- Alkenes are a Homologous Series composed of unsaturated Hydrocarbons, since they contain at least one C=C Double Bond. They have a general formula CnH2n. Since they are unsaturated, they are quite reactive.
Alkenes are named with an ‘ene’ suffix. Often, in naming Alkenes with more than three Carbons, the position of the Double Bond is identified. For example, Penta-2-ene means the double bond starts on Carbon 2.
If an Alkene has more than one Double Bond, this is shown in the name, and the position of all the double bonds are identified. For example, Hex-1,2,4-triene represents a 6 Carbon Alkene with Double Bonds on Carbons 1, 2 and 4.
Like Alkanes, Alkenes can form Branched Structures, and Cyclic Structures, called Cycloalkenes.
Electrophilic Addition Mechanism
The Double Bond in and Alkene is a region of high density negative charge. Positive Ions or molecules with a partially positively charged region may be attracted to this Double Bond, and act as Electrophiles, accepting a pair of Electrons from it.
The Electrophilic Addition Mechanism describes the way in which some molecules are thought to react with the Double Bond in an Alkene. One example of such a reaction is the reaction with Bromine, which is a test for Unsaturation.
The Bromine molecule is thought to become Polarised by the negative charge of the Double Bond, so that electrons are pushed to the other end of the Bromine molecule. This means that the Bromine atom nearest the Double Bond becomes slightly positive.
This Bromine atom then reacts with the Double Bond like an Electrophile, accepting a pair of electrons and bonding to one of the Carbon atoms. This leaves a negatively charged Bromine Ion, and a positively charged Carbon atom in the Alkene, called a Carbocation.
The Bromine Ion reacts with the Carbocation so that it bonds with it. This mechanism produces Dibromoethene.
This is the mechanism for numerous reactions with Alkenes. Water reacts with Alkenes, slitting into OH- and H+ Ions, forming an Alcohol when bonded to the Alkene. Hydrogen will react like this as well. This process is known as Hydrogenation, and occurs at 150°C, 5atm, with a Platinum or Nickel Catalyst.
If an Alkene is reacted with Bromine in the presence of Chlorine atoms, some of the products will contain a Chlorine atom instead of one of the Bromine atoms. This gives evidence in support of the mechanism for Electrophilic Addition.