(1) The 'alcohol' of the homologous series of alcohols!
What we call alcohol in everyday life is a
substance whose chemical name is ethanol.
Ethanol is just one member of
a family of substances, the homologous series we call alcohols, which
all have the C–OH 'hydroxy' functional
group in their structure.
The full displayed formula for the
first five members of the homologous series of ALCOHOLS
diagrams show ALL the covalent bonds (C-H, C-C, C-O and O-H)
in alcohol molecules, C-OH being the alcohol functional group
The structural formulae can also be written as:
CH3OH, CH3CH2OH, CH3CH2CH2OH, CH3CH2CH2CH2OH and
The general formula for this
particular alcohol series is
(n = number of carbon atoms in molecule)
(Note: the technical names for the 3rd,
4th and 5th displayed above are propan-1-ol, butan-1-ol and pentan-1-ol)
Methanol, ethanol, propanol and butanol
are all colourless flammable liquids that dissolve in water to give neutral solutions (pH
Don't write alcohol formula as
molecular formula, always give the displayed or structural formula clearly
showing the alcohol functional group, which is
CH3OH, NOT CH4O and
C2H5OH, NOT C2H6O
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(2) The structure,
physical properties and uses of ethanol
and other alcohols
- molecular structure, the physical and chemical properties of ethanol
- The first formula is the simplest formula to
show the structure of ethanol and the last image is the full graphic
structural displayed formula of ethanol (full structural formula), i.e. it shows
all the atoms
and the bonds linking them together in the molecule.
2D displayed formulae
and 3D impression of the displayed formula
3D ball and stick model of methanol CH3OH AND 3D space-filling model of methanol
Dot and cross diagram for the methanol molecule showing the inner shell
electrons of carbon and oxygen. Carbon's electrons are 2.4 (4
outer valency electrons, 4 short of noble gas), oxygen 2.6 (6
outer valency electrons, 2 short of noble gas) and hydrogen just
and stick model of the ethanol molecule read as CH3-CH2-OH.
2D displayed formula of ethanol, full structural formula,
3D version of the displayed formula of ethanol
dot and cross diagram of the ethanol molecule.
Only the outer electrons are shown in the dot
and cross diagram above. The 2 inner electrons of carbon and oxygen
are not shown. Again, carbon's electrons are 2.4 (4
outer valency electrons), oxygen 2.6 (6
outer valency electrons) and hydrogen just
1. This is
perhaps going beyond GCSE level?
- Ethanol is used as
a solvent, as a
biofuel (can be mixed with petrol or used directly), and used to make 'ethyl esters' (see
Esters page) as well as the 'potent' chemical present in alcoholic
- The % alcohol in wines, spirits and
beer varies from 1–40%.
- The alcohol (ethanol) used in
beer and wines is made by fermentation, NOT from ethene derived
from cracking crude oil.
- The fermentation chemistry to
produce alcoholic drinks is
- Note that ethanol can be made
from waste biomass (see
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(3) Ethanol can be produced by
and a class experiment to
illustrate the manufacture of ethanol ('alcohol') from sugar
- The raw material is usually a
sugar from a carbohydrate source e.g. sugar cane or sugar beet, molasses - a
by-product from processing sugar cane and sugar beet into sugar, starch
from maize, potatoes or wheat.
- The sugar is mixed with water and yeast at just above
room temperature (30-40oC) in a reactor vessel (a big vat!)
from which air (oxygen) is restricted.
- The fermentation is carried out in steel
containers which are easier to sterilise than traditional wooden barrels
- The fermenters must be sterilised to stop
other micro-organisms growing e.g. Acetobacter produces ethanoic
acid ('acetic acid') from sugars - great for vinegar production,
but who wants to drink sour beer or wine!
- The yeast contains an
enzyme called zymase which acts as the biological catalyst to convert
sugar to ethanol in fermentation.
- The enzyme works best at an optimum of pH ~6.
- Under anaerobic conditions at an optimum temperature of 30oC to
40oC, the sugars react via the enzymes in the yeast
cells to form ethanol and carbon dioxide gas.
- If the temperature is too low, the reaction is too
slow. If the temperature is to high the enzyme is denatured - the molecular
structure is distorted so the active site doesn't work properly.
- Oxygen from air must be excluded to maintain
conditions - the reaction is an example of anaerobic respiration.
