Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School


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

● Generally:

○ 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 

Bromine, present in bromine water, reacts with the double bond, forming the dihalides, the orange-brown color from Bromine disappears and the solution becomes colorless.
Ethene, as all alkenes, decolorize bromine water by producing 1,2 dibromoethane 

CH2=CH2 + Br2  equilibrium arrow  CH2BrCH2Br  

Ethene  + Bromine  equilibrium arrow 1,2-diBromopropene

colorless  +  orange/brown  equilibrium arrow    colorless                       

● 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.

CH2=CH2 + H2O  equilibrium arrow  CH3CH2OH

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)
    • catalyst

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.