ALCOHOL
Chemistry of -OH
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Dr Seemal Jelani
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Alcohols
Propan- 1- ol
Propan- 2- ol
Butan- 1- ol
Butan - 1, 4 - diol
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These all have the formula C4H9OH
butan-1-ol
2-methylpropan-2-ol
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butan-2-ol
2-methylpropan-1-ol
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Bond angles in alcohol groups
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Solubility in water
The alcohol groups form
hydrogen bonding which
makes the short chain
molecules soluble in water.
The solubility in water
decreases as the chain
length increases.
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Low-mass alcohols are soluble
in water (because they
hydrogen bond with water).
As the hydrocarbon chain
lengthens, the solubility
decreases.
Ethanol
Propan-1-ol
Butan-1-ol
This photo shows ethanol, propan-1-ol and butan-1-ol in water. The first
two are completely miscible in water, while butan-1-ol is not miscible in
water.
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Boiling Points of Alcohols
Increases with molecular size due to increased instantaneous dipoles
• Alcohols have higher
boiling points than
similar molecular mass
alkanes
• This is due to the added
presence of intermolecular hydrogen
bonding
•
More energy is required
to
separate the
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molecules
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CLASSIFICATION OF ALCOHOLS
Aliphatic • general formula CnH2n+1OH - provided there are no rings
• the OH replaces an H in a basic hydrocarbon skeleton
NB. Aliphatic - straight chain molecule (not a ring / cyclic)
Structural
differences • Alcohols are classified according to the environment of
the OH group
• Chemical behaviour, e.g oxidation, often depends on the
structural type
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PRIMARY 1°
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SECONDARY 2°
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TERTIARY 3°
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Luca’s Reagent
• Solution of anhydrous zinc chloride in
concentrated hydrochloric acid
• This solution is used to classify alcohols of
low molecular weight.
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Distinguishing alcohols
Lucas reagent can be used to distinguish between low mass
primary, secondary and tertiary alcohols.
Lucas reagent contains anhydrous zinc chloride dissolved in
concentrated hydrochloric acid. It contains a very high
concentration of chloride ions and the Zn2+ ion acts as a
catalyst.
Take 1–2 mL of Lucas reagent in a dry test tube, add a few
drops of the alcohol and shake. If there is no reaction, place
the test tube in a beaker of boiling water for a few minutes.
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Distinguishing alcohols - Lucas test
Lucas reagent = conc. HCl and ZnCl2
Primary alcohol - remain unchanged
Secondary alcohol - will turn cloudy but takes a bit of time
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tertiary
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Jelani
alcohol - turns cloudy
immediately
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Tertiary alcohols turn
cloudy immediately.
Once heated, the
secondary alcohol
quickly turned cloudy.
The primary alcohol
tube is unchanged.
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OXIDATION OF PRIMARY ALCOHOLS
Primary alcohols are easily oxidised to aldehydes
e.g.
CH3CH2OH(l) + [O]
ethanol
——>
CH3CHO(l) + H2O(l)
ethanal
it is essential to distil off the aldehyde before it gets oxidised to the
acid
CH3CHO(l) + [O]
ethanal
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——>
CH3COOH(l)
ethanoic acid
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Practical details
• The alcohol is dripped into a warm solution of acidified k2cr2o7
• Aldehydes have low boiling points - no hydrogen bonding - they distil
off immediately
• If it didn’t distil off it would be oxidised to the equivalent carboxylic
acid
• To oxidize an alcohol straight to the acid, reflux the mixture
•
•
•
•
Compound
Formula
Ethanol
C2H5OH
Ethanal
CH3CHO
Ethanoic acid CH3COOH
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Intermolecular bonding
hydrogen bonding
dipole-dipole
hydrogen bonding
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boiling point
78°C
23°C
118°C
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Oxidising a primary alcohol to an aldehyde
Full oxidation is not wanted:
use dilute acid and less dichromate. The
reaction mixture is heated gently,
ethanal vapourises (21°C) as soon as it is
formed and distils over. This stops it being
oxidised further to ethanoic acid.
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Apparatus for the oxidation of ethanol to ethanoic acid
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Oxidising a primary alcohol to a carboxylic acid
reflux
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Distil to separate
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Oxidising a secondary alcohol to a ketone
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Oxidation of alcohols
Primary and secondary alcohols are oxidised by acidified
potassium dichromate.
A beaker of hot water speeds up the reaction.
There is no reaction with tertiary alcohols.
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Oxidation of alcohols
Primary alcohols
Secondary alcohol
aldehydes
Carboxylic acid
Ketones
Don’t oxidise
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tertiary alcohol
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Formation of ethanol by fermentation
Conditions
yeast
warm, but no higher than 37°C (optimum temp. for
yeast)
Advantages
LOW ENERGY PROCESS
USES RENEWABLE RESOURCES - PLANT MATERIAL
SIMPLE EQUIPMENT
Disadvantages
SLOW
PRODUCES IMPURE ETHANOL - will need distilling to purify
BATCH PROCESS
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Formation of haloalkane
Ethanol and PCl5
Phosphoryl chloride
C2H5OH(l) + PCl5(s)

C2H5Cl(g) + POCl3(l) + HCl(g)
solid
fumes
Thionyl chloride
Ethanol and SOCl2
C2H5OH(l) + SOCl2(l)
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
C2H5Cl(g) + SO2(g) + HCl(g)
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Formation of ethanol from ethene
Advantages
Fast
Pure ethanol produced
Continuous process
Disadvantages
high energy process
Expensive plant required
Uses non-renewable fossil fuels to make ethene
Uses of ethanol
alcoholic drinks
SOLVENT - industrial alcohol / methylated spirits
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FUEL - petrol substitute in countries with limited oil reserves
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Dehydration of alcohols
Reagent:
concentrated sulphuric acid
or
passing the alcohol over aluminium oxide
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Reaction with sodium
The reaction is similar to the reaction of alkali metals
with water, but less vigorous.
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Esterification
Catalyst: concentrated H2SO4 (dehydrating agent - it removes water
causing the equilibrium to move to the right and increases the yield
Conditions:
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reflux
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Esters
Methyl Ethanoate
Ethanoate
Uses of esters
Methyl
Esters are fairly unreactive but that doesn’t make them useless
Used as flavourings
Naming esters
Named from the alcohol and carboxylic acid which made them...
CH3OH + CH3COOH
from ethanoic acid
CH3COOCH3 + H2O
CH3COOCH3
from methanol
METHYL ETHANOATE
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