- Other enzymes present can catalyse the
of ethanol to ethanoic acid (acetic acid, vinegar), not a nice taste.
- The fermentation reaction of the sugar
obtained from the sugar cane or sugar cane is ...
- glucose (sugar) == enzyme ==> ethanol + carbon
====> 2C2H5OH(aq) + 2CO2(g)
- The carbon dioxide is
allowed to escape and air is prevented from entering the reaction vessel
to stop oxygen oxidising ethanol to ethanoic acid ('acetic acid' or
vinegar!) ensuring the reaction occurs under anaerobic conditions. The
acid would also lower the pH lowering the effectiveness of the enzyme
and it wouldn't do the taste of beer any good either.
- The fermentation reactions stop when the
concentration of alcohol reaches a maximum of 10-20% when the alcohol kills
the yeast cells.
- When the reaction is over, the mixture is
filtered, or the 'dead yeast
sludge' allowed to settle out and the mixture decanted (poured off leaving
the sludge behind) prior to distillation.
- The fractional distillation (more details below)
concentrates the alcohol for particular products.
- e.g. wine is distilled to
make brandy, whisky is made from fermented grain like barley, vodka is very
concentrated ethanol and is also made from fermented grain or potatoes.
- Some spirits have up to 40% alcohol in them, beers
have about 3-4% alcohol in them - but distillation isn't necessary.
- The boiling point of ethanol, 78oC,
is lower than water, 100oC, so theoretically, the ethanol
should distil off first. However, there is a maximum of ~96% ethanol
in the distillate.
- An azeotropic mixture is formed of 95.6%
ethanol and 4.4% water, which cannot be separated by fractional
distillation, but it is sufficiently concentrated to be used as the
fuel bioethanol e.g. 5-20% of petrol (see section 9B on
alternatives to fossil fuels).
Teacher note - class fermentation experiment
- A simple recipe for small groups of pupils:
- Dissolves ~10g of granulated sugar in 100 cm3 of water
to make a ~10% solution.
- Pour into a 250 cm3 conical flask.
- Add a teaspoon of dried yeast.
- Seal the conical flask with a rubber bung and an inverted U tube
- You can just use a cotton wool plug, but more awkward
to get a gas sample to test.
- Keep in a warm place (if possible) between lessons.
- In fact after a few hours (maybe by the end of the
lesson?) the student can test for carbon dioxide with limewater - the carbon
dioxide should give a white precipitate with the limewater.
- I used to ask the students to bring some fruit juice
in their school bag and add it to the solution, or make up the fruit juice
to 100 ml and add the sugar and yeast.
- Not sure on H & S regulations on this point these days?
- In the next lesson I would then gather all the
fermented mixtures and filter them all into a large distillation flask.
- I'd then perform 'with great ceremony' the fractional
distillation described below.
- Then you demonstrate the flammability of the
'firewater' by burning the odd drop to show it was fuel.
- They were also allowed to smell the product, which
had a lovely fruity odour.
details on how is the ethanol separated from a
- The ethanol is separated from the reaction
mixture by fractional distillation to make a petrol additive fuel
or whisky! Ethanol has a lower boiling point (78oC) than water
(100oC) and distils off first giving a concentration of up to 95%
ethanol. The two vapours separate out in the fractionating column, the lower
boiling ethanol rising to the top, passing out into the condenser,
condensing to a liquid for collection in some suitable container.
- Most of
the higher boiling water condenses back into a liquid in the fractionating column and runs
back into the flask. This distillation is needed to make spirits like brandy
and whisky. The laboratory process is illustrated in the diagram on
the right using a glass column filled with glass beads and connected to the
distillation flask and a Liebig condenser (see
separation of mixtures -
distillation for more explanation).
Extra notes on the fermentation
- The progress of the fermentation
can be followed by measuring the density of the fermented liquid with a
hydrometer. Ethanol/alcohol is less dense than water/sugar so the
density changes as the sugar is converted into alcohol.
- When the concentration of alcohol
reaches about 10–20% the fermentation reaction stops because the yeast
cells are then killed by this high concentration of ethanol.
ethanol is classed as a toxic poison just like cyanide and arsenic!
