Reactions of Alkenes

1
Reactions of Alkenes

• 1.7.3
• understand that they are unsaturated hydrocarbons
  with a carbon-carbon double bond which consists
  of a ? and a ? bond
• describe the addition reactions of alkenes

Independent Learning (HW) pp 129-133 and ALL
questions therein
Connector Identify whether the compounds below
are E or Z isomers
Z
E
Z

• Crowe2008

2
The structure of methane
In methane all the C-H bonds are the same and due
to mutual repulsion they are arranged in a
tetrahedral structure How can we explain this?
3
Electronic configuration and orbital theory
Carbon At. No. 6 Electron arrangement (GCSE)
2.4 Electronic configuration of carbon is
1s22s22p2
Orbitals are a way of showing the probability of
finding an electron in a certain position
s orbitals are spherical and p orbitals are
dumbbell shaped
4
Shapes of orbitals
Spherical s orbitals are centred on the nucleus
Dumbbell shaped p orbitals are also centred on
the nucleus, but are arranged at right angle to
each other.
5
The order of filling orbitals
2s orbital has a slightly lower energy than the
2p orbitals, so fills first.
The energies of the various orbitals in the first
and second levels.
Electrons fill low energy orbitals (closer to the
nucleus) before they fill higher energy ones.
6
Bonding in Carbon
How many unpaired electrons?
In this arrangement carbon would form two bonds.
For carbon to form four bonds all four electrons
must be unpaired, so one of the 2s electrons is
promoted to the vacant 2pZ orbital
7
Bonding in methane
Remember - in methane all the C-H bonds are the
same
In this arrangement the electrons are not in the
same environment.
To account for the observation that all bonds are
equal - The electrons rearrange themselves again
in a process called hybridisation. This
reorganises the electrons into four identical
hybrid orbitals called sp3 hybrids (because they
are made from one s orbital and three p
orbitals).
8
Bonding in methane
sp3 hybrid orbitals All equal, arranged in a
tetrahedral shape
Bonds formed in this way are called sigma (s)
bonds
9
The nature the CC bond
Ethene is a planar, 2D structure With bond angles
of approximately 120o
Once again the 2s orbitals are hybridised, but in
this case sp2 hybrid orbitals form
The sp2 hybrid orbitals are shown in green they
are in the same plane and roughly 120o apart The
p orbital contains an unbonded electron
10
The bonding in ethene
The two carbon atoms and four hydrogen atoms
would look like this before they joined together
The various atomic orbitals which were pointing
towards each other now merge to give molecular
orbitals, each containing a bonding pair of
electrons. These are sigma bonds (s)
11
The pi (p) bond
The p orbitals shown in red overlap to produce a
different type of molecular orbital - a pi (p)
bond, that lies above and below the C-C s bond
A pi bond is a region of space in which you can
find the two electrons which make up the bond.
Those two electrons can live anywhere within that
space.
Note The presence of the p bond makes the CC
bond rigid
12
Addition reactions of alkenes
Chemical compounds of the type XY will add
across the double bond of an alkene.
X Y
X Y
H2C CH2
H2C CH2
Br Br
Br Br
H2C CH2
dibromoethane
H OH
H OH
H2C CH2
ethanol
13
Write equations and name the products formed when
ethene reacts with the following reagents
the addition of hydrogen with a nickel catalyst
to form an alkane
the addition of halogens to produce
di-substituted halogenoalkanes (Cl2 Br2)
the addition of hydrogen halides to produce
mono-substituted halogenoalkanes (HCl HBr)
14
Oxidation of the double bond by potassium
manganate (VII) to produce a diol
Write the corresponding reaction for cyclohexene,
and name the product