Lec preparations of alkenes

• Preparation of alkenes

  • From Alkynes
  • From alkyl halide
  • From vicinal dihalide
  • From alcohol
• From alkyne

  Alkynes can be reduced to alkenes using palladium charcoal catalyst deactivated with poison like sulphur compounds or quoline

  Partially deactivated palladised charcoal isknown as Lindlar's catalyst

  Alkynes can also be reduced to alkenes with sodium in liquid ammonia (birch reduction)
• From alkyl hallides

  Alkyl can be prepared from alkyl hallide by treating with alcoholic postash(potassium hydroxide dissolved in ethanol)

  This reaction removes a molecule of Hx and therefore, the reaction involved is dehydrohalogenation

  In this reaction, the hydrogen atom is eliminated from b carbon atom C next to carbon which halogen is attached.

  Therefore, this reaction is called B elimination reaction
• Nature of halogen atom andalkyl group determine rate of the reaction

  It is observed that for halogens the rate is iodine > bromine > chlorine

  While for alkyl groupfroups tertiary > secondary > primary
• From vicinal dihalide

  dihalogen derivatives of alkanes in which two halogens are attached adjacent carbon atom (called vicinal dihalide derivatives) are coneverted to alkenes by heating with zinc dust in ethyl alcohol
• From alcohol

  Alkenes are prepared from alcohols by heating with protonic acid such as sulphuric acid at about 442 k

  this reaction is called as dehydration of alcohol

  This reaction is also an example of b elimination reaction because Oh group takes out one hydrogen atom
• From alcohol

  Alkenes are prepared from alcohols by heating with protonic acid such as sulphuric acid at about 442 k

  this reaction is called as dehydration of alcohol

  This reaction is also an example of b elimination reaction because Oh group takes out one hydrogen atom