Transcript Organic Pathways

Chapter 10 – Organic Reactions:
Pathways to New Products
Reactions of Alkanes
• Alkanes are insoluble in water but are soluble in
non-polar solvents.
• There are weak dispersion forces between
molecules as evidenced by their low melting and
boiling points.
• The stability of the carbon-carbon bonds and the
non polar nature of the molecules means that
alkanes are very resistant to reaction.
• Most reactions involving alkanes are either
combustion or substitution.
Combustion
• Alkanes are used as fuels.
• Combustion reactions involving alkanes release
large amounts of heat energy.
• Methane is the major component in natural gas
and octane is an important component of petrol.
• The combustion equations for these two alkanes
are:
CH4(g) + 2O2(g)  CO2(g) + 2H2O(g) + energy
2C8H18(g) + 25O2(g)  16CO2(g) + 18H2O(g) + energy
Substitution Reactions
• In substitution reactions, one or more of the hydrogen
atoms in an alkane is replaced by a different atom or
functional group.
• This involves breaking the carbon-hydrogen bonds and
making new bonds with the substituted group or atom.
• An example of this is the reaction between a chlorine
molecule and a hydrocarbon.
• The chlorine molecule breaks into separate atoms and
because these are unstable with only 7 outer electrons
the chlorine free radicals attack the carbon-hydrogen
bonds.
RH + Cl2  RCl + HCl
Reactions of Alkenes
• Ethene:
–
–
–
–
–
–
Is unsaturated
Is a non-polar molecule
Is insoluble in water
Is a flammable gas
Participates in addition reactions
Polymerises to produce polyethene.
• Since ethene is a small, non-polar molecule, the only
attractive forces between its molecules are dispersion
forces and ethene therefore has a very low boiling
temperature.
Addition Reactions of Alkenes
• Ethene reacts more readily and with more
chemicals than ethane.
• The reaction of ethene usually involve
addition of a small molecule to produce a
single product.
• In these situations, the double carbon-carbon
bond is broken and the new molecule bonds
to each carbon.
Addition Reaction Example…
Addition Reactions cont…
• Ethanol can be produced by an addition
reaction of ethene and water using a catalyst
to speed up the reaction.
• Ethene with water is known as a hydrolysis
reaction.
• The other alkenes that undergo similar
addition reactions to produce alkanols and
chloralkanes.
Hydrolysis Reaction Example…
Addition Polymerisation
• A type of addition reaction of ethene is
involved in making polyethene.
• The number, n, in this reaction is very large.
• A molecule made by linking a large amount of
small molecules is called a polymer.
• The singular small molecule is called a
monomer.
• A reaction where many monomer react
together to produce a polymer is called
addition polymerisation.
Addition Polymerisation cont…
• When the polymer is formed, the ethene
molecules add to the end of growing polymer
chains.
Addition Polymerisation cont…
• Ethene is also used to synthesise other
monomers which are used to manufacture
addition polymers, for example PVC and
polystyrene.
• PVC is the abbreviated name for the polymer
polyvinyl chloride.
• Polyvinyl chloride is manufactured by an
addition polymerisation reaction of the
monomer chloroethen (vinyl chloride).
Addition Polymerisation cont…
• Polystyrene is made from the monomer
styrene, which in turn is made from ethene.
• A copolymer is a polymer made of more than
one monomer.
Reactions of Functional Groups
• The influence of functional groups on the
chemistry of organic molecules may be seen
by studying their reactoins.
• When considering how new substances can be
made, we should think about how the
structure of the functional group determines
the way a particular molecule reacts and the
conditions needed for the reaction to occur.
Reactions of Chloroalkanes
• When a more electronegative atom such as chlorine has been
substituted for a hydrogen, the hydrocarbon becomes polar.
• Electrons are more attracted to the chlorine atom, which
makes the carbon atom at the other end of the bond
susceptible to be attacked by anions.
• For example, chloromethane is converted to methanol when
it reacted with hydroxide ions.
• The chlorine atom is substituted by an OH functional group.
• The carbon-chlorine bond is also susceptible to attack by the
negatively charged end of a polar molecule, like in the case of
ethanol.
