Alkenes

         These are the acyclic hydrocarbon in which carbon-carbon contain double bond. These are also defined as olefins, due to lower alkene react with halogens to form oily substances. General formula is  C_nH_2n. Examples, C₂H₄, C₃H₆, C₄H₈.

         (1) Preparation methods

         (i) From Alkynes :

         q Poison's catalyst such as BaSO₄, CaCO₃ are used to stop the reaction after the formation of alkene.

         (ii) From mono halides :

         q If we use alc. NaOH in place of KOH then trans product is formed in majority because of its stability. According to saytzeff rule.

         (iii) From dihalides

(a)   From Gem dihalides

         q If we get two different kinds of gemdihalides then we get three different types of alkenes .

         q Above reaction is used in the formation of symmetrical alkenes only.

         (b) From vicinal dihalides :

         q Alkene is not formed from 1, 3 dihalides. Cycloalkanes are created by dehalogenation of it.

         (iv) By action of NaI on vicinal dihalide : 

         (v) From alcohols [Laboratory method] :

         (vi) Kolbe's reaction :

         (vii) From esters [Pyrolysis of ester] :

         (viii) Pyrolysis of quaternary ammonium compounds :

(ix) Action of copper alkyl on vinyl chloride :

         (x) By Grignard reagents :

         (xi) The wittig reaction :

         (xii) From b bromo ether [Boord synthesis]

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