13. Alkenes Flashcards
Molecules with C=C are unsaturated because
They can make more bonds with extra atoms In addition reactions
Explain why sigma bonds have high bond enthalpy
This a type on bond formed when s orbitals overlap in a straight line
This gives highest possible electron density so strong electrostatic attraction
Sigma bonds have high bond enthalpy- strongest type of covalent bond
Explain why pi bonds have low enthalpy
Pi bonds formed by sideways overlap of two adjacent p-orbitals
It got two parts above and below molecular axis because p orbitals are dumb-bell shaped
Electron density is spread out.
Electrostatic attraction between nuclei and shared pair electrons are weak
Pi bonds are low bond enthalpy
In ethene C2H4 the atoms around double bond can’t rotate because
Of the way p-orbitals overlap to form pi bonds
The restricted motion around the C=C causes alkenes to form stereoisomers
In z-isomers
The priority groups lie either above or below the double bond
In E-isomers the priority groups lie
Across the double bond
Atoms with larger atomic…. In E/Z isomers
Atoms with larger atomic number are given higher protein
If atoms bonded directly bonded to carbon double are same…
You have to look at the next atom in the groups to work our higher priority
How to carry out the addition of alkenes to alkanes
Adding hydrogen to double bond
Reaction of alkenes with hydrogen to produce alkenes
Need nickel catalyst and 150 degrees
In electrophillic addition of Alkenes.
The double bond…
Opens up and the atoms are added to carbon atoms
Electrophiles are…
Electron pair acceptors
How can you make alkanes from alkenes
Hydrogenation and cracking
You can use bromine water to carry a test for alkenes explain how might carry it out??
When you shake orange bromine water, solution quickly decolourises
Add bromine adds across double bond
How to carry out steam hydration (alkene ~> alcohol)
Alkanes are hydrated by steam at 300 degrees and a pressure of 60-70 atm
Phosphoric acid catalyst needed
Reaction is reversible and yield is 5% but you can recycle the unreacted alkene gas
What is Markownikoff’s rule
The major product from addition of hydrogen halide
To an unsymmetrical alkene is the one where the hydrogen adds to the carbon with most hydrogen
