Transcript 10.4 Alcohols - SCIS Teachers

Alcohols
IB Chemistry Topic 10.4
10.4 Alcohols Asmt. Stmts
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10.4.1 Describe, using equations, the
complete combustion of alcohols.
10.4.2 Describe, using equations, the
oxidation reactions of alcohols.
10.4.3 Determine the products formed by the
oxidation of primary and secondary alcohols.
Properties of Alcohols
•Alcohols have the general formula: CnH2n+1OH
•The physical properties of alcohols are similar to
those of both water and hydrocarbons
•The shorter chain alcohols such as methanol and
ethanol are similar to water, in general they
•have higher boiling points than hydrocarbons but
lower than water
•dissolve in water to some degree
•are more polar than hydrocarbons but less polar
than water
Reactions of Alcohols
Alcohols undergo several types of reactions
including:
Combustion
Oxidation
Esterification
Dehydration
Reactions with active metals
We will focus on the first 2 reactions… for now.
10.4.1
Describe, using equations, the
complete combustion of alcohols.
Reactions of Alkenes:
The general mechanism
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Alcohols burn in an excess supply of oxygen
to form carbon dioxide and water in an
exothermic reaction:
2CH3OH(l) + 3O2(g) → 2CO2(g) + 4H2O(l)
∆H = -726kJmol-1
2C5H11OH(l) + 15O2(g) → 10CO2(g) + 12H2O(l)
∆H = -3330kJmol-1
10.4.2
Describe, using equations, the
oxidation reactions of alcohols.
Oxidation
Alcohols are oxidized to alkanals
(aldehydes) or alkanones (ketones)
Oxidizing Agents
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The most common oxidizing agents are KMnO4 in
basic solution, K2Cr2O7 in acidic solution, or oxygen
from the air.
[O] over the yields arrow indicates an oxidizing
agent
The most commonly used for organic reactions is
acidified potassium dichromate (VI)
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H+/Cr2O7 is bright orange
When heated, it oxidizes the alcohol and changes color, as
Cr+7 is reduced to Cr+3
The reduced form of chromium is green
Figure 10.55
Primary Alcohols are Oxidized
to Alkanals
Primary alcohols are oxidized to become
carboxylic acids in a two-step process.
Step 1: alcohol → aldehyde
Ex. Ethanol is oxidized to become ethanal
Primary Alcohols are Oxidized
to Alkanals
Primary alcohols are oxidized to become
carboxylic acids in a two-step process.
Step 2: aldehyde → carboxylic acid
Ex. Ethanal is further reduced to become
ethanoic acid
Mechanism of the
Oxidation of Primary Alcohols
Oxidation of Primary Alcohols
Experimental conditions may be altered to yield the
desired product…
Aldehydes can be removed by distilling it as it forms
(due to its lower boiling point)
If the carboxylic acid is the desired product, then the
aldehyde should be exposed to [O] for a long period
of time and heated under reflux
See Fig. 10.56
Secondary Alcohols are oxidized
to Alkanones
Secondary Alcohols are
oxidized to Alkanones
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Since secondary alcohols only have one
hydrogen attached to the carbon atom, the
only possible product is a ketone
Oxidation of Secondary
Alcohols
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Propan-2-ol will oxidize into propanone
Tertiary Alcohols are not easily
oxidized
10.4.3
Determine the products formed by
the oxidation of primary and
secondary alcohols.
Testing Solutions
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2,4-dinitrophenylhydrazine solution tests for the
presence of aldehydes and ketones by forming
orange crystals
To differentiate between the two classes:
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Fehling’s solution starts blue and will turn orange-brown in
the presence of aldehydes. It will remain blue for ketones.
Tollens’ reagent produces a silver mirror effect on the
inside of the test tube in the presence of aldehydes, but will
remain clear for ketones.
See Fig. 10.58
Testing Solutions
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Fehling’s Solution
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Aldehyde is orange
Ketone is blue (unreacted)
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Tollen’s Reagent
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Aldehyde is produces the
“silver mirror”
Ketone is clear (unreacted)
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