Reactions of Chloroalkanes
example…
Reactions of Chloroalkanes cont…
• Chloroalkanes will also reaction with ammonia
to form amines.
RCl + NH3  RNH2 + HCl
Reactions of Alkanols
• Alkanols can undergo substitution reaction.
• The amino functional group can be introduced
to the chain by a substitution reaction
between ammonia and an alkanol.
• Ethylamine is formed by passing ammonia and
ethanol vapour over aluminium oxide heated
to about 400 degrees.
CH3CH2OH(g) + NH3(g)  CH3CH2NH2(g) + H2O(l)
Reactions of Alkanols cont…
• Alkanols can be oxidised to form carboxylic acids.
• Not all alkanols will oxidise to form carboxylic acids.
• The position of the OH in an alkanol determines the oxidation
product.
• Carboxylic acids are produced from the oxidation of primary
alkanols.
• Primary alkanols have an OH functional group attached at the
end of a chain of carbon atoms, or at the end of a side chain.
CH3CH2OH(as)CH3COOH(aq)
O2
• Alkanols react with carboxylic acids to form esters.
• Structures of the isomers of C4H9OH:
• a a primary alkanol, butan-1-ol
• b a secondary alkonal, butan-2-ol
• c a tertiary alkonal, 2-methylpropan-2-ol.
Reactions of Carboxylic Acids
• All carboxylic acids are weak acids, reacting
with water to form a weakly acidic solution:
CH3COOH(aq) + H2O(l)  CH3COO-(aq) + H3O+(aq)

• Carboxylic acids such as ethanoic acid react
with bases, reactive metals and carbonates.
Esters
• Esters are a group of organic compounds
responsible for some of the natural and
synthetic flavours and smells in ice cream,
lollies, flowers and fruits.
Ester
Smell of Flavour
Pentyl Propanoate
Apricot
Ethyl Butanoate
Pineapple
Octyl Ethanoate
Orange
2-Methylpropyl Methanoate
Raspberry
Ethyl Methanoate
Rum
Pentyl Ethanoate
Banana
Esters cont…
• Esters composed of small molecules are volatile and
have distinctive odours.
• They have low boiling points that allow them to
evaporate easily and reach your nose.
• Esters of larger molecular size are oils and waxes.
• Esters are made by a condensation reaction between
a carboxylic acid and an alkanol.
• Reactions that involve the combination of two
reactants and the elimination of a small molecule,
such as water, are called condensation reactions.
Esters cont…
• For example, gently heating a mixture of ethanol and pure
ethanoic acid with a trace amount of sulfuric acid added
produces an ester (ethyl ethanoate) and water.
• The sulfuric acid acts as a catalyst.
The general equation for the esterification reaction involving a
carboxylic acid and an alkanol is shown below.
Esters cont…
• Esters have two part names.
• The first part is derived from the name of the
alkanol from which is is made, where yl
replaces anol.
• The second part comes from the carboxylic
acid, where ic is replaced by the suffix ate.
Reaction Pathways
• The reaction pathway selected needs to take into
account the yield and purity of the product and also
minimise any unwanted side-products and waste
materials.
• Time and cost factors also need to be considered.
• There is also a lot of current interest in working out
green synthetic routes- ones that minimise waste,
use more environmentally friendly solvent, require
less energy and help to preserve the world’s
resources.
Example: synthesis of ethyl
propanoate
• Ethyl propanoate is to be formed using alkanes and alkenes as
starting materials.
• Looking at the structure, it indicates that it is an ester
produced by a condensation reaction between propanoic acid
and ethanol.
• Ethanol is a two carbon compound that can can be
synthesised directly from ethene, or from ethene via the
intermediate product chloroethane.
• In this case, the more direct route is selected.
• Propanoic acid is a carboxylic acid containing three carbon
atoms.
• It is prepared by the oxidation of the primary alkanol propan1-ol.
• This in turn can be formed by the reaction of 1-chloropropane
with sodium hydroxide.
• 1-chloropropane can be prepared by reacting propane with
chlorine.
• A number of chlorine-substituted products will be formed.
• These are separated by fractional distillation.
• The substitution reaction of propane is chosen
rather than an addition reaction of propene
because the addition of HCl to propene with
result in the formation of unwanted 2chloropropane.
