Transcript 10.4 Alcohols

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.
10.4.1 Reactions of Alkenes:
Combustion
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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.
Primary, Secondary & Tertiary
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Draw and name a 5-carbon…
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Primary alcohol
Secondary alcohol
Tertiary alcohol
10.4.2 Oxidation
Alcohols are oxidized to alkanals
(aldehydes) or alkanones (ketones)
10.4.2 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), written as
H+/Cr2O7
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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
10.4.2 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
10.4.2 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
10.4.2 Mechanism of the
Oxidation of Primary Alcohols
10.4.2 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
Distill to obtain an aldehyde
water out
aldehyde vapor
condensing tube
water in
boling solution
condensed
aldehyde out
Heat under reflux to obtain a
carboxylic acid
condensing tube
water in
boling solution
water out
vapor condenses
and falls back
into flask
10.4.2 Secondary Alcohols are
oxidized to Alkanones
10.4.2 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
10.4.2 Oxidation of Secondary
Alcohols
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Propan-2-ol will oxidize into propanone
10.4.2 Tertiary Alcohols are not
easily oxidized
10.4.2 Esterification
Alcohol + Carboxylic acids  Esters
A quick note…
O
OH
+
O
propionic acid
OH
O
+
O
propan-1-ol
acetic acid
OH
acid
+
OH
ethyl acetate
acid
O
O
propan-1-ol
H2O
propyl propionate
+ H2O
10.4.3
Determine the products formed by
the oxidation of primary and
secondary alcohols.
10.4.3 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 colourless for ketones.
See Fig. 10.58
10.4.3 Testing Solutions
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Fehling’s Solution
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Aldehyde is orange
Ketone is blue (unreacted)
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http://www.youtube.com/watch?v=WmwTRbQLIVo
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Tollen’s Reagent
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Aldehyde is produces the “silver mirror”
Ketone is colourless (unreacted)
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http://www.youtube.com/watch?v=F-Emzzls6Io