Chapter 4 - Part 2
CHAPTER 4 HYDROCARBONS HYDROCARBO NS (P (PART ART 2) Prepared By: Mrs. Shyamala.A
Alkenes are unsaturated hydrocarbons (compounds containing carbon and hydrogen only) containing a carbon-carbon double bond. The first member of the alkene family family is ethene, C2H4. You can work out the formula for any of them using: CnH2n
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 “ene”.
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 atoms in the double bond have the lowest possible number.
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 Alkene with four double bonds – tetraene
Reactions of alkenes
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.
Reactions with halogens 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 reacts with the alkene.
Reactions with hydrogen halides
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, you get chloroethane.
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 CH 2 group has more hydrogens than the CH group.
Notice that only the hydrogens directly attached to the carbon atoms at either end of the double bond count.
Reaction of sulphuric acid 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.
This is in line with Markownikoff's Rule
Reaction of hydrogenation
Ethene reacts with hydrogen in the presence of a finely divided nickel catalyst at a temperature of about 150 C. Ethane is produced. This process is used in the manufacture of margarine , where unsaturated oils are converted to saturated oils.
Reaction with water (hydration)
Ethanol is manufactured by reacting ethene with steam. The reaction is reversible.
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 acidified with dilute sulphuric acid, the purple solution becomes colourless.
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 ethane1,2-diol.
Oxidation – with oxygen
Alkenes burn in excess oxygen to produce carbon dioxide and water. For example: C6H12
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 360 0C) or concentrated sulphuric acid at 170 0C, the ethanol is essentially cracked to give ethene and water vapour. 24
To make a few test tubes of ethene, you can use this apparatus:
b) Elimination of HX from halogenoalkanes
Halogenoalkanes undergo elimination to produce alkenes when heated with alcoholic potassium hydroxide. C
+ H2O + KX