Download reactions of alcohols

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Reactions of alcohols
N Goalby
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Solubility of alcohols in water
•
•
The smaller alcohols (up to 3 carbons) are
soluble in water. The longer the hydrocarbon
chain the less soluble the alcohol
The hydrocarbon part of the chain cannot
hydrogen bond with water molecules whereas
the OH part can form hydrogen bonds with
water. As the hydrocarbon chain becomes
longer the OH group is blocked from forming
hydrogen bonds with water and the alcohol is
no longer soluble.
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Different types of alcohols
H
H
H
H
C
C
C
H
H
H
Propan-1-ol
O
Primary
O
H
C
C
H
H
H
C
H
H
2-methylpropan-2-ol
Tertiary
H
H
C
C
C
H
O
H
H
Primary alcohols are alcohols where 1
carbon is attached to the carbon adjoining
the oxygen
H
C
Propan-2-ol
Secondary
H
H
H
H
H
H
H
Secondary alcohols are alcohols where 2
carbon are attached to the carbon adjoining
the oxygen
Tertiary alcohols are alcohols where 3
carbon are attached to the carbon adjoining
the oxygen
Oxidation reactions of the alcohols
• potassium dichromate K2Cr2O7 is an
oxidising agent that causes alcohols to
oxidise.
The exact reaction, however, depends on the type of
alcohol i.e. whether it is primary, secondary, or tertiary,
and on the conditions.
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Partial Oxidation of Primary Alcohols
Reaction: primary alcohol
aldehyde
H
H
C
O
C
H
H
Reagent: potassium dichromate(VI) solution and dilute
sulphuric acid.
Conditions: (use a limited amount of dichromate) warm
gently and distil out the aldehyde as it forms:
Distillation for partial
oxidation
thermometer
liebig
condenser
water in
Alcohol +
acidified
dichromate
mixture
water out
anti-bumping
granules
HEAT
Aldehyde
distillate
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Equation for partial oxidation : Structural
H
H
H
H
C
C
C
H
H
H
O
H
H
+ [O]
H
H
C
C
H
H
O
C
+ H2O
H
propanal
propan-1-ol
An aldehyde always has the C=O bond on the last carbon of the
chain so it does not need an extra number
General Equation for partial oxidation reaction :
CH3CH2CH2OH + [O]
CH3CH2CHO+ H2O
oxidising agent
Observation: the orange dichromate ion (Cr2O72-) reduces to the green
Cr 3+ ion
Full equation showing changes in dichromate
Here Cr is +3 and is
green coloured
Here Cr is +6 and is
orange coloured
Na2Cr2O7 + 3C3H7OH + 4H2SO4
Na2SO4 + Cr2(SO4)3
+ 3C2H5CHO + 7H2O
Propan-1-ol
Propanal
You don’t need to learn this equation.
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Full Oxidation of Primary Alcohols
• Reaction: primary alcohol
carboxylic acid
H
H
H
C
C
H
H
O
C
O
H
• Reagent: potassium dichromate(VI) solution and
dilute sulphuric acid.
•
Conditions: use an excess of dichromate, and heat
under reflux: (distill off product after the reaction
has finished)
Reflux Apparatus for
use with full oxidation
water OUT
water IN
Reflux mixture of
alcohol and acidified
dhichromate
anti-bumping
granules
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Equation for full oxidation : Structural
H
H
H
H
C
C
C
H
H
H
O
H
+ 2 [O]
H
H
H
C
C
H
H
O
+ H2O
C
O
H
Propanoic acid
propan-1-ol
Equation for full oxidation reaction :
CH3CH2CH2OH + 2 [O]
CH3CH2CO2H+ H2O
Observation: the orange dichromate ion (Cr2O72-) reduces
to the green Cr 3+ ion
Oxidation of Secondary Alcohols
• Reaction: secondary alcohol
ketone
H
H
O
H
C
C
C
H
H
H
• Reagent: potassium dichromate(VI) solution and
dilute sulphuric acid.
