Download Reactions of Molecules with Oxygen

Organic Chemistry 5
Reactions of Molecules with Oxygen
IB Topic 10.4 and 20.4.1
Reference: Higher Level Chemistry, p. 390-392 and 401-403
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Alcohols - Main Types of Reactions
combustion reactions
oxidation reactions
alcohols
biofuels
condensation
reactions
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Combustion and Oxidation
Reactions of Alcohols
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Combustion of Alcohols
When alcohols undergo complete combustion in excess
oxgyen gas, they produce carbon dioxide and water.
One group of “biofuels” are alcohols (mainly ethanol)
produced by the fermentation of food crops.
C2H5OH + 7/2 O2 → 2CO2 + 3H2O
Alcohols tend to be “cleaner burning”
fuels than larger hydrocarbons.
Suggest a meaning for the term “cleaner burning”.
Provide evidence for your suggestion.
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Oxidation Reactions of Alcohols
this reactant is the
oxidizing agent
http://www.aquahealthproducts.com/free-radicals-and-antioxidants
this reactant is oxidized
Oxidation reactions occur when an
atom in a substance “gives up” electrons
during a reaction.
Another reactant known as an
“oxidizing agent” is required.
It is a reactant that removes an electron
from the other reactant.
Because alcohols are covalent compounds, they will usually not form ions when they lose electrons.
But electrons will shift away from the atom that is oxidized. (You will learn more about this in Gr 12!)
Two signs that an organic compound has undergone oxidation are:
1. The compound gains oxygen atoms
C2H6O → C2H4O2
2. The compound loses hydrogen atoms
C2H6O → C2H4O
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Oxidation Reactions of Alcohols
Combustion Reactions
Complete combustion reactions are
examples of complete oxidation of
organic compounds.
http://zenstoves.net/PressureSideBurner.htm
Each carbon atom in the alcohol bonds to the
maximum number of oxygen atoms possible, forming CO2.
C3H7OH + 5O2 → 3CO2 + 4H2O
O2 is the oxidizing agent in a combustion reaction.
The organic compound is converted into inorganic CO2.
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Oxidation Reactions of Alcohols
Selective Oxidation Reactions
Alcohols may also be selectively oxidized, where only the carbon
atom attached to the functional hydroxyl group changes.
The other carbon atoms in the R group stay the same.
These oxidation reactions use a
different oxidizing agent rather than O2.
The symbol [O] is used for any oxidizing agent.
C3H7OH + [O] → C3H6O
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Oxidation Reactions of Alcohols
What oxidizing agent is used in the selective oxidation of alcohols?
Potassium dichromate, K2Cr2O7, is the oxidizing agent [O]
used in many alcohol oxidation reactions.
The K2Cr2O7 solution must be acidified first by adding sulfuric acid, H2SO4.
This provides an appropriate environment for the oxidation reaction.
Acidified potassium dichromate is often represented as H+/Cr2O72-.
K2Cr2O7 is an ORANGE compound.
During the oxidation of an alcohol, the K2Cr2O7
changes and produces the green Cr3+ ion.
http://www.visualphotos.com/image/1x6037951/potassium_dichromate_test_for_alcohols
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Oxidation of Primary Alcohols
Primary alcohols can undergo TWO successive oxidation steps:
1.
RCH2OH + [O] → RCHO
primary alcohol
aldehyde
2. RCHO + [O] → RCOOH
aldehyde
carboxylic acid
Remember [O] (oxidizing agent) = H+/Cr2O72– (acidified dichromate)
details →
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Oxidation of Primary Alcohols
1º alcohol + [O] → aldehyde + [O] → carboxylic acid
If you want to only produce the aldehyde, a process known as distillation is used.
Distillation takes advantage of the
different boiling points of substances.
Aldehydes have a lower boiling point
than either alcohols or carboxylic acids
with the same number of carbon atoms.
Explain why.
reaction mixture
BP ethanal = 21ºC
BP ethanol = 79ºC
http://commons.wikimedia.org/wiki/File:Fractional_distillation_lab_apparatus.png
BP ethanoic acid = 118ºC
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Oxidation of Primary Alcohols
1º alcohol + [O] → aldehyde + [O] → carboxylic acid
Oxidation of ethanol to ethanal only by distillation
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As long as the temperature of the reaction mixture in
the flask is below 79ºC, only the ethanal will vaporize.
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2
The ethanal vapour rises into the fractionating column,
and then moves into the condensor.
3
Cold water surrounding the condensor lowers the
temperature of the aldehyde vapour, turning into a liquid.
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Purified liquid aldehyde is collected in the flask.
