ALKENESHydrogen (H 2 ) H–H
The addition of hydrogen to an alkene is called hydrogenation and the product is an
alkane. Hydrogenation is carried out in the presence of a catalyst (nickel, palladium
or other platinum metals) and at a high pressure of hydrogen:
HH
Ni ||
RCH=CH 2 H—H—RCH—CH 2
500 °CHydrogen halides (HCl) H–Cl, (HBr) H–Br and (HI) H–I
The gaseous hydrogen halides react directly with the alkene to produce halogenoalkanes:
Cl H
||
RCH=CH 2 H—ClRCH—CH 2Sulfuric acid (H 2 SO 4 ) H–OSO 3 H
Alkenes react with cold, concentrated sulfuric acid to form alkyl hydrogensulfates:
OSO 3 HH
||
RCH=CH 2 H—OSO 3 HRCH——CH 2- Oxidation
Alkenes can be oxidized by cold, slightly alkaline potassium permanganate (man-
ganate(VII)) to make substances called 1,2 diols, containing two OH groups on
adjacent carbon atoms. The potassium permanganate solution turns from purple to
brown. This is another test (theBaeyer test) that is often used to detect unsatura-
tion in a hydrocarbon:
OH OH
KMnO 4 ||
CH 2 =CH 2 —CH 2 —CH 2
ethane-1,2-diol- Polymerization
Under the right conditions, alkenes will undergo addition reactions with each other.
The reaction is called polymerization. For example, many ethene molecules will join
together to form polythene – the double bonds break and the molecules link up:
O 2 , heat, pressure
n(CH 2 =CH 2 ) —————···CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 ···
wheren= a large number or catalyst
(typically 20 000)The product can also be written as
···(—CH 2 —CH 2 —)n···Long chain molecules called polymers are formed from the small ethene
molecules, themonomers. The long, thin molecules of a polymer may also be called
linear macromolecules. Polythene is an unreactive solid; it can be easily moulded
and is used for plastic bags, bottles, washing-up bowls and plastic piping. The word
‘plastic’ means ‘easily moulded’. Substituted ethene molecules can undergo the same
addition reaction to give other polymers:
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