chapter 10 - organic pathways

8/12/2019 Chapter 10 - Organic Pathways

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Chapter 10Organic Reactions:

Pathways to New Products


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Reactions of Alkanes

Alkanes are insoluble in water but are soluble innon-polar solvents.

There are weak dispersion forces between

molecules as evidenced by their low melting andboiling points.

The stability of the carbon-carbon bonds and thenon polar nature of the molecules means that

alkanes are very resistant to reaction.

Most reactions involving alkanes are eithercombustion or substitution.


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Combustion

Alkanes are used as fuels.

Combustion reactions involving alkanes releaselarge amounts of heat energy.

Methane is the major component in natural gasand octane is an important component of petrol.

The combustion equations for these two alkanesare:

CH4(g) + 2O2(g)CO2(g) + 2H2O(g) + energy

2C8H18(g) + 25O2(g)16CO2(g) + 18H2O(g) + energy


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Substitution Reactions

In substitution reactions, one or more of the hydrogenatoms in an alkane is replaced by a different atom orfunctional 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 electronsthe chlorine free radicals attack the carbon-hydrogenbonds.

RH + Cl2RCl + HCl


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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 onlyattractive forces between its molecules are dispersionforces and ethene therefore has a very low boilingtemperature.


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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.


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Addition Reaction Example


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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.


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Hydrolysis Reaction Example


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Addition Polymerisation

A type of addition reaction of ethene isinvolved in making polyethene.

The number, n, in this reaction is very large.

A molecule made by linking a large amount ofsmall 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.


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Addition Polymerisation cont

When the polymer is formed, the ethene

molecules add to the end of growing polymer

chains.


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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).


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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.


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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 theconditions needed for the reaction to occur.


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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.


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Reactions of Chloroalkanes

example


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Reactions of Chloroalkanes cont

Chloroalkanes will also reaction with ammonia

to form amines.

RCl + NH3

RNH2

+ HCl


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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)


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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.


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Structures of the isomers of C4H9OH:

aa primary alkanol, butan-1-ol

ba secondary alkonal, butan-2-ol

ca tertiary alkonal, 2-methylpropan-2-ol.


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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.


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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


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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.


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Esters cont For example, gently heating a mixture of ethanol and pure

ethanoic acid with a trace amount of sulfuric acid addedproduces an ester (ethyl ethanoate) and water.

The sulfuric acid acts as a catalyst.

The general equation for the esterification reaction involving acarboxylic acid and an alkanol is shown below.


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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.


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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 worlds

resources.


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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.


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Propanoic acid is a carboxylic acid containing three carbon

atoms.

It is prepared by the oxidation of the primary alkanol propan-

1-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.


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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 2-chloropropane.

Having synthesised ethanol and propanoic

acid, ethyl propanoate can be prepared usinga condensation reaction.


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The reaction pathways for the preparation of

ethanol, propanoic acid and ethyl propanoate

can be summarised.


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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.


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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 ofthe 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.


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Synthesis cont

The formation of isomers and other by-

products 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.


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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.


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Worked Example

In a particular synthesis, the yield of AB is 80% andthe yield of BC 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%


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Fractional Distillation

Is a technique used to separate liquids thathave 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


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Fractional Distillation cont


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The principle on which fractional distillation is based is that if amixture 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 re-

heated by vapours rising from the distillation flask.


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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.


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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

chapter 10 - organic pathways

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