Alkenes
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What is an electrophile?
Attracted to a negativity, and electron pair accepter
How many products are formed in an electophilic addition reaction?
1
Where are electrophiles attracted to on alkenes?
The carbon to carbon double bond, which is electron dense
What are 3 examples of electrophiles?
• Br2 • HBr • H2SO4
What is the first thing that happens in electrophilic addition (between Br2 and ethene)?
A dipole is induced in Br2, which is caused by repulsion from the electron dense double bond in ethene
Draw the reaction mechanism when an alkene reacts with HBr, include intermediate steps
http://alevelchem.com/img/electrophilic_addition_hydrogen_bromide.gif
What is a common reaction in alkenes? Why?
Electrophilic addition As the C=C bond creates and electron dense area that attracts electrophiles
What are the 4 steps of electrophilic addition reactions?
1) electrophile is attracted to the double bond 2) Electrophiles accept a pair of electrons from the double bond 3) A carbocation is formed 4) a negatively charged ion forms a bond with the carbocation
Why don’t pi bonds rotate?
Any rotation would break the pi bond, because the p orbitals above and below would no longer overlap
Why are products in electrophilic substitution formed in different amounts?
• The major product is formed via a tertiary carbocation intermediate and the minor product is formed via a secondary carbocation intermediate • The tertiary carbocation is more stable than the secondary/primary carbocation • because it has a greater positive inductive effect
What classification is the most stable carbocation?
tertiary (primary is the least)
In addition polymerisation, what will high temperatures and pressures cause?
branched chain polymers with weak intermolecular forces
What is the positive inductive effect?
more carbon atoms cause e- density to be shifted towards the carbocation, making it more stable
Why is a tertiary carbocation more stable than a secondary carbocation?
As it has a greater positive inductive effect
What 2 step reaction can form alcohols from alkenes?
1) reaction with cold, conc. H2SO4 2) reaction with cold water
Name the mechanism for the reaction of concentrated H2SO4 with an alkene…
electrophilic addition
Why does E-Z isomerism occur in alkenes?
C=C bonds cannot rotate