13. Alkenes
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What is a pi bond?
the sideways overlap of the 2 p orbitals creating electron density above and below the plane of atoms
Can C=C double bonds rotate?
no <3
What is the double bond in alkenes?
an area of localised high electron density which attracts electrophiles
What is the relative strength of pi to sigma bonds
pi is weaker so is relatively easy to break
What is the bond angle around a C=C bond?
120 degrees because there are 3 bonded regions around each atom and they repel as far apart as possible all atoms are in the same plane
Define a stereoisomer
isomers have the same structural formula, but the atoms occupy a different position in space
Why does E/Z isomerism occur?
due to the restricted rotation of C=C bonds
What is the difference between E/Z and cis/trans isomerism?
E/Z just must contain a C=C bond where each carbon is bonded to 2 different groups cis/trans requires one of those groups to be a the same on each carbon
How does cis/trans relate to E/Z ?
cis=Z : same side of C=C trans=E : opposite side of C=C
Define an electrophile
an electron pair acceptor : usually possesses a d+ dipole or positive charge somewhere in its structure
Why does a pi bond attract an electrophile?
a pi bond is an area of high electron density and an electrophile wants to “accept” those electrons
What is a C=C bond made of?
a sigma bond and a pi bond, with the electron density above and below the plane of the sigma bond
What are the conditions for the hydrogenation of alkenes?
alkene + hydrogen + nickel/platinum catalyst @ 423K
How do you test for unsaturation?
bromine water
addition reactions of alkenes with halides
learn
What are the conditions for reaction of alkene hydration to alcohols?
alkene + steam + phosphoric acid catalyst
What breaks in electrophilic addition?
the pi bond
How does a dipole become induced in Br2 in electrophilic addition?
as the Br2 approaches the area of high electron density around the C=C bond, the electrons of the Br2 molecule move as far away as possible, creating a d +ve charge at the end of the molecule closer to the alkene
Where should the + go on a forming carbocation to get the major product?
on the carbon bonded to the most other carbons
Which is the most stable carbocation?
tertiary
Why is a tertiary carbocation always the major product when possible?
it is the most stable so is formed more readily
What are the conditions for the dehydration of alcohols to alkenes?
alcohol + conc. sulphuric/phosphoric acid + reflux
What are the problems with polymers?
unreactive and non-biodegradable (carbon chain is non-polar and covalent bonds are strong) adds to landfill problem on disposal
How can we dispose polymers safely/less dangerously? (4 ways)
recycling : but is high cost burn waste : saves landfill and produces energy purification: toxic fumes can be removed from burnt chlorinated polymers feedstock : reuse polymers as useful organic compounds