reaction. It is observed that for halogens, the rate is: iodine > bromine > chlorine, while for alkyl groups it is : tertiary > secondary > primary. . From vicinal dihalides : Dihalides in which two halogen atoms are attached to two adjacent carbon atoms are known as vicinal dihalides .
Vicinal dihalides on treatment with zinc metal lose a molecule of ZnX to form an alkene. This reaction is known as dehalogenation . CH Br–CH Br + Zn CH =CH + ZnBr ( . ) CH CHBr–CH Br + Zn CH CH=CH +ZnBr ( .
) . From alcohols by acidic dehydration : You have read during nomenclature of different homologous series in Unit that alcohols are the hydroxy derivatives of alkanes. They are represented by R–OH where, R is C n H 2n+ . Alcohols on heating with concentrated sulphuric acid form alkenes with the elimination of one water molecule.
Since a water molecule is eliminated from the alcohol molecule in the presence of an acid, this reaction is known as acidic dehydration of alcohols . This reaction is also the example of β -elimination reaction since –OH group takes out one hydrogen atom from the β -carbon atom. ( . ) .
. Properties Physical properties Alkenes as a class resemble alkanes in physical properties, except in types of isomerism and difference in polar nature. The first three members are gases, the next fourteen are liquids and the higher ones are solids. Ethene is a colourless gas with a faint sweet smell.
All other alkenes are colourless and odourless, insoluble in water but fairly soluble in non-polar solvents like benzene, petroleum ether. They show a regular increase in boiling point with increase in size i.e., every – CH group added increases boiling point by – K. Like alkanes, straight chain alkenes have higher boiling point than isomeric branched chain compounds. Chemical properties Alkenes are the rich source of loosely held pi ( π ) electrons, due to which they show addition reactions in which the electrophiles add on to the carbon-carbon