9.3 Alkenes and Alkynes
• Recall that the carbon atoms in alkanes all
have single covalent bonds between them.
• Alkane compounds are said to be saturated
hydrocarbons since all of their bonding
electrons are taken up in bonds with
hydrogen atoms.
• In alkenes and alkynes there are double
and triple covalent bonds, respectively.
• These compounds are therefore referred
to as unsaturated.
• I.e. Not all of the bonding electrons are
taken up by hydrogen and there are fewer
than the maximum number of hydrogen
atoms in these molecules.
• This makes alkenes and alkynes much more
reactive than their alkane counterparts.
• They can react with small diatomic
molecules like hydrogen, this reaction is
called hydrogenation, and converts
unsaturated hydrocarbons to saturated
ones.
Hydrogenation
Alkenes
• Hydrocarbons containing carbon to carbon
double bonds (C=C). These form a
homologous series with the general formula
CnH2n
• The simplest alkene is ethene C2H4 which
has Hthe
common name ethylene.
H
I
I
C=C
I
I
H H
Or CH2 = CH2
Properties of Alkenes
• The physical properties of alkenes are the same
as for alkanes.
• The presence of the C = C double bond has little
effect on boiling point since the dispersion forces
are about the same.
• Since they are also non-polar molecules, their
solubility is similar.
• Chemical properties of alkenes are also similar to
those of alkanes, but alkenes are more reactive
due to the presence of the double bond(s).
Naming Alkenes
1.
Identify the longest continuous chain of
carbon atoms containing the double bond,
and the chain is numbered from the end
closest to the multiple bond.
2. The number indicates the position of the
multiple bond on the parent chain and
follows the prefix of the name of the
compound’ parent chain. Eg. but-1-ene
CH2=CH-CH2-CH3
3. Name and number the branches as for
alkanes.
Example
CH3
I
CH2
I
CH3 - CH= CH - CH2 - C - CH2 – CH2 - CH2 - CH3
I
CH3
This molecule is called 5-ethyl-5-methylnon-2-ene
Alkynes
• Hydrocarbons containing carbon to carbon
triple bond(s) (C C).
• These compounds are also unsaturated.
The general formula is CnH2n -2
• There are not many alkynes in nature due
to the reactivity of these compounds.
• The simplest is ethyne C2H2 which has the
common name acetylene.
H–C C–H
• This compound is used in oxyacetylene
welding. It also used to be used in mining
and cycle lamps.
Naming Alkynes
1.
Identify the longest continuous chain of
carbon atoms containing the triple bond,
and the chain is numbered from the end
closest to the multiple bond.
2. The number indicates the position of the
multiple bond on the parent chain and
follows the prefix of the name of the
compound’s parent chain. Eg. but-1-yne
CH C-CH2-CH3
3. Name and number the branches as for
alkanes.
Example
CH3
I
CH2
I
CH3 - CH2 – C C - C - CH2 - CH2 - CH3
I
CH3
•This molecule is called 5-ethyl-5-methyloct-3-yne
Properties of Alkynes
• Alkynes are non-polar molecules with
weak dispersion forces between
molecules.
• They have low boiling points.
• The introduction of a triple bond
does not affect the physical
properties of alkynes.
• Alkynes are highly reactive, due to
the occurrence of the triple bond
that makes the molecule unstable.
Alkynes and Alkenes Worksheet
hex-2-ene
3,3-dimethylpent-1-ene
oct-3-ene
2-methylbut-2-ene
4-ethyl-2-methylhex-2-ene
2,4-dimethylpent-2-ene
4-ethyl-3-methylhex-2-ene
pent-1-ene
pent-2-ene
2-methylbut-1-ene
2-methylbut-2-ene
3-methylbut-1-ene
3-methylbut-1-yne
4,4-diethylhex-1-yne
4-methylhex-2-yne
5-ethyl-4-propylhept-2-yne
pent-1-yne
3-methylbut-1-yne
pent-2-yne
Summary of alkanes, alkenes and alkynes
hex-2-ene
but-1-yne
hept-3-ene
Summary of alkanes, alkenes and alkynes
3-methylhex-3-ene
but-2-yne
pent-2-ene
buta-1,3-diene
Cycloalkenes
• It is possible for there to be double bonds
in the ring structure of cyclic
hydrocarbons.
• These compounds are named cycloalkenes.
• They are structural isomers of alkynes.
• Cycloalkenes have similar
properties to cycloalkanes but
are a little more reactive.
Cycloalkynes
• It is also possible to have triple bonds in a
ring structure of cyclic hydrocarbons.
• These are called cycloalkynes.
• There are few cycloalkenes and cycloalkynes,
because of the bond angle stress put on the
double and triple bonds when a cyclic
hydrocarbon is created.
Cycloalkanes and Cycloalkenes
Cycloalkanes and Cycloalkenes
cyclopenta-1,3-diene
Ethane Cracking
• This is not conventional Cracking, which we
look, but is the term used in the industry.
• The low boiling points in natural gas make it
difficult to separate these components.
• Methane is separated at -100°C and
pressures up to 1500 to 5000 kPa.
• The components are known as LPG’s (liquid
petroleum gases)
• The most familiar LPG is propane, but
ethane is the most important LPG.
• Ethene (ethylene) is produced by cracking
either ethane or Naptha (a mixture of C5 C7 hydrocarbons)
• Cracking is an industrial process in which
larger hydrocarbon molecules are broken
down at temps, with or without catalysts,
to produce smaller hydrocarbon molecules.
• Ethane can be stored in large
salt caverns
• A catalyst is used to increase the rate of
reaction and the hydrogen product is used in
the plant.