Alkenes
Structure and Bonding in Alkenes
• Alkenes all contain a double carbon-carbon bond, which is responsible for their reactivity.
• The C=C bond contains a strong σ bond and a weaker π bond.
• The 3 pairs of bonding electrons around each carbon creates a flat molecule, as all of the atoms lie in one
plane.
• The double bond is an area of high electron density.
Electrophilic Addition
Electrophiles
• An electrophile is a species that is attracted to electron-rich regions in other molecules or ions.
• They can either carry a full or partial positive charge.
• The double bond in alkenes is an area of high electron density and so will attract electrophiles, such as HBr or
H2SO4.
In the addition reaction, the pi bond breaks, with the pair of electrons
being used to join other atoms, or groups, onto the alkene, creating a
saturated compound.
E.g. HBr is a polar molecule, with a δ+ charge on the H, which will be
attracted to the alkene.
14
, Electrophilic Addition
Electrophilic Addition with Hydrogen Bromide (HBr) - Haloalkanes
Step 1:
• The δ+ charge of H in HBr acts as an electrophile and strongly attracts the electrons in the pi bond (which are
relatively exposed).
• The electrons from the pi bond move towards the δ+ H, forming a covalent bond, which repels the electrons in
the HBr bond even futher towards Br, until the bond breaks.
Step 2: The lone pair on Br- is strongly attracted to the carbocation.
Ethene + HBr
Cyclohexene + HBr
Electrophilic Addition Reactions with Unsymmetrical Alkenes
Ethene+ HBr → bromoethane (one possible product)
Propene + HBr → 1-bromopropane OR 2-bromopropane (two possible products)
If an alkane is unsymmetrical about its double bond, such as propene, two different products could result, depending
on which end of the double bond reacts with the electrophile.
The products will likely be in unequal measure.
Example: Addition of H-X to Propene
Possibility 1:
Possibility 2:
15
Structure and Bonding in Alkenes
• Alkenes all contain a double carbon-carbon bond, which is responsible for their reactivity.
• The C=C bond contains a strong σ bond and a weaker π bond.
• The 3 pairs of bonding electrons around each carbon creates a flat molecule, as all of the atoms lie in one
plane.
• The double bond is an area of high electron density.
Electrophilic Addition
Electrophiles
• An electrophile is a species that is attracted to electron-rich regions in other molecules or ions.
• They can either carry a full or partial positive charge.
• The double bond in alkenes is an area of high electron density and so will attract electrophiles, such as HBr or
H2SO4.
In the addition reaction, the pi bond breaks, with the pair of electrons
being used to join other atoms, or groups, onto the alkene, creating a
saturated compound.
E.g. HBr is a polar molecule, with a δ+ charge on the H, which will be
attracted to the alkene.
14
, Electrophilic Addition
Electrophilic Addition with Hydrogen Bromide (HBr) - Haloalkanes
Step 1:
• The δ+ charge of H in HBr acts as an electrophile and strongly attracts the electrons in the pi bond (which are
relatively exposed).
• The electrons from the pi bond move towards the δ+ H, forming a covalent bond, which repels the electrons in
the HBr bond even futher towards Br, until the bond breaks.
Step 2: The lone pair on Br- is strongly attracted to the carbocation.
Ethene + HBr
Cyclohexene + HBr
Electrophilic Addition Reactions with Unsymmetrical Alkenes
Ethene+ HBr → bromoethane (one possible product)
Propene + HBr → 1-bromopropane OR 2-bromopropane (two possible products)
If an alkane is unsymmetrical about its double bond, such as propene, two different products could result, depending
on which end of the double bond reacts with the electrophile.
The products will likely be in unequal measure.
Example: Addition of H-X to Propene
Possibility 1:
Possibility 2:
15
