General Methods of Preparation of Alkenes

 

(i)         By cracking :

 

                                                CH3—CH2—CH3 10_image001.png C2H4 + CH4

 
 

(ii)        By dehydration of alcohols :

 

(a)       Removal of H2O from a substrate molecule by a suitable dehydrating agent e.g. conc. H2SO4, AI2O3, H3PO4, P2O5. Some other dehydrating agents are KHSO4, BF3, dry HCI, CaCI2etc.

 

(b)       This involves α–β elimination.

 

(c)        e.g.                            604_image002.png   RCH = CH2 

 

            RCH2CH2OH            320_image003.png  RCH=CH2

 

                                                1382_image004.png     RCH = CH2

 
 
                        CH3CH2CH—CH3802_image005.pngCH3CH=CHCH3 + CH3CH2CH=CH2
                                    |                                           Major                   Minor
                                  OH                                            (Cf. Saytzeff rule)
 

(d)       The case of dehydration shows the order : tertiary > secondary > primary alcohols.

 

(iii)       By dehydrohalogenation of monohalides:

 

(a)          Removal of HX from a substrate by alcoholic KOH or NaNH2

 

(b)          This too is α–β elimination

 

(c)          RCH2CH2X 1019_image006.png RCH=CH2

 

              CH3CH2CHCH3 733_image007.png CH3CH = CHCH3 + CH3CH2CH=CH2
                       |                                      Major                    Minor
                      X                                          (Cf. Saytzeff rule)
  

            (d)       The case of dehydrohalogenation shows the order:

 

                        For alkyl halides: tertiary > secondary > primary alkyl halides

 

                        For halogens in halides : iodide > bromide > chloride

 

(iv)      By halogenations of dihalides:

 

(a)       Removal of X2 from a substrate Zn dust/CH3OH or Zn—Cu couple in alcoholic solution.

 
(b)       e.g.                  CH3CHX2 733_image007.png CH2=CH2
                                            gem dihalide

 

 

                                    CH2XCH2X 733_image007.png CH2=CH2
                                                                     vicinal dihalide
 

Note : Metallic sodium can also be used in place of zinc.

 

(v)       By Kolbe’s electrolysis :

 

         (a)       Electrolysis of aqueous solutions of sodium or potassium salts of saturated dicarboxylic acids gives alkene.

 

1671_04.JPG

 

(vi)      By partial hydrogenation of alkynes:

 

            (a)       Hydrogenation of alkynes in presence of Lindlar catalyst gives alkenes.

 

                                                CHCH 17_image013.png CH2=CH2

 

                                                RCCH 17_image013.png RCH=CH2

 

(b)       Lindlar catalyst is Pd on CaCO3 deactivated by lead acetate which prevents further hydrogenation.

 

(c)        Use of Pd or Pd-charcoal poisoned by BaSO4 and quinoline give better results. (Cram et. Al) 

 

(vii)      By heating quaternary ammonium compounds : 

 

1162_05.JPG

 

(viii)    By Grignard reagents :

 

                          739_06.JPG

 

(ix)      Action of Copper alkyl on Vinyl Chlorides : Vinyl chloride on alkylation with copper alkyl form higher alkenes.

 
                        2H2C = CHCI 793_image028.png 2H2C=CHR + CuCI2
                                                               alkene

 

                        2H2C = CHCI 1703_image029.png 2H2C=CHR + CuCI2
                                                                                      propene
Related Resources
Miscellaneous Reactions Shown by Alkynes

Alkenes and Alkynes Miscellaneous Reactions shown...

Chemical Nature of Alkynes

Alkenes and Alkynes Fundamentals of alkenes...

Chemical Nature of Alkenes

Alkenes and Alkynes Fundamentals of alkenes...

Fundamentals of Alkynes or Acetylenes

Fundamentals of Alkynes or Acetylenes (i) Alkynes...

Addition Reactions of Alkenes

Addition Reactions of Alkenes Addition reactions...

Physical properties of alkynes

Alkenes and Alkynes Physical Properties of Alkynes...

Test for Unsaturation

Alkenes and Alkynes Fundamentals of alkenes...

Ozonolysis of Alkenes

Ozonolysis of Alkenes Ozonolysis of alkenes : A...

Oxidation of Alkenes

Alkenes and Alkynes Fundamentals of alkenes...

Miscellaneous Reactions of Alkenes

Following are the miscellaneous reactions of...

Addition Reactions of Alkynes

Alkenes and Alkynes Fundamentals of alkenes...

Methods of Preparation of Alkynes

Alkenes and Alkynes Fundamentals of alkenes...

Dienes or Diolefins or Alkadienes

Alkenes and Alkynes Fundamentals of alkenes...

Oxidation of Alkynes

Alkenes and Alkynes Oxidation of Alkynes: (a)...

Fundamentals of Alkenes

Alkenes and Alkynes Fundamentals of alkenes...

Physical Properties of Alkenes

Alkenes and Alkynes Physical Properties of Alkenes...