Hydrocarbons Class 11 Notes Chemistry Chapter 9

 Page 1


 ? Preparation of Alkanes :
  (1) From unsaturated hydrocarbons :
  (2) Wurtz reaction 
        R–X + 2Na + X–R   R–R + 2 NaX
  (3) Decarboxylation :
  [Sodium salt of carboxylic acid + NaOH + CaO ? Alkane [n
c
 = 1 less]
           
Page 2


 ? Preparation of Alkanes :
  (1) From unsaturated hydrocarbons :
  (2) Wurtz reaction 
        R–X + 2Na + X–R   R–R + 2 NaX
  (3) Decarboxylation :
  [Sodium salt of carboxylic acid + NaOH + CaO ? Alkane [n
c
 = 1 less]
           
       (4) Kolbe’s Electrolysis :
 
Current
Potassiumsalt of carboxylicacid(aq) Higher alkane
??
?????
??
??
? Chemical Properties of Alkanes :
 (1) Halogenation : One (H) atom is replaced by halogen at a time.
CH
4
(g) + Cl
2
(g)  CH
3
Cl + CH
2
Cl
2
 + CHCl
3
 + CCl
4
 (2) Aromatization : n-Hexane 
? Preparation of alkenes :
(1) From alkynes [Alkyne + H
2
 — ? Alkene]
(2) From alkyl halide by (dehydrohalognation)
Page 3


 ? Preparation of Alkanes :
  (1) From unsaturated hydrocarbons :
  (2) Wurtz reaction 
        R–X + 2Na + X–R   R–R + 2 NaX
  (3) Decarboxylation :
  [Sodium salt of carboxylic acid + NaOH + CaO ? Alkane [n
c
 = 1 less]
           
       (4) Kolbe’s Electrolysis :
 
Current
Potassiumsalt of carboxylicacid(aq) Higher alkane
??
?????
??
??
? Chemical Properties of Alkanes :
 (1) Halogenation : One (H) atom is replaced by halogen at a time.
CH
4
(g) + Cl
2
(g)  CH
3
Cl + CH
2
Cl
2
 + CHCl
3
 + CCl
4
 (2) Aromatization : n-Hexane 
? Preparation of alkenes :
(1) From alkynes [Alkyne + H
2
 — ? Alkene]
(2) From alkyl halide by (dehydrohalognation)
 Carbon attached with halogen is a-carbons
Carbon attached with a-carbons is ß-carbons 
Halogen is removed and ‘H’-atom is removed from ß-carbon to form  
(C = C) double bond.
(3) By dehydration of alcohols (Loss of water molecule)
 Carbon attached to alcohoic group is a-carbon.
 Carbon attached to a-carbon is ß-carbon.
(4) From vicinal dihalides [Compounds in which halogen atom are 
attached with adjacent carbons]
? Chemical Properties of Alkenes :
(1) Addition of Halogens :
(2) Addition of H—X : 
 (A) Markownikov’s rule (M.R.) : During electrophillic addition of  
	 hydrogen	 halide,	 the	 electron 	 deficient	 electrophile	 (E
+
) always attack 
Page 4


 ? Preparation of Alkanes :
  (1) From unsaturated hydrocarbons :
  (2) Wurtz reaction 
        R–X + 2Na + X–R   R–R + 2 NaX
  (3) Decarboxylation :
  [Sodium salt of carboxylic acid + NaOH + CaO ? Alkane [n
c
 = 1 less]
           
       (4) Kolbe’s Electrolysis :
 
