Mechanisms of addition reactions
The first point we must consider here is what is happening in addition reactions (the movement of atoms) more specifically the movement of electrons. We can represent the movement of electrons with curly arrows.
Curly arrows must either:
- Start from a bond and move to an atom, or
- Start from a lone pair of electrons and move to an atom.
Curly arrows must either:
- Start from a bond and move to an atom, or
- Start from a lone pair of electrons and move to an atom.
Ethene an alkene has a Pi bond which is a region of high electron density. This makes the alkene molecule attractive to other species that are electron deficient, including molecules with polar bonds.
Electrophiles
The term electrophile comes from the term electron loving. An electrophile likes negative charges, specifically they attack the center of a negative charge.
The term electrophile comes from the term electron loving. An electrophile likes negative charges, specifically they attack the center of a negative charge.
When a hydrogen bromide molecule approaches and ethene molecule the H end (slightly positive) is attracted to the electrons in the Pi bond in C=C. The H-Br molecule is described as being an electrophile when it acts in this way. This is called electrophilic addition.
The hydrogen bromide molecule breaks, electrons in the H-Br bond go to one atom in this case the Bromine (the reason for this is it being more electronegative).
This process is known as heterolytic fission.
(This is one form of breaking a bond which results in the formation of ions. In a covalent bond the pair of electrons are shared between 2 atoms and in heterolytic fission this pair of electrons moves to one of the atoms).
This process is known as heterolytic fission.
(This is one form of breaking a bond which results in the formation of ions. In a covalent bond the pair of electrons are shared between 2 atoms and in heterolytic fission this pair of electrons moves to one of the atoms).
This entire process is known as the electrophilic addition of hydrogen halides.
Step 1
Two ions are formed. The bromine ion (-) and the carbocation which has a (+) charge on the carbon.
Step 1
Two ions are formed. The bromine ion (-) and the carbocation which has a (+) charge on the carbon.
Step 2
The two oppositely charged ions attract each other and react to form a new covalent bond as one of the lone pairs of electrons forms a covalent bond with the carbon atom in carbocation
The two oppositely charged ions attract each other and react to form a new covalent bond as one of the lone pairs of electrons forms a covalent bond with the carbon atom in carbocation
Electrophilic addition of halogens
This reaction is almost identical to the reaction with hydrogen halides. However the only difference is the Br-Br molecule is not polar. Instead as it approaches the C=C bond, the electrons in the pi bond repel electrons in the Br-Br bond and induce (cause) the molecule to become polar.
This reaction is almost identical to the reaction with hydrogen halides. However the only difference is the Br-Br molecule is not polar. Instead as it approaches the C=C bond, the electrons in the pi bond repel electrons in the Br-Br bond and induce (cause) the molecule to become polar.
