Alkenes Flashcards
Alkene definition
An alkene is an unsaturated hydrocarbon.
Why are alkenes much more reactive than alkanes?
- Because they are unsaturated there is scope to add atoms onto the molecule on either end of the double bond to make it saturated
- Because the double bond has a high electron density it makes alkenes susceptible to reactions with molecules which have positive or δ+ charges (i.e. electrophiles)
Structure of Alkenes
Ethene is a planar molecule (and the bonding about all C=C bonds is also planar). This makes the bond angles approximately 120⁰.
Why do double bonds not rotate?
*The first bond of the double bond is formed by the overlap of 2s orbitals.
*The second bond of the double bond is formed by the overlap of one of the p orbitals from each carbon atom.
*These orbitals overlap above and below the plane of the double bond and merge together to form a π–orbital.
*The two p orbitals need to remain aligned with one another in order to maintain the π–orbital. If the bond did rotate the p orbitals would no longer overlap and it would break the second of the bonds in the alkene
Physical properties of Alkenes
Alkenes have no polar bonds and so only have Van der Waals intermolecular forces.
Each alkene has very similar physical properties to their equivalent alkane. (i.e. ethane is similar to ethene etc).
As with alkanes, the melting point and boiling point or alkenes increases as their chain length increased because the Van der Waals forces get larger.
Alkenes are insoluble in water.
How do alkenes react?
A double bond is almost twice as strong as a single bond, but because it is an area of high electron density it is susceptible to attack by ELECTROPHILES.
Electrophiles are electron deficient and so tend to have positive or δ+ charges. They are electron pair acceptors. Because alkenes are unsaturated other atoms can be added onto them to saturate the double bond.
What is a carbocation?
These are ions which have a positive charge on a carbon atom, and only exist for a very short amount of time before reacting further.
Positive inductive effect definition
The Positive Inductive Effect is the ability of an alkyl group to release electron density through a covalent bond
Stability of Carbocations & The Positive Inductive Effect
The more alkyl groups which are attached to the positively charged carbon, the more stable the carbocation is. This is due to the POSITIVE INDUCTIVE EFFECT
Electrophilic Addition reagents
alkene → haloalkane
HBr, HCl, Br2, Cl2
Electrophilic Addition with hydrogen halides
ethene + HBr → Bromoethene
Mechanism:
- arrow from double bond to H
-arrow from bond to Br
-+ on C
- arrow from Br- lone pair to C with +
Why is 2-bromopropane the major product in this reaction?
2-bromopropane is produced via a 2⁰ carbocation. This is more stable than a 1⁰ carbocation due to the positive inductive effect
Electrophilic Addition with halogens
ethene + Br2 → dibromoethene
Mechanism:
- arrow from double bond to Br
-arrow from bond to Br
- on C
- arrow from Br- lone pair to C with +
Explanation of Electrophilic Addition with halogens
- C=C is e- rich
- Br2 becomes polarised
- Brδ+ attracted to C=C
Bromine test
Bromine water decolourises from orange to colourless, when added to an alkene
Electrophilic Addition with H2SO4
ethene + H2SO4 → ethylhydrogensulfate
Mechanism:
- arrow from double bond to H
-arrow from bond to O
- on C
- arrow from O- lone pair to C with +
ethylhydrogensulfate + H2O → ethanol + H2SO4
Direct Hydration
ethanol produced
ethene + H2O(g) → ethanol (H3PO4 catalyst)
H2O = steam
Addition Polymers
Monomer= ethene
Polymer= polyethene ( H branches straight up no slanted)
one repeat unit:
-m‘n’ represents a big number
- repeating unit has a C-C single bond, not a double bond
- Single bonds extend beyond the brackets on either side of the repeating unit.