- Its important to have the optimum
temperature (30oC – 40oC) otherwise the efficiency
of the process is affected. If the temperature is to high the enzymes in
the yeast cells are denatured and if the temperature is too low, the
reaction is too slow (see graph of rate of reaction below).
- The diagram
graphically illustrates the idea of the optimum temperature by showing
the rate of reaction varies for a fermentation process.
- The diagram
graphically illustrates the idea of the enzyme zymase working best at
around a pH of 4-8, average pH 6-7. In very acid or moderate to strong alkaline conditions
the zymase enzyme becomes very ineffective and the rate of reaction for
fermentation becomes extremely slow, inefficient and uneconomic (same
argument for temperature too).
- For more
details about enzymes see
in a solution made from
fermented sugar cane or sugar beet, can be concentrated by fractional distillation.
- The fermentation process is used to make
wines and beers for the food and drinks industry. Brandy is made from
distilling wine to concentrate the alcohol. Whisky is distilled from
fermented grain (e.g. barley) and vodka is distilled from fermented
grain or potatoes. Beers typically have 3–4% ethanol, most wines in the
supermarket seem to be ~15% and spirits may have a concentration of up
- In Brazil ethanol is blended with petrol to give an alternative motor
vehicle fuel (gasohol) i.e. an example of a biofuel. See also 9b.
- C2H5OH(l) +
3O2(g) ====> 2CO2(g) + 3H2O(l)
- Heat energy released from the exothermic reaction
- You produce various blends of petrol by
mixing ethanol from fermentation with petrol from the fractions of
distilled crude oil.
- The natural fermentation process would
have discovered by accident after its products were sampled and so beer
has been brewed for thousands of years. Most people in medieval times
would have drunk weak beer every day because it was less harmful than
polluted water supplies apart from pure natural spring water.
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The social and medical issues associated
with drinking alcoholic beverages
('alcohol') is the major ingredient in the drinks industry producing
beers, wines and spirits from fermentation processes.
- Ethanol has a powerful physiological
effect on the body, particularly the brain.
- Alcohol containing drinks initially make
you feel relaxed and less inhibited.
- However, there are health and social issues
about the medical and behavioural aspects of alcohol consumption e.g.
- Ethanol reduces brain activity e.g.
slower thinking and slower reaction responses to a changing situation
hence the obvious dangers from drink driving! Many serious injuries and
deaths result from 'drink driving' accidents.
- Your judgement is impaired and your
general physical coordination, including balance, are much affected.
- Imbibing large quantities of alcohol can
produce unconsciousness and a potentially fatal coma.
- Alcohol causes dehydration and brain
cell damage leading to decrease in brain function and long–term memory
- Alcohol causes liver damage and a very
serious condition, a liver disease called cirrhosis of the liver, you
may even need a liver transplant if lucky enough to obtain one if your
liver eventually fails to function.
- and addiction problems
adding unnecessary extra costs for the NHS (in the UK).
- Binge drinking and
alcohol dependency, especially among young people, can cause major social problems both within a family
and for the wider community with anti–social behaviour. This again cost
society in policing and doctors in A & E.
- Drinking too much can lead to
violent behaviour, silly and sometimes dangerous 'loutish'
- Alcoholism can lead to family
breakdown, loss of job and eventually homelessness.
- Irresponsible sexual behaviour
ranging from not using contraception, increasing chance of pregnancy
or passing on sexually transmitted diseases to the worst case
- Liver disease from alcohol abuse is now
showing up in young men and women in their 20s, but there are plenty of
older people drinking to much too.
- Just out of interest, doc b is
actually allergic to alcohol and can become quite ill after one
drink! maybe its a blessing? maybe not!, but I'm very relaxed and
relatively unstressed in my retirement and thoroughly enjoying to
continue to write this website without the need of either relaxants
- Methylated spirit is
mainly ethanol but
poisonous and nasty tasting chemicals like methanol
are added so it is not used as a
- Deaths have occurred from drinking
'meths' and from contaminated illicit alcoholic drinks.
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manufacture of ethanol from ethene and compared to fermentation
- Ethanol can also be produced by the reaction of
steam and ethene (an alkene from oil cracking)
in the presence of a strong acid catalyst (phosphoric
acid, H3PO4). Beyond the alcoholic drinks industry,
this is the main industrial method of manufacturing ethanol.
