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Chemistry: Unit F322: Chains, Energy and Resources
Alcohols
Ethanol
Alcohols are a homologous series with the functional group CnH2n+1OH.
Alcohols contain the atoms hydrogen and oxygen, and because of the
electronegativity of these atoms, alcohols are able to form hydrogen
bonds.
Other molecules of water
are then attracted to it; the
δ+ hydrogen of one
molecule is attracted to the
δ- oxygen of another
molecule.
Oxygen has a greater electronegativity than
hydrogen; the bonding pair of electrons in the
covalent bond is therefore pulled slightly
towards the oxygen.
A Hydrogen Bond is a strong intermolecular force of attraction formed between the lone pair of
electrons on an electronegative oxygen (nitrogen or fluorine) and an electron deficient hydrogen atom.
Miscibility with Water
Miscibility is a measure of the property of liquids to mix to form a homogenous
solution. Alcohols are miscible because some of the hydrogen bonds between the
ethanol molecules and some of the hydrogen bonds between the water molecules
are replaced by hydrogen bonds between the water and the alcohol.
As chain length increases, the alcohol
becomes less miscible.
Hydrogen
bonds
between
Ethanol and
water.
Only the –OH group of the alcohol can form
hydrogen bonds, so as the hydrocarbon gets
longer, less of the hydrogen bonds broken are
replaced, and all that is created are weak Van
der Waals’ forces. Most of the chain in the
longer alcohol molecules is made up from the
non-polar hydrocarbon tail. Because this cannot
make any hydrogen bonds, the alcohol is less
miscible.
Boiling Points of Alcohols


The boiling point of an alcohol is always
much higher than the alkane of the same
carbon chain length
The boiling point of the alcohols increases
as the carbon chain length increases. The
longer the carbon chain, the less volatile
the alcohol is.
Alkanes have a lower boiling point than alcohols because they can only form Van
der Waals forces, and these are much weaker than the hydrogen bonds which
alcohols can form.
Alcohols have hydrogen bonding, Van der Waals forces and dipole-dipole
interactions. As the length of the carbon chain increases and there are more
electrons, the strength of the temporary dipoles that are set up increases, so
more energy is needed to overcome the forces, thus giving a higher boiling point.
The preparation of Alcohols
1. Fermentation of Sugars
For
alcoholic
drinks
Raw material: Starch
Equation: C6H12O6  2C2H5OH + 2CO2
Reaction Conditions: yeast (zymase enzyme), absence
of oxygen, 25-37 degrees temperature
Purity of product: Not pure – reprocessing by
distillation is required
Advantages: Renewable; no high temperatures required; biological catalyst
Disadvantages: Not 100% atom economy; processing required to obtain pure
alcohol
2. Hydration of Ethene [mechanism of electrophillic addition]
Raw material: Ethene
Equation: C2H4 + H2O  C2H5OH
Reaction Conditions: Phosphoric acid catalyst, 330 degrees, high pressure
Purity of product: Pure
Advantages: 100% atom economy – addition reaction; fast, continuous
process
Disadvantages: Requires Ethene from crude oil, so not renewable; chemical
catalyst required; lots of fuel needed to create high pressure and high temperature.
For
industrial
alcohol
Homologous
Series
Alkenes
Functional
Group
C=C
Example
Name
Alcohols
C – OH
Propan-1-ol
Aldehydes
CHO
Propanal
Ketones
C=O
Propan-2-one
Carboxylic Acid
COOH
Propanoic acid
Esters
COOC
Ethyl Methanoate
Propene
Alcohols can be classified into primary, secondary and tertiary;
Primary alcohols have one carbon attached to the C-OH
Carboxylic Acids have the highest
boiling point; alcohols the next, and
Propan-1-ol
Aldehydes the lowest. This is because
carboxylic acids have hydrogen
bonds, dipole-dipole interactions and
Secondary alcohols have two carbons bonded to the C-OH
Van der Waals’ forces. Alcohols have
hydrogen bonding and Van der
Butan-2-ol
Waals’ and Aldehydes have only
dipole-dipole and Van der Waals’
interaction. This means that the most
Tertiary alcohols have three carbons bonded to the C-OH.
energy is needed to overcome the
2 methylbutan-2-ol
forces in the carboxylic acid, so this
creates the highest boiling point.
Reactions of Alcohols
1. Combustion
Alcohols can be used as fuels; they are burned in combustion
to produce energy. Ethanol is often used as a petrol substitute
in countries with limited oil reserves.
C2H5OH + 3O2  2CO2 + 3H2O
2. Elimination: Dehydration
Alcohols can be dehydrated by the elimination of a water molecule, forming an
alkene. Under reflux and hot concentrated sulphuric acid or by passing alcohol
vapour over hot aluminium oxide, this reaction occurs;
Ethanol

Ethene
+
Water
3. Esterification
Esters are made when an alcohol reacts with a carboxylic acid under a hot
concentrated sulphuric acid catalyst, forming an ester and water.
Ethanol
CH3CH2OH
Propanoic Acid
+
CH3CH2COOH
Ethyl Propanoate [+ Water]
 CH3CH2COOCH2CH3 + H2O
4. Oxidation (of primary and secondary alcohols)
A primary alcohol can be oxidised to form an Aldehyde, and then further oxidised
to form a carboxylic acid. The reagents used are acidified potassium dichromate
(K2Cr2O7) and concentrated sulphuric acid H2SO4 (source of H+ ions). If an
oxidisation reaction occurs, the acidified potassium dichromate turns from
orange to green.
1. Alcohol to Aldehyde
2. Aldehyde to Carboxylic Acid (under
reflux: continuous heating of the
reactant mixture, followed by
evaporation and condensation of the
mixture back into the flask)
Complete oxidation of a Primary Alcohol
Combining the two steps above to give one procedure which turns alcohols into
carboxylic acids (with a by-product of water).
Primary Alcohol + 2 [O]  Carboxylic Acid + Water
This is done under reflux; a continuous process with high temperatures.
Oxidation of a Secondary Alcohol
Secondary Alcohol + [O]  Ketone + Water
Reagents: Acidified Potassium
Dichromate (K2Cr2O7)
H2SO4 (source of H+)
Reflux
Colour change from orange to
green.
Propan-2-ol + [O] 
Propan-2-one
+
Oxidation of a Tertiary Alcohol
Tertiary alcohols cannot be oxidised. There are no hydrogens attached to the
carbon with the –OH functional group. This therefore means that no hydrogens
can be lost in oxidation, so a tertiary alcohol cannot be oxidised.
Water