Alkenes Flashcards
Alkenes
A homologous series of hydrocarbon which contains a double bond
What type of hydrocarbon are alkenes?
Unsaturated because they contain a double bond thus don’t have the maximum number of hydrogen atoms bonded to it
Alkenes functional group
C=C
General formula of alkenes
C2H2n
Naming alkenes
Number of carbon atoms in longest chain
- number(s) of the carbon that the double bond is on
-ene (if 2 double bonds is diene etc)
How are double bonds formed?
When a sigma covalent bond forms
So a pi covalent bond can form
How are sigma bonds formed in carbon?
When the 2s orbitals in 2 carbon atoms which each contain a single electron overlap in a straight line
Where is the electron density in the sigma bond?
Between the 2 nuclei due to the overlap of orbitals side on in a straight line, on the molecular axis
Where is the electron density in a pi bond?
Above the molecular axis and below the molecular axis
Therefore spread out above and below the nuclei
Electrostatic attraction in a pi bond
Because the electron density is spread out above and below the nucleus, it is weaker than in sigma bonds where the electron density is in between
Why are sigma bonds stronger than pi bonds?
Because in sigma bonds the high electron density is between 2 nuclei thus stronger electrostatic attraction
Whereas in pi bonds the electron density is spread above and below the nucleus
Double bond strength vs single bond strength
Even though pi bonds are weaker than sigma bonds, having combined strength of both means they are stronger than single bonds
What reagent are alkenes considered?
Nucleophiles because they contain a double bond where there is high electron density thus an area of negative charge
What reagents do alkenes usually react with?
Electrophiles (electron-pair acceptors) which usually are attracted to high electron density areas
Reaction of alkenes + hydrogen halogens or hydrogen halides
Electrophilic addition because an electrophile is being added to the Alkene without substituting any atoms to make a larger molecule
Alkene + hydrogen reaction name
Hydrogenation
What does Alkene + hydrogen form
An alkane by adding 2 hydrogen and breaking the double bond
Conditions for hydrogenation of alkenes
Nickel catalyst
150°c
What happens to the melting point after hydrogenation of Alkene?
You remove the double bond by adding hydrogen to a carbon
Which raises the melting point
Therefore alkane is a solid at room temperature
Halogen + Alkene forms?
Dihalogenoalkane
So opening up the double bond to attach 2 halogen atoms = DI
How are halogen atoms added to the Alkene when undergoing electrophilic addition?
Adds across the double bond when it breaks
So ensure they are added to different carbons and not the same
How is halogen molecule form an induced dipole (and therefore act as electrophile)
The high electron density (negative) area around double bond of Alkene repels electrons in the halogen molecule. Induces a dipole across halogen molecule where closest halogen atom is positively charged
Breaking of halogen bond and double bond in alkene in electrophilic addition
There is heterolytic fission of the halogen molecule
Where the 2 electrons in this bond are given to furthest halogen atom
And so the double bond in Alkene breaks because the 2 electrons in pi bond form bond with closest halogen atom
Arrow mechanisms in the heterolytic fission of halogen molecule
An arrow from the bond between halogens to the furthest halogen atom to show how 2 electrons in the bond go there
Arrow from double bond in Alkene to closest halogen to show how these electrons are shared thus bonded