- The reversible reaction is carried out at a moderately high temperature
(e.g. 300oC) and a
high pressure (e.g. 60–70 times atmospheric pressure).
- The higher temperature and phosphoric acid catalyst
speed up the reaction and increasing pressure moves the equilibrium to the
right (side least gaseous molecules at 300oC)
- ethene + water
CH2=CH2 + H2O
====> CH3CH2OH (or C2H5OH)
- This is an example of an alkene addition reaction and a
hydration reaction because it
involves the addition of water to another molecule.
- Since there is only one product, the atom economy is
- This process can be carried out fast, efficiently
and continuously on a large industrial scale to produce high quality ethanol
compared to the slow fermentation process producing impure ethanol (see
- This is done on an industrial scale, and the
products from the reaction chamber are passed into a condenser where the
higher boiling ethanol and unreacted water readily condense
- When the gases are first passed over the catalyst,
the yield is only 5%.
- The more costly unreacted ethene can be recycled
because it remains as a gas (much lower boiling
point than water or ethanol) and so non is wasted.
- The separated ethanol/water mixture is concentrated and purified by
- This means there is no waste and the
atom economy is 100%.
- The ethene is obtained from catalytic or
cracking reactions at high temperatures of 450oC to 900oC
of alkane hydrocarbons from the fractional distillation of crude oil e.g.
- butane ==> ethane + ethene
- the ethane can be further cracked to make
- ethane ==> ethene + hydrogen
- so you can have, for example, a synthetic
route for ethanol as follows ...
- crude oil ====> ethane ====> ethene ====>
- Note that ethanol made from ethene is NOT a
renewable method of alcohol production because ethene is made from cracking
hydrocarbons from crude oil.
- Ethanol from plant material like sugar cane or
sugar beet can be considered renewable.
- Advantages and disadvantages of the two
methods of making ethanol
- We are talking fermentation and hydration of
ethene and the 'pros' and 'cons' of the ways of making
- See also 9b.
- BUT first for instance, are the two methods
of ethanol production 'green' and 'sustainable'
- Factors to consider include listed
which you can merge in with the 'pros' and 'cons' discussion that
- What is the source of raw material? Will
it run out?
- Fermentation: Sugar beet and sugar cane grow quickly,
particularly in warm climates and labour may be very cheap in third
world countries. So we have a sustainable renewable resource thanks to
- Cracking & ethene hydration: Crude
oil, from which ethene is obtained by cracking, will eventually run
out, and oil is a non-renewable resource, so not sustainable in the
- What are the energy costs? and catalyst
- Fermentation: Some energy is required to keep the
fermenting mixture at the optimum temperature of 30-40oC.
Yeast is relatively cheap to produce, since it reproduces and grows
- Cracking & ethene hydration: Both
processes need energy to sustain high pressure and high temperature
reaction conditions. There is also an extra cost for catalysts which
would cost a lot more than yeast.
- Are there any implications for climate
change? Are there any environmental issues?
- Fermentation: Carbon dioxide is produced in the
process, contributing to global warming, but, isn't it recycled via
photosynthesis when more sugar beet or sugar cane is grown?
- Cracking & ethene hydration: Neither
processes directly harms the environment, though there are dangers
from oil spillages in transporting oil in tankers.
- What is the atom economy? Is there much
- Fermentation: The atom economy is only 51% (see
calculation) because 49% by mass of the
sugar is lost as carbon dioxide. Not only that, as the yeast cells are
killed off by the high concentration of ethanol, not all of the sugar is
actually fermented further decreasing the efficiency of the process.
- Cracking & ethene hydration:
Cracking ethane and other hydrocarbons is quite high (93%) with only
hydrogen gas as the waste product (but this can be used in
hydrogenation processes and making ammonia). The atom economy is
very high for the hydration of ethene (theoretically 100% with just
- Is it a profitable process?, does it
make a profitable product?
- Fermentation: It would seem so from the point of view
of the food and drinks industry, breweries and vineyards make good
profits, though a vineyard's economy-profits can be dependent on the
weather. BUT, is it profitable to use the alcohol as a biofuel? e.g.
blended with petrol from oil.
- Cracking & ethene hydration: Both
processes are fast and efficient and can be run on a continuous
basis and at the moment the raw materials, from oil, are relatively
cheap, but the price will increase as oil reserves become depleted
in the future.
