4.1.3 - Alkenes Flashcards
What are stereoisomers?
Compounds with the same structural formula but with a different arrangement in space.
What is E/Z isomerism?
An example of stereoisomerism, in terms of restricted rotation about a double bond and the requirement for two different groups to be attached to each carbon atom of the C=C group.
What is cis-trans isomerism?
A special case of E/Z isomerism in which two of the substituent groups are attached to each carbon atom of the C=C group are the same.
How do you know whether a compound is cis or trans?
When both hydrogens are on the same side of the double bond then the molecule is cis.
How can you work out whether something is an E or Z isomerism?
If the molecule with the highest atomic number on each side of the double bond is on the same side then it is Z.
How can you remember what Z and E mean?
Z = zusammen, E = entgehen.
General formula?
CnH2n
What are all alkenes?
Hydrocarbons which are unsaturated as they all contain at least one double bond.
What is a double bond made up of?
Sigma and pi bond.
When is a sigma bond formed?
When two s orbitals overlap.
How do s orbitals over lap?
In a straight line.
What does the overlapping of the s orbitals mean?
There is the highest possible electron density between the two nuclei.
What does high electron density lead to?
A strong electrostatic attraction between the nuclei and the shared pair of electrons.
Due to the strong electrostatic attraction is sigma bonds what does this lead to?
Sigma bonds having a high bond enthalpy.
How are pi bonds formed?
By the sideways overlap of two adjacent p orbitals.
Why are pi bonds much weaker than sigma bonds?
The electron density is spread out above and below the nuclei. Therefore the electrostatic attraction between the nuclei and the shared pair of electrons is weaker.
Why do pi bonds have relatively low bond enthalpy?
The electrostatic attraction between the nuclei and the shared pair of electrons is weaker.
Why can’t C=C bonds rotate?
Because of the way the p orbitals overlap to form a pi bond.
What is the shape and bond angle of alkenes?
Trigonal planar and 120 degrees.
How can waste polymers be reused?
They can be combusted, turned into organic feedstock or melted and remoulded.
Why can waste polymers be combusted?
They have a high calorific value, so when burnt in power stations the chemical energy transferred can be used to drive turbines and generate electricity.
What are the conditions needed for hydrogenation?
150 degrees C and nickel catalyst.
What is added in hydrogenation?
Hydrogen.
What is added in halogenation?
A halogen such as Br2 or Cl2.
What is test for alkenes?
Shake bromine water with an alkene. The solution will turn colourless.
What kind of mechanisms are halogenation and hydrogenation?
Electrophilic addition.
What are electrophiles?
Electron pair acceptors.
What is added during hydration.
Steam
What are the conditions for hydration?
H3PO4 catalyst, 65atm and 300 degrees C.
What are the conditions for hydrohalogenation?
Room temperature.
What is produced in hydrohalogenation?
More than 1 product, a minor and a major one.
How can you tell which product is major?
The one with the larger carbocation.
What is a tertiary carbocation?
More stable than primary and secondary and it is connected to three other carbons and then the cation.
