Alkenes Flashcards
What are alkenes
- Unsaturated hydrocarbons with the general formula CnH2n
Unsaturated-Has at least one double carbon bond (C=C)
How many sigma and Pi bonds can occur in a double bond (C=C)
one sigma bond and one Pi bond
Explain what is meant by the term electrophile and by the term addition.
Lone pair acceptor
where two molecules form one molecule
Why are electrophiles attracted to alkenes
- The double bonds in alkenes are areas with high electron density.
- This causes the electrophiles to become polarised
- (And then the delta positive on one of the elements in the) electrophile is attarcted to the double C=C
What is the The order of stability for carbocation and why is it like this
What makes it more likely for a carbocation forming
- Tertiary > secondary >primary
- Because the more alkyl groups bonded to the carbocation the more stable the intermediate is
–> The alkyl groups push electrons towards the positive carbocation stablaising it - The more stable the carbocation is the more likely it will f orm
Spmething you mayneed to know
In exam answers
*Draw out both carbocations and identify as primary, secondary and tertiary
*State which is the more stable carbocation e.g. secondary more stable than primary
*State that the more stable carbocation is stabilised because the methyl groups on either (or one) side of the positive carbon are electron releasing and reduce the charge on the ion.
*(If both carbocations are secondary then both will be equally stable and a 50/50 split will be achieved)
Testing for alkenes with bromine water
Bromine water decolourises in the presence of a double bond.
Forms dibromoalkane
Combustion of alkenes
Alkenes are too valuable to burn
Alkenes burn with a sootier flame
what is heterolytic fission
One atom takes both electrons in a covalent bond
Why is more of the major product made?
The major product is always more stable due to the positive inductive effect
What is the use of plasticisers
Makes them more flexible
What is the use of PVCs
Drains pipes
Used for aprons
Cycloalkenes
They have 2 fewer hydrogens than cycloalkanes
Also double bonds have a high electron density and this makes alkenes fairly reactive
Give examples of electrophiles
Br-Br
H-Br
H2SO4
What happens when a bromine molecule appraoches an area of high electron density
Br2 becomes polarised as the electrons in the double bond repel the electrons in Br2 (delta - and delta+)
What would be step 1 in an electrophillic addiction reaction
A curly arrow is drawn from the double bond to the electrophile (delta positive)
What would be stage 2 in an electrophillic addition reaction (using H-Br as an exmaple)
After the double bond breaks the electrophile (H+) should be on one of the carbons
The other carbon that was attached to the double bond should now be a carbocation (draw a + on the C)
the electrophile (Br-) should be drawn as a negative ion with one lone pair and drawn an arrow from the lone pair to the carbocation(+)
What is a polymer
Many monomers bonded together
Repeat units
When asked to draw a repeat unit DO NOT INCLUDE THE N outside the bracket