Current
Potassiumsalt of carboxylicacid(aq) Higher alkane
??
?????
??
??
? Chemical Properties of Alkanes :
 (1) Halogenation : One (H) atom is replaced by halogen at a time.
CH
4
(g) + Cl
2
(g)  CH
3
Cl + CH
2
Cl
2
 + CHCl
3
 + CCl
4
 (2) Aromatization : n-Hexane 
? Preparation of alkenes :
(1) From alkynes [Alkyne + H
2
 — ? Alkene]
(2) From alkyl halide by (dehydrohalognation)
 Carbon attached with halogen is a-carbons
Carbon attached with a-carbons is ß-carbons 
Halogen is removed and ‘H’-atom is removed from ß-carbon to form  
(C = C) double bond.
(3) By dehydration of alcohols (Loss of water molecule)
 Carbon attached to alcohoic group is a-carbon.
 Carbon attached to a-carbon is ß-carbon.
(4) From vicinal dihalides [Compounds in which halogen atom are 
attached with adjacent carbons]
? Chemical Properties of Alkenes :
(1) Addition of Halogens :
(2) Addition of H—X : 
 (A) Markownikov’s rule (M.R.) : During electrophillic addition of  
	 hydrogen	 halide,	 the	 electron 	 deficient	 electrophile	 (E
+
) always attack 
on that doubly/triply bounded carbon atom. which already has greater 
number of hydrogen atoms.
 (B) Peroxide/Kharasch effect (Anti M.Rule) : This 	 effect	 takes	 place 	 in	
presence of organic peroxides when the hydrogen free radical  attacks 
on that doubly bonded carbon which has lesser number of hydrogen  
atoms.
(3) Ozonolysis
 In this reaction all those carbons which form double bonds get finally 
converted into carbonyl carbons.  If alkenes are symmetrical then both 
carbonyl compounds are same.
If more than two double bonds are present then we get atleast one 
compound which has two carbonyl groups at the end. Such bifunctional 
compounds are formed from that part of alkene which is in between 
the double bonds.
For cyclic alkenes : 
Page 5


 ? Preparation of Alkanes :
  (1) From unsaturated hydrocarbons :
  (2) Wurtz reaction 
        R–X + 2Na + X–R   R–R + 2 NaX
  (3) Decarboxylation :
  [Sodium salt of carboxylic acid + NaOH + CaO ? Alkane [n
c
 = 1 less]
           
       (4) Kolbe’s Electrolysis :
 
Current
Potassiumsalt of carboxylicacid(aq) Higher alkane
??
?????
??
??
? Chemical Properties of Alkanes :
 (1) Halogenation : One (H) atom is replaced by halogen at a time.
CH
4
(g) + Cl
2
(g)  CH
3
Cl + CH
2
Cl
2
 + CHCl
3
 + CCl
4
 (2) Aromatization : n-Hexane 
? Preparation of alkenes :
(1) From alkynes [Alkyne + H
2
 — ? Alkene]
(2) From alkyl halide by (dehydrohalognation)
 Carbon attached with halogen is a-carbons
Carbon attached with a-carbons is ß-carbons 
Halogen is removed and ‘H’-atom is removed from ß-carbon to form  
(C = C) double bond.
(3) By dehydration of alcohols (Loss of water molecule)
 Carbon attached to alcohoic group is a-carbon.
 Carbon attached to a-carbon is ß-carbon.
(4) From vicinal dihalides [Compounds in which halogen atom are 
attached with adjacent carbons]
? Chemical Properties of Alkenes :
(1) Addition of Halogens :
(2) Addition of H—X : 
 (A) Markownikov’s rule (M.R.) : During electrophillic addition of  
	 hydrogen	 halide,	 the	 electron 	 deficient	 electrophile	 (E
+
) always attack 
on that doubly/triply bounded carbon atom. which already has greater 
number of hydrogen atoms.
 (B) Peroxide/Kharasch effect (Anti M.Rule) : This 	 effect	 takes	 place 	 in	
presence of organic peroxides when the hydrogen free radical  attacks 
on that doubly bonded carbon which has lesser number of hydrogen  
atoms.
(3) Ozonolysis
 In this reaction all those carbons which form double bonds get finally 
converted into carbonyl carbons.  If alkenes are symmetrical then both 
carbonyl compounds are same.
If more than two double bonds are present then we get atleast one 
compound which has two carbonyl groups at the end. Such bifunctional 
compounds are formed from that part of alkene which is in between 
the double bonds.
For cyclic alkenes : 
(D) With potassium paramagnate :
(i) Cold dilute alkaline KMnO
4
 = Bayer’s reagent.
 
[ ]
4
Alkene+colddil.KMnO Diol ?
Bayer’s test for the presence of (C = C) bond 
Compound + cold dil. alk. KMnO
4 
—? Purple colour decolourised
 ? Compound is alkene.
 (ii) Hot KMnO
4
 
 + Hot KMnO
4
 
+ 
Alkynes
? Preparation :
 (1) CaC
2
 + 2H
2
O —? Ca(OH)
2
 + HC = CH
Calcium carbide        Acetylene
(2) From vicinal dihalides
? Chemical properties :
(1) Hydration [Addition of water]
—
—