Alkenes Flashcards
(25 cards)
What’s the general formula of an alkene?
CnH2n
What are the two bonds in the carbon-carbon double bond?
Sigma bond and pi bond
What shape is the carbon in the double bond and what’s the bond angle?
Trigonal planar 120 degrees
Which bond in the double bond is more reactive and why?
The pi bond because it has a lower bond enthalpy
What is the definition of structural isomerism and what different types of structural isomer are there?
Same molecular formula different structural formula
Functional group, chain, position
What’s the definition of stereoisomerism?
Same molecular formula and same structural formula but different arrangement of atoms in space
When are cis and trans used?
When the groups on each carbon are the same
What does trans mean?
Across/opposite
What does cis mean?
On the side/next to
When are E and Z used?
When the two groups on the carbon atoms are different
How are E and Z assigned?
The are assigned priority by which has the largest Mr. If the two largest groups are on the same side we use Z, if they’re on different sides we use E
What does hydrogenation do to an alkene?
The addition of hydrogen breaks the double bond to from a saturated alkane
When is hydrogenation used in industry?
In hydrogen sting vegetable oils to make things like margarine
What happens when you react an alkene with steam?
The steam (H2O) forms bonds with the broken pi bond forming an alcohol
What’s the mechanism associated with alkenes?
Electrophilic addition
Why are alkenes more reactive than alkanes?
Alkenes contain a double bond which is a reason of high electron density (will attract anything positive easily)
What is an electrophile?
An electron deficient species which accepts an electron pair
Explain electrophilic addition using a bromine molecule in four steps
- The c=c induced a dipole in the bromine molecule
- The pi bond electrons are attracted to the delta positive bromine, and the bonding pair of electrons in the bromine molecules are repealed down the molecule to the delta negative bromine.
- The pi bond breaks and bonds with the bromine atom, the other bromine is left with both electrons and a negative charge. The unbonded carbon has a positive charge and is called the carbocation
- The bonded carbon sends some electrons down to the carbocation (the inductive effect) to stabilise it, the bromine ion is attracted to the positive charge on the carbocation and they bond covalently
What’s Markovnikov’s rule?
When a compound of different atoms is added to an unsymmetrical alkene two products are formed the hydrogen in the molecule becomes attached to the carbon with the most hydrogen me already attached to it
What’s a primary carbocation?
When a carbon receives the inductive effect from one other carbon
What’s a secondary carbocation?
When a carbocation can receive the inductive effect from 2 other carbons forming a more stable product (more likely to be the major product)
What’s a tertiary carbocation?
A carbocation that receives the inductive effect from 3 other carbons forming the most stable (major) product
What is a polymer?
A long chained molecule made by the joining of small chained monomers
What’s addition polymerisation?
When alkenes undergo polymerisation to produce saturated chains. The pi bond is broken and two new sigma bonds are formed