13. ALKENES Flashcards
(26 cards)
Are Alkenes saturated or unsaturated hydrocarbons?
Unsaturated (C=C)
What is the general formula for alkenes?
CnH2n
Describe the nature of the double bond
- Carbons have 4 outer electrons
- 3 electrons are involved in sigma bonds (one of which forms a sigma bond to the other carbon atom of the double bond)
- There are 2 left over electrons (1 from each carbon atom) that are not involved in sigma bonds.
- These electrons are in p-orbitals
- The sideways overlap of the p-orbitals above and below the sigma bond forms a π-bond which locks the carbon atoms in place and prevents rotation
What makes up a double bond?
1 sigma bond
1 pi-bond
Describe the shape and angle around a double bond
There are 3 regions of electron density which repel each other as far apart as possible.
This forms a trigonal planar shape, with a bond angle of 120˚
Define Stereoisomers
Compounds with the same structural formula but different arrangements of atoms in space
What are the different types of stereoisomerism?
- E-Z isomerism
- Cis-Trans isomerism (a special case of E-Z isomerism)
- Optical isomerism
Why does stereoisomerism occur around a double bond?
Rotation around the double bond is restricted due to the position of the π-bonds electron density above and below the plane of the sigma-bond.
Therefore the groups attached to the carbon atoms are fixed relative to each other.
When will a molecule have E-Z isomerism?
- it contains a double bond
- there are different groups attached to the carbons
(there cannot be multiple of the same group on a carbon)
When will a molecule have Cis-Trans isomerism?
- it contains a double bond
- each carbon is attached to a single hydrogen atom
How do we know if a molecule is E, Z, Cis, or Trans?
groups with higher priority on same side = Z or Cis
groups with higher priority on different sides = E or Trans
How does the reactivity of alkenes compare to the reactivity of alkanes?
Alkenes are more reactive than alkanes
Why are Alkenes more reactive than alkanes?
Alkenes contain π-bonds and sigma bonds.
Alkanes only contain sigma bonds.
π-bonds have a lower bond enthalpy than sigma bonds, so π-bonds readily break.
What are the different addition reactions alkenes undergo?
- Alkene + Hydrogen in the presence of a nickel catalyst, 423K —> Alkane
- Alkene + Halogen, room temp —> Dihaloalkane
- Alkene + Hydrogen halide, room temp —> Haloalkane
- Alkene + H2O (steam) / H3PO4 or conc H2SO4, 300˚C 60 atm, —> Alcohol
Alkene —> Alkane
Alkene + H2 —> Alkane
Hydrogen in the presence of a nickel catalyst
423K
The reaction is Hydrogenation
Alkene —> Dihaloalkane
Alkene + Br2 or Cl2 —> Dihaloalkane
Room temp
How can the reaction of alkenes and bromine be used to test for unsaturation?
When bromine water (an orange solution) is added dropwise to a sample of an alkene, bromine adds across the double bond, and the orange colour disappears.
Any compound containing a C=C bond will decolourise bromine water
Alkene —> Haloalkane
Alkene + Hydrogen halide (g) —> Haloalkane
room temp
2 different products are made
Alkene —> Alcohol
Alkene + H2O (g) —> Alcohol
H2O (steam)
H3PO4 or Conc. H2SO4
300˚C, 60 atm
2 different products are made
What is an electrophile?
An atom or group of atoms which is attracted to an electron-rich centre or atom, where it accepts a pair of electrons to form a new covalent bond.
(an electron pair acceptor)
Draw the mechanism of electrophilic addition of methene + HBr
- Partial charges on H and Br
- Curly arrow from double bond to H
- Curly arrow from bond to Br
- No double bond, H attached to molecule
- Carbocation on carbon
- Negative charge on Br
- Curly arrow from Br to carbocation
- Bromomethane
Draw a primary carbocation
C attached to 2 H and a R
positive charge on C
Draw a secondary carbocation
C attached to 1 H, R and R’
Positive charge on C
Draw a tertiary carbocation
C attached to R, R’ and R’’
Positive charge on C