• Having synthesised ethanol and propanoic
acid, ethyl propanoate can be prepared using
a condensation reaction.
• The reaction pathways for the preparation of
ethanol, propanoic acid and ethyl propanoate
can be summarised.
• The ester can be separated from the reaction
mixture and purified by fractional distillation.
• Its identity can then be verified using
instrumental analysis such as IR, NMR or Mass
Spectroscopy.
Considerations in Devising a
Synthesis
• When planning a reaction pathway, the structure of
the required compound or target molecule is studied
and the functional groups are identified.
• A synthetic pathway is devised using knowledge of
the reactions of functional groups.
• The synthesis may require the preparation of a
number of intermediate compounds.
• More than one possible pathway may need to be
considered as a desired product may be synthesised
via a number of pathways.
Considerations in Devising a
Synthesis cont…
• The formation of isomers and other byproducts also needs to be considered.
• The methods of separation of the desired
intermediate and final product from isomers
and other by-products must be determined.
• The final product then needs to be purified
and the purity evaluated.
• The yield must also be taken into account, as
not all of the reactants are necessarily
converted to product.
Considerations in Devising a
Synthesis example…
• For example, in the reaction:
CH3CH2Cl + OH-  CH3CH2OH + Cl• You would expect 6.45g of chloroethane to produce 4.60g of
ethanol if all the chloroethane was converted to ethanol.
• If only 2.30g of ethanol was obtained, then the yield would be
50%.
• Where there are a number of intermediate steps involved in a
synthesis, the yield for each step must be taken into account.
• A low yield in one of the intermediate reactions can have a
dramatic effect on the overall yield.
Worked Example…
• In a particular synthesis, the yield of AB is 80% and
the yield of BC is 70%. Calculate the overall
percentage yield for the preparation of C from A.
When A reacts to form B, only 80% or 80/100 of the
theoretical mass of B is formed.
Then, when B forms C, only 70% of 70/100 of C is
formed.
Hence the overall yield of C = (80/100) x (70/100) x
100%
= 56%
Fractional Distillation
• Is a technique used to separate liquids that
have different boiling points.
• It is commonly used in the lab to separate
volatile liquids from a reaction mixture.
• Industrial applications of fractional distillation
include:
– Separation of the fraction from crude oil
– Production of oxygen and nitrogen by fractional
distillation of air
– Extraction of ethanol in the fermentation of sugar
Fractional Distillation cont…
Fractional Distillation cont…
• The principle on which fractional distillation is based is that if a
mixture of volatile liquids is heated, the vapour contains a higher
concentration of the lower boiling point components.
• In fractional distillation the components of a mixture of volatile
liquids are separated by what can be considered to be a succession
of simple distillations.
• When the mixture of liquids is heated in the distillation flask, the
vapours rise up the fractioning column.
• These vapours contain a higher concentration of the more volatile
component than the liquid in the distillation flask.
• Eventually the vapours reach a height in the fractioning column
where the temperature is low enough for condensation to occur.
• As the condensed liquid trickles back down the column it is reheated by vapours rising from the distillation flask.
Fractional Distillation cont…
• As a result, some of the condensed liquid evaporates, and the resulting
vapour has an even higher concentration of the low boiling point
component.
• This process is repeated many times throughout the length of the column,
and the concentration of the more volatile substance in the vapour
increases in each evaporation-condensation cycle.
• At the same time, the concentration of the less volatile (higher boiling
point) substances in the distillation flask will increase.
• When the vapour reaches the top of the fractioning column it will ideally
consist of only the more volatile component.
• When the component reaches the top of the column the temperature
remains stable.
• The material that condenses over a small temperature range near the
boiling point of the substance of interest is collected.
• It is not always possible to achieve a complete separation.
Using Fractional Distillation
• Pure ethyl ethanoate can be extracted from the
reaction mixture by fractional distillation.
• The reaction in the mixture is heated in the
distillation flask.
• The vapours rise up the fractioning column.
• The temperature at the bottom of the column slowly
increases until it stabilises at about 57 degrees,
which is the boiling point of ethyl ethanoate.
• The fraction condensing over a small range of
temperatures, 55-59 degrees, near the boiling point
of ethyl ethanoate is collected.