•
Conditions: heat under reflux
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Equation for oxidation of secondary alcohol : Structural
H
H
H
H
C
C
C
H
O
H
H
+ [O]
H
H
O
H
C
C
C
H
+ H2O
H
H
Propanone
H
propan-2-ol
Equation for oxidation reaction of secondary alcohol :
CH3CHOHCH3 + [O]
CH3COCH3+ H2O
Observation: the orange dichromate ion (Cr2O72-) reduces to the green
Cr 3+ ion
There is no further oxidation of the ketone under these
conditions.
Oxidation of tertiary alcohols
• Tertiary alcohols cannot be oxidised at all by
sodium dichromate
Remember to learn
- Potassium Dichromate is an oxidising agent
and changes colour from the orange Cr2O7 2ion to the green Cr 3+ ion
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Summary of Oxidation reactions of the alcohols
• potassium dichromate K2Cr2O7 is an oxidising
agent that causes alcohols to oxidise.
• When it reacts it changes from orange to green
The exact reaction, however, depends on the type of alcohol
i.e. whether it is primary, secondary, or tertiary, and on the
conditions.
Primary alcohol
Secondary alcohol
tertiary alcohol
Does not oxidise
aldehyde
ketone
Carboxylic acid
Reaction of alcohols with Dehydrating agents
Dehydration Reaction: Definition removal of a water
molecule from a molecule
Change in functional group : Alcohol
Alkene
Reagents : Concentrated Sulphuric or Phosphoric acids
(classed as dehydrating agents)
Conditions: warm (under reflux)
Type of reaction: acid catalysed elimination
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Equation for dehydration of alcohol : Structural
H
H
H
H
C
C
C
H
H
H
O
C
H
H
C
H
H
+ H2O
C
H
H
H
Propene
propan-1-ol
Equation for dehydration reaction of alcohol :
CH3CH2CH2OH
CH3CH=CH2 + H2O
This reaction can be classified as either dehydration
or elimination
Some 2o and 3o alcohols can give more than one product,
when the double bond forms between different carbon atoms
H
H
H
H
H
C
C
C
C
H
H
O
H
H
H
H
C
C
Butan-2-ol
H
H
H
C
C
H
H
H
H
But-1-ene
H
Butan-2-ol can form
both alkenes although
more but-2-ene would
be formed
H
H H
C
C
H
H
C
But-2-ene
H
C
H
Butan-1-ol only forms one alkene
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Test for presence of aldehyde :
Tollen’s Reagent
• Reagent: Tollen’s Reagent formed by mixing aqueous
ammonia and silver nitrate. The active substance is the
complex ion of [Ag(NH3)2]+ .
• Conditions: heat gently
• Reaction: aldehydes only are oxidised by Tollen’s reagent
into a carboxylic acid and the silver(I) ions are reduced to
silver atoms coating the inside of the test tube .
The silver coating the inside of
test tube is called a silver mirror.
(ketones do not react)
CH3CHO + 2Ag+ + H2O
CH3COOH + 2Ag + 2H+
Test for presence of aldehyde :Fehlings
• Reagent: Fehling’s Solution containing blue Cu 2+ ions.
• Conditions: heat gently
• Reaction: aldehydes only are oxidised by Fehling’s
Solution into a carboxylic acid and the copper ions are
reduced to a red precipitate of copper(I) oxide .
(ketones do not react)
.
• Learn the colour change of Blue Cu 2+ ions in solution
to red precipitate of Cu2O
CH3CHO + 2Cu2+ + 2H2O
Blue
solution
CH3COOH + Cu2O + 4H+
red
precipitate
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Test for presence of a Carboxylic Acid
• reaction: carboxylic acid
carboxylic acid salt
• Reagents: sodium carbonate solid or solution
of sodium hydrogen carbonate solution
Conditions: room temp.
2CH3COOH + Na2CO3
2CH3COO-Na+ +H2O + CO2
Ethanoic Acid
Sodium Ethanoate
• Observation : fizzing will occur and the gas
given off will make Lime Water turn cloudy
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