ALKENES
- Unsaturated hydrocarbons (double bond)
- The double bond makes them more reactive than alkanes because of the high concentration of
electrons between the two carbon atoms possessing the double bond.
- Produced in large quantities when crude oil is thermally cracked.
- 𝐶𝑛𝐻2𝑛
In alkanes the bond rotates due to the movement of electrons, but in alkenes there is none. This is because of
the p orbital on each carbon. These p orbitals produce the pi-orbital as they overlap and form a cloud of
electron density above and below the single bond. (*as the p orbitals spin and rotate between the bond they
end up overlapping and joining together to form a double bond and a pi-orbital*)With the presence of the
pi-orbital, the bond cannot rotate. (aka restricted rotation)
Q: If pi orbital cause restricted rotation, how is twisting/ switching of the attached functional groups
possible?
A: Stereoisomerism is still able to happen because of the 50:50 chance of the group being on one side or the
other before it is made into an alkene because it is constantly spinning. Once it becomes an alkene it stays in
a fixed position because of the pi orbitals.
However, if it is enzyme catalysed then that will create a preference to one type of isomer.
ISOMERISM
1. Structural isomers
2. Stereoisomerism
- MOLECULES WITH THE SAME MOLECULAR AND STRUCTURAL
FORMULAS BUT A DIFFERENT ARRANGEMENT OF THE ATOMS IN SPACE.
- GEOMETRICAL ISOMERS - example: E-Z isomers - two stereo isomers have the same
structural formula but the bonds are arranged differently in space.
E/Z isomerism:
- Impossible to happen if the functional group/alkyl is attached to the SAME carbon atom
- Z isomers occur when attached group/atoms are on the same side of the molecule (when switched)
- E isomers occur when attached group/ atoms are on the opposite sides of the molecule (when
switched)
- Unsaturated hydrocarbons (double bond)
- The double bond makes them more reactive than alkanes because of the high concentration of
electrons between the two carbon atoms possessing the double bond.
- Produced in large quantities when crude oil is thermally cracked.
- 𝐶𝑛𝐻2𝑛
In alkanes the bond rotates due to the movement of electrons, but in alkenes there is none. This is because of
the p orbital on each carbon. These p orbitals produce the pi-orbital as they overlap and form a cloud of
electron density above and below the single bond. (*as the p orbitals spin and rotate between the bond they
end up overlapping and joining together to form a double bond and a pi-orbital*)With the presence of the
pi-orbital, the bond cannot rotate. (aka restricted rotation)
Q: If pi orbital cause restricted rotation, how is twisting/ switching of the attached functional groups
possible?
A: Stereoisomerism is still able to happen because of the 50:50 chance of the group being on one side or the
other before it is made into an alkene because it is constantly spinning. Once it becomes an alkene it stays in
a fixed position because of the pi orbitals.
However, if it is enzyme catalysed then that will create a preference to one type of isomer.
ISOMERISM
1. Structural isomers
2. Stereoisomerism
- MOLECULES WITH THE SAME MOLECULAR AND STRUCTURAL
FORMULAS BUT A DIFFERENT ARRANGEMENT OF THE ATOMS IN SPACE.
- GEOMETRICAL ISOMERS - example: E-Z isomers - two stereo isomers have the same
structural formula but the bonds are arranged differently in space.
E/Z isomerism:
- Impossible to happen if the functional group/alkyl is attached to the SAME carbon atom
- Z isomers occur when attached group/atoms are on the same side of the molecule (when switched)
- E isomers occur when attached group/ atoms are on the opposite sides of the molecule (when
switched)
