Alkenes Flashcards
Are alkenes saturated or unsaturated
Unsaturated because they contain at least one carbon - carbon double bond
What is the general formula for an alkene
CnH2n
Do alkenes form a homologous series
Yes
For a carbon of the double bond, how many sigma bonds are formed and between which atoms are they formed
3 sigma bonds, 1 between the carbon in which it’s double bonded too and 2 to the other atoms e.g. hydrogen or carbon
How is a Pi bond formed
Formed by the sideways overlap of 2 p-orbitals, one from each carbon atom of the double bond
Where is the Pi- electron density concentrated in a double bond
Above and below the line joining the nuclei of the bonding atoms
What bond (sigma or Pi) locks the carbon atoms in an alkene in place, preventing them from rotating around the double bond
Pi bond
How does the geometry differ in alkanes and alkenes
Rotation is possible in alkanes since there is no Pi bond whereas rotation cannot happen in alkenes due to the presence of a Pi bond.
What is the bond angle and shape around the carbon atoms in the double bond in an alkene
Trigonal planar (120)
- There are 3 regions of electron density around each carbon atom
What is a stereoisomer
A compound with the same structural formula but a different arrangement of atoms in space
What type of isomerism occurs in compounds with C=C double bond
E/Z isomers
Why does stereoisomers arise around a double bond
Because rotation around the double bond is restricted so the groups attached to each carbon is fixed
Why are the groups attached to each double bonded carbon restricted
Due to the position of the Pi bond above and below the C=C double bond
What 2 conditions are required for a molecule to have E/Z isomerism
- A C=C double bond
- Different groups attached to each carbon atom of the double bond
Why can But-1-ene not form E/Z isomers whereas But-2-ene can
Because one of the carbons in the double bond in But-1-ene is attached to 2 hydrogens meaning that there isn’t a different group attached to each carbon
What is cis-trans isomerism
The name commonly used to describe a special case of E/Z isomerism.
What extra rule differs E/Z isomerism and cis-trans isomerism
In cis-trans isomerism one of the attached groups on each carbon atom of the double bond must be hydrogen
Is But-2-ene an example of a molecule with E/Z isomers or cis-trans isomers
Both, because there is hydrogen attached to each carbon involved in the double bond
In a cis isomer, where are the hydrogens
On the same side of the molecule
In a trans isomer, where are the hydrogens
Diagonally opposite each other in the molecules
In an E isomer, are the same atoms/groups opposite each other in the molecule or on the same side of the molecule
Opposite
In a Z isomer, are the same atoms/ groups opposite each other in the molecule or on the same side of the molecule
The same side
In cis-trans isomerism, is the cis isomer the E or the Z
Z isomer
In cis-trans isomerism, is the trans isomer the E or the Z
E isomer
When naming isomers, how do you assigns priority
Upon their atomic number
If the groups of higher priority are on the same side of the double bond, what isomer is the compound
Z isomer
If groups of higher priority are diagonally placed across the double bond, what isomer is the compound
E isomer
If the 2 atoms attached to a carbon atom in the double bond are the same, what do you do when trying to identity the isomerism
Find the first point of difference
Why are alkenes much more reactive than alkanes
Because of the presence of the Pi bond