Lec preparations of alkenes

Cards (7)

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

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