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1
reaction mixture
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http://commons.wikimedia.org/wiki/File:Fractional_distillation_lab_apparatus.png
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Oxidation of Primary Alcohols
1º alcohol + [O] → aldehyde + [O] → carboxylic acid
Further oxidization of the aldehyde to a carboxylic acid by reflux
1
As in distillation, the aldehyde produced in
the reaction mixture evaporated and
aldehyde vapour rises.
2
In the condensing tube, now positioned
above the reaction vessel, the aldehyde
vapor turns into liquid.
In a reflux apparatus, this condensed liquid
is returned to the reaction mixture.
3
The aldehyde that returns to the reaction
mixture is now in further contact with the
oxidizing agent. This pushes the reaction
further to the right to form a carboxylic acid.
1
2
reaction mixture
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http://chem-ilp.net/labTechniques/SimpleRefluxAnimation.htm
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Oxidation of Primary Alcohols
Example: Butan-1-ol reacts with acidified K2Cr2O7 .
Write the chemical equations showing the two step oxidation of this alcohol.
Include conditions.
Draw the structural formula of all reactants and products and name the products.
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Oxidation of Primary Alcohols
Example: Butan-1-ol reacts with acidified K2Cr2O7 .
Write equations showing the oxidation of this alcohol. Include conditions.
Draw the structural formula of all reactants and products and name the products.
CH3CH2CH2CH2OH + [O]
H+ / Cr2O72–
distillation
butanal
butan-1-ol
CH3CH2CH2CHO + [O]
butanal
CH3CH2CH2CHO
H+ / Cr2O72–
reflux
CH3CH2CH2COOH
butanoic acid
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Oxidation of Secondary Alcohols
2º alcohol + [O] → ketone + [O] → no further reaction
A secondary alcohol can only be oxidized to produce a ketone.
RCH(OH)R + [O] → RCOR
secondary alcohol
ketone
The C-C bonds and the C=O bonds
are strong and too difficult to
break. No further atoms can be
added to this carbon.
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Oxidation of Secondary Alcohols
Example: Pentan-2-ol reacts with acidified K2Cr2O7 .
Write the chemical equations showing the oxidation of this alcohol.
Include conditions.
Draw the structural formula of all reactants and products and name the products.
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Oxidation of Secondary Alcohols
Example: Pentan-2-ol reacts with acidified K2Cr2O7 .
Write the chemical equations showing the oxidation of this alcohol.
Include conditions.
Draw the structural formula of all reactants and products and name the products.
CH3CH(OH)CH2CH2CH3 + [O]
pentan-2-ol
H+ / Cr2O72–
distillation
CH3COCH2CH2CH3
pentan-2-one
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Oxidation of Tertiary Alcohols
3º alcohol + [O] → no further reaction
CH3
H3C C CH3
OH
The C-C bonds are strong and too difficult
to break. The C-O bond cannot be made
into a double C=O bond because carbon
atoms can only make 4 bonds.
A mixture of a tertiary alcohol and acidified potassium
dichromate would NOT show any colour change.
The mixture would remain orange.
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Condensation Reactions:
Alcohols + Carboyxlic Acids
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Condensation Reactions
Condensation reactions involve the joining of two small
molecules to form one larger molecule
with the loss of a small molecule (such as water).
A – OH
+H–B
A–B
+ H2O
Types of condensation reactions you should know:
formation of esters and polyesters
formation of amides and polyamides
(In this subtopic we will only look at condensation reactions that produce ESTERS.
Polyesters, amids and polyamides will be looked at later.)
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Condensation Reactions
formation of esters
carboxylic acid + alcohol
ester + water
+
conditions:
+
catalyst = conc. H2SO4
heat
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Condensation Reactions
example: ester formation
Methanoic acid reacts with ethanol in the presence
of concentrated H2SO4 at 80ºC.
Using structural diagrams, show the condensation reaction.
Name the products that form.
+
Note: Draw the reactants so that the
functional groups are “facing” each other.
+
products
ethyl methanoate + water
Note: The O atom in water is always
removed from the carboxylic acid.
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Condensation Reactions
importance of esters
← synthetic (& natural) flavourings
synthetic (& natural)
fragrances →
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Condensation Reactions
examples of esters and their odours
methyl butanoate
pentyl ethanoate
ethyl propanoate
2-methylpropyl butanoate
ethyl butanoate
propyl ethanoate
propyl octanoate
propyl butanoate
butyl heptanoate
propyl methanoate
octyl methanoate
butyl ethanoate
octyl ethanoate
butyl propanoate
octyl propanoate
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