + Br^- → R – CHBr - CH₃
Anit- Markovnikov’s Addition (Peroxide Effect):
R - CH = CH₂ + HBr + (C₆H₅CO)₂O₂ → R – CHBr – CH₃
Mechanism
Initiation:
R - O - O - R  →  2RO^•
RO^. +  HBr  →  Br^.   +   ROH
Propagation
CH₃CH = CH₂   + Br^• →   CH₃•CH - CH₂Br
CH₃^•CHCH₂Br + HBr→ CH₃CH₂CH₂Br + Br^•
Termination:
2RO^•  →  R - O - O - R  
Br^• + Br^•→Br₂

3. Addition of Water to Alkenes: Acid Catalyzed Hydration:

4. Oxymercuration-Demercuration:

Examples:

5. Hydroboration-Oxidation:

Examples:

6. Halogen Addition in Non-polar Solvent:

7. Halogen Addition in Aqueous Medium:

8. Syn – Hydroxylation: Formation of di-oles.

9. Ozonolysis of Alkenes:
 

Alkyne

• Saturated open chain hydrocarbon with general formula (C_nH_2n-2).
• At least one -c≡c-  (triple bond) group i.e. sp hybridisation, is present throughout the chain.
• Physical properties of alkynes are similar to those of the corresponding  alkenes

Preparation
1. Dehydrohalogenation of vic-Dihalides or gem-Dihalides

2. Dehalogenation of vic-Tetrahalogen Compounds

3. Alkyl Substitution in Acetylene; Acidity of º C-H
 

4. From Calcium Carbide:
CaC₂ +2H₂O →   Ca(OH)₂+ C₂H₂

5. Kolbe’s Electrolysis:

Chemical Properties
1. Hydrogenation: RC ≡ CCH₂CH₃ + 2H₂ →  CH₃CH₂CH₂CH₂CH₃
2. Hydro-halogenation:
Markovnikov addition: RC≡CH +HBr → RCBr=CH₂ +HBr→ RCBr₂-CH₃
Anti-markovnikov addition: RC≡CH +HBr +peroxide → RCH=CHBr
  

Aromatic Hydrocarbons: 
For being aromatic a hydrocarbon should

• be a cyclic compounds.
• have planarity in geometry.  
• have complete delocalization of electrons over ring.
• follow Huckel Rule i.e. number of ?? electrons in ring = (4n+2).
  

Benzene (C₆H₆)
1. Structure:

2. Chemical Reactions of Benzene:

Anti-aromatic Hydrocarbons:
Highly unstable compounds.
Number of π electrons in ring = 4n.
Example: