Alkenes are hydrocarbons with double bonds between 2 consecutive Carbon atoms
Physical Properties of Alkenes
- They have the General Formula: CnH2n with all C–C bonds. Example: C2H4 – Ethene.
- The simplest alkenes are gases at room temperature, then liquids, finally solids, due to increased molecular mass. This decrease in volatility is due to increasing Van der Waal’s forces
- They have typical covalent, physical properties (i.e. almost insoluble in water, soluble in organic solvents)
- The boiling point of each alkene is just a bit lower but very similar to that of the alkane with the same number of carbon atoms. Ethene, propene and most of butenes are gases at room temperature
Describe the manufacture of alkenes and of hydrogen by cracking
● Hydrocarbons can be cracked to produce smaller, more useful molecules. This
process involved heating the hydrocarbons to vaporise them.
● The vapours are:
o Either passed over a hot catalyst (silica or alumina)
o Mixed with steam and heated to a very high temperature (temperature
in the range of 600-700˚C) so that thermal decomposition reactions can
● The products of cracking include shorter chain alkanes and alkenes (or hydrogen)
Cracking an ALKANE to obtain an ALKENE
Distinguish between saturated and unsaturated hydrocarbons: from
molecular structures, by reaction with aqueous bromine
● From molecular structures:
o Unsaturated = contain one or more C=C double bonds e.g. alkenes
o Saturated = contain no C=C double bonds e.g. alkanes
● By reaction with aqueous bromine:
o Unsaturated hydrocarbons react with bromine in an addition reaction,
decolourising it (orange to colourless) – shown above with the example
of ethene reacting with bromine
o Saturated hydrocarbons do not react with bromine and therefore the
solution will remain orange
(Extended only) Describe the properties of alkenes in terms of addition
reactions with bromine, hydrogen and steam
○ Addition reactions
○ Involves the removal of C=C double bond
○ C=C is very reactive and can easily react to form –C-C-
● reaction with bromine:
○ alkene + bromine → dibromoalkane
○ E.g. Ethene + bromine → 1,2-dibromoethane
Testing ethene with bromine water
● reaction with steam:
○ alkene + steam → alcohol
○ E.g. Ethene + steam → ethanol
Adding steam to the double bond: Preparing ethanol
Ethanol is manufactured by reacting ethene with steam. The reaction is reversible.
conditions: 300 ºC and 60-70 Atm. pressure
Removing the ethanol as it forms shifts the equilibrium towards the formation of more alcohol.
● reaction with hydrogen:
○ alkene + hydrogen → alkane
○ E.g. Ethene + hydrogen → ethane
Adding Hydrogen to the double bond
We will work with ethene but the same reaction occurs with any alkene.
Ethene reacts with hydrogen using a nickel catalyst at around 150°C.
The product of this reaction is ethane.
Ni/150° to 200°
CH2=CH2 + H2 CH3CH3
For this reaction or similar you should remember
- what saturated means
- conditions of the reaction (temperature and pressure if specified)
This reaction is used to hydrogenate vegetable oil to produce margarine.
Not all margarines are made by hydrogenation of the double bonds in fat acids. When an oil is hydrogenated and loses all the double bonds, it is SATURATED. It cannot accept more hydrogens in the molecule. If the molecule has one double bond is MONOUNSATURATED, and if it has several double bonds is called POLYUNSATURATED.