alkenes have a high electron density at the C=C bonds.
And so they can be attacked by substances we call electrophites (electron deficit species) some substances are added to the carbon atoms with the double bond.
hydrogenation/addition of hydrogen
alkenes react with hydrogen in the presence of a nickel catalyst at about 152 degrees to form alkanes e.g
this type of reaction is useful in converting unsaturated oils into margarine.
they react with steam in the presence of phosphoric acid (catalyst) to form alcohols.
ethene reacts to form ethanol:
with hydrogen halide
react with hydrogen halide to form a halogenoalkane the reactivity decreases with the bond strength of the hydrogen halide.
Ethene reacts with hydrogen chloride chloroethane:
they react very fast with halogens to form (di)halogeno alkane. the reaction between bromine and alkenes is used as a test for unsaturation. And also potassium promanganate.
with dilute manganate 7
alkenes are quickly oxidised by dilute manganate 7 to form diol (contains OH) in this reaction the OH group is added to the carbons which had the C=C.
self addition polymerisation
in the presence of a catalyst, right temperature and pressure they undergo polymerisation to form polyalkenes.
some compounds containing C=C may also undergo polymerisation to form polymers.