Download Alkenes

Alkenes
Bonding
The electron configuration of carbon is
When carbon bonds in alkenes, the
electron configuration changes
The 2s electron and two of the 2p
electrons combine to form three sp2
hybrid orbitals, leaving a spare porbital on each of the carbon atoms
1s22s22p2
1s22s12p3
H
H
C
H
C
H
A hybrid orbital from each carbon
overlaps to form a σ-bond
The p-orbitals overlap causing areas of
electron density spanning across the top and
the bottom of the σ-bond joining the two
carbon atoms.
This is the π-bond.
Test for unsaturation
• If an alkene is shaken with bromine
water (Br2) the colour changes from
reddy-brown to colourless as
dihalogenoalkanes are formed.
Reaction Mechanism
This is a two step reaction;
1) Formation of a carbocation by electrophillic
addition
2) Rapid reaction with the negative Br ion
Reactions with hydrogen halides
If an alkene is bubbled through concentrated
HCl(aq) at room temperature, a
monosubstituted chloro-alkane is formed.
Addition of steam
When steam and a gaseous alkene are passed
over a solid phosphoric acid catalyst (H3PO4)
at 600K and 6MPa, alcohols are formed.
Addition of hydrogen
If a gaseous alkene and H2 gas are passed
over a finely divided nickel catalyst at high
pressure and temperature (420K and 500kPa
for making margarine!) an alkane is formed.