C13 - Alkenes Flashcards
What are alkenes?
Unsaturated hydrocarbons with at least 1 carbon to carbon double bond.
Aliphatic alkenes with a double bond have the formula CnH2n.
They can be branched, contain more than one double bond or be cyclic.
(Only branched will obey the general formula)
How is a double bond formed?
For each of the carbon atoms belonging to the double bond, three of the four electrons are used in three sigma bonds (one to the other carbon atom of the double bond and the other two to two other atoms).
This leaves one electron on each carbon atom of the double bond not involved in the sigma bond.
This electron belongs in a p-orbital.
This forms a pi bond (the sideways overlap of 2 p-orbitals). The pi electron density is concentrated above and below the line joining the nuclei of the bonding atoms.
Why are atoms joined by double bonds unable to rotate?
The pi bond locks the 2 carbon atoms in position and prevents them from rotating.
This makes the geometry of the alkenes different from that of alkanes where rotation is possible around every atom.
What is the shape of atoms around a double bond?
Trigonal planar
This is because:
- there are 3 regions of electron density around each of the carbon atoms
- the three regions repel each other as far apart as possible so the bond angle around each carbon atom is 120°
- all of the atoms are in the same plane
What are stereoisomers?
Isomers which have the same structural formula (as well as molecular formula) but in a different arrangement of the atoms in space.
What are the two types of stereoisomers?
E/Z isomerism
Optical isomerism
What is the difference between E/Z and optical isomerism?
E/Z isomerism only occurs in compounds with a C=C bond whereas optical isomerism can occur in a much wider range of compounds.
Why do double bonds not allow free rotation?
The pi bond has regions of electron density above and below the plane of the sigma bond.
What is necessary for E/Z isomerism to occur?
- a C=C double bond
- different groups attached to each carbon atom of the double bond
Do but-1-ene and but-2-ene have E/Z isomerisms?
But-2-ene can has it has a methyl group and H atom on each of its binding carbon atoms however but-1-ene cannot as it has two H atoms in one side.
What is an E and Z isomer?
E: the higher priority groups are on opposite sides of the bond.
(=)
Z: the higher priority groups are both on the same side.
(=/)
What is cis-trans isomerism?
A special case of E/Z isomerism where one of the attached groups on each carbon of the double bond MUST be hydrogen
The cis isomer has the H atom on the same side of the molecule whereas the trans isomer has the H atoms on opposite sides of the isomer.
How does cis-trans isomerism link to E/Z isomers?
The cis isomer is the Z isomer - the higher priority groups are both on the same side.
The trans isomer is the E isomer - the higher priority groups are on opposite sides of the
When is cis-trans isomerism used?
When there is a C=C double bond with different groups on both carbons and a hydrogen atom bonded to both carbons.
They are determined by the priority of of atoms bonded to the carbons.
How is priority of bonding atoms in isomerism determined?
Based on atomic number.
The greater the atomic number, the higher the priority.
High priority on same side = Z isomer
High priority on opposing sides = E isomer
