CHAPTER 4
CHAPTER 4
HYDROCARBO
HYDROCARBO
NS
NS
(P
(P
ART 2)
ART 2)
Prepared By: Mrs. Shyamala.A
Introduction
Introduction
•
• Alkenes are unsaturated hydrocarbonsAlkenes are unsaturated hydrocarbons (compounds(compounds
containing carbon and hydrogen only) containing a containing carbon and hydrogen only) containing a carbon-carbon double bond.
carbon-carbon double bond.
•
• The first member of the The first member of the alkene falkene family is ethene,amily is ethene,
C
C22HH44. You can work out the formula for any of them. You can work out the formula for any of them using:
Naming of alkene
• Name the parent hydrocarbon:
– select the longest continuous carbon chain that
contains the double bond.
- Prefix is the number of carbon atoms and suffix is
• Indicate the position of the double bond:
- when the carbon chain contains more than 3 carbon atoms, a number is used to indicate the position of the double bond.
- The carbon atoms in the chain are numbered, beginning from the closest to the double bond, so that the carbon
• Indicate the position of the side chains
– the side chains are numbered according to their
position in the chain and are listed alphabetically. Note: Alkene with two double bonds – diene
Alkene with three double bonds – triene
• Being unsaturated, the main reaction of alkene is
addition.
• The electron density of the two carbon atoms,
which are joined by the double bond is high.
• Hence, they are susceptible to attack by
electrophiles.
• Alkenes undergo electrophilic addition.
Ethene and fluorine:
• Ethene reacts explosively with fluorine to give carbon and
hydrogen fluoride gas.
• Ethene and chlorine or bromine or iodine;
in each case you get an addition reaction.
For example, bromine adds to give 1,2-dibromoethane.
• The reaction with bromine happens at room
temperature.
• If you have a gaseous alkene like ethene, you can
bubble it through either pure liquid bromine or a solution of bromine in an organic solvent like
tetrachloromethane.
• The reddish-brown bromine is decolourised as it
• The reaction of the carbon-carbon double bond in
alkenes such as ethene is commonly with hydrogen halides such as hydrogen chloride and hydrogen
bromide.
• This rule states that in the reaction between an
unsymmetrical alkene and an unsymmetrical
reagent (example: A-Y) the more electropositive atom (usually H atom) or group will attach itself to unsaturated carbon atom which carries the most number of hydrogen atoms.
Addition to symmetrical alkenes
• All alkenes undergo addition reactions with the
hydrogen halides. A hydrogen atom joins to one of the carbon atoms originally in the double bond,
and a halogen atom to the other.
• For example, with ethene and hydrogen chloride,
• With but-2-ene you get 2-chlorobutane:
• Reaction rates increase in the order HF - HCl - HBr - HI.
Hydrogen fluoride reacts much more slowly than the other three
Addition to unsymmetrical alkenes
• If HCl adds to an unsymmetrical alkene like propene, there
are two possible ways it could add. However, in practice, there is only one major product.
This is in line with Markownikoff's Rule which says:
• When a compound HX is added to an unsymmetrical
alkene, the hydrogen becomes attached to the carbon with the most hydrogens attached to it already
.
• In this case, the hydrogen becomes attached to the CH2
group, because the CH2 group has more hydrogens than the CH group.
• Notice that only the hydrogens directly attached to the
The reaction with ethene
• Alkenes react with concentrated sulphuric acid in
the cold to produce alkyl hydrogensulphates.
• Ethene reacts to give ethyl hydrogen sulphate.
The reaction with propene
• If sulphuric acid adds to an unsymmetrical alkene like
propene, there are two possible ways it could add.
• However, in practice, there is only one major product.
Reaction of hydrogenation
• Ethene reacts with hydrogen in the presence of a finely
divided nickel catalyst at a temperature of about 150 Ethane is produced.
• This process is used in the manufacture of margarine,
where unsaturated oils are converted to saturated oils. C.
Reaction with water (hydration)
• Ethanol is manufactured by reacting ethene with
Oxidation
–with potassium manganate
(VII)
• Alkenes react with potassium manganate (VII)
solution in the cold. The colour change depends on whether the potassium manganate (VII) is used
under acidic or alkaline conditions.
• If the potassium manganate(VII) solution is
• If the potassium manganate(VII) solution is made
slightly alkaline (often by adding sodium carbonate solution), the purple solution first becomes dark green and then produces a dark brown precipitate
• Manganate(VII) ions are a strong oxidising agent,
and in the first instance oxidise ethene to ethane-1,2-diol.
Oxidation
–
with oxygen
• Alkenes burn in excess oxygen to produce carbon
dioxide and water.
• For example:
Reaction that produce alkenes
a) Dehydration of alcohols
• The dehydration of ethanol to give ethene.
• This is a simple way of making gaseous alkenes like
ethene.
• If ethanol vapour is passed over heated
aluminium oxide powder (at 3600C) or
• To make a few test tubes of ethene, you can use this
b) Elimination of HX from halogenoalkanes
• Halogenoalkanes undergo elimination to produce alkenes
when heated with alcoholic potassium hydroxide.
C C + KOH C=C + H2O + KX H X