- Does the fermentation process have any
issues with society? e.g. are there particular benefits or risks?
- Fermentation: There are no particular health and
safety issues or great risks for the surrounding local communities,
unlike the potential hazards of running an oil refinery. The risks come
later with alcohol abuse! Benefits may include jobs for the local
economy and revenue for local farmers growing the sugar cane or sugar
- Cracking & ethene hydration: There
are important health and safety issues to deal with in the
petrochemical industry. You are dealing with highly flammable and
explosive gases being processed at high temperatures and pressures.
This poses dangers at all the time and so all the processes must be
carefully monitored and controlled, this is also increases the costs
of the processes because it requires very standards of engineering
and safety measures.
- Are there any issue with waste products?
- Fermentation: The waste carbon dioxide can be safely
released into the atmosphere, but it could be used in fizzy carbonated
drinks or even pumped into greenhouses to increase the rate of
photosynthesis - case of good recycling?
- Cracking & ethene hydration: The
only waste product from cracking is hydrogen gas, but this can be
used to hydrogenate vegetable oils to make margarine or reacted with
nitrogen to make ammonia.
Summing up the advantages of
fermentation to make ethanol
- In third world
countries and more advanced developing countries sugar cane/sugar beet
is a common crop and labour
is cheap and
the process uses a cheap renewable resource eg sugar cane grown
in Brazil or sugar beet in England.
- It does not require any costly advanced
technology compared to a large petrochemical complex based on crude oil.
- There is no need for high pressure chemical
reactors and less energy needed at the lower reaction temperature
- It does not require the importation of
expensive crude oil, a non–renewable resource and since based on an
agricultural system, it aught to be sustainable with a long–term
- It is also possible to make a range of
organic chemicals from ethanol itself.
Summing up the disadvantages of
fermentation to make ethanol
- Its a slow
reaction and made by an inefficient batch process, poor quality
product e.g. low aqueous concentration of ethanol. A batch process
means you have to keep on emptying the reaction vessel (e.g.
fermentation vat or tank) and clean it out and refill with reactants
i.e. yeast and sugar solution.
- The yield of the reaction is less
than that from the hydration process.
- The product is not very pure and
expensive purification via fractional distillation is required, and
even that has a limit of 95% purity.
- The resulting
ethanol ('alcohol') solution is not very concentrated.
- It only has 4–10%, rest water and waste products e.g. other organic chemicals formed
to, and yeast cell residues to remove.
- Therefore the alcohol must be
distilled from the fermentation mixture, so this purification is an
extra costly process requiring
lots of energy.
- The atom economy is lower than
that from the hydration of ethene.
- atom economy = 100 x mass of
useful products / mass of reactants
- from the equation:
====> 2C2H5OH + 2CO2
- and molecular masses: Mr(glucose)=
180 and Mr(ethanol) = 46, (C = 12, H = 1, O = 16)
atom economy = 100 x (2 x
46) / 180 = 51% for fermentation
- It is theoretically 100% for the ethene
- Large areas of agricultural land
are needed and tends towards monoculture agriculture (lack of diversity)
– in many countries
more food should be grown.
- Brazil has allowed the cutting down of large
areas of valuable rain forest.
- Therefore, producing ethanol in this way
does have quite an environmental impact.
Advantages of the hydration of ethene
route to ethanol manufacture
- Its a fast and
efficient continuous process in the petrochemical industry which
produces a relatively pure product in bulk quantities. Its NOT a batch
process, the ethene and water can be rapidly fed into the reactor
chamber and the product collected, such a system may run for months
without any need to replenish the catalyst or carry out maintenance
- Some countries may
have local oil supply (e.g. North Sea for UK, US and Middle East countries).
- It is much cheaper to produce ethanol
from ethene derived from cracking crude oil fractions compared to any
plant material and fermentation – oil is still relatively cheap, even if
it doesn't seem so when petrol prices go up!
- The product formed is much purer
than that from fermentation and requires less processing to obtain
100% pure ethanol, known as 'absolute alcohol'.
- On the initial pass of the
ethene–water mixture over the catalyst only a small percentage is
converted to ethanol, BUT, it is possible to recycle the unreacted
ethene and so the eventual yield is up to 95%, much higher than the
yield from fermentation.
- The reaction has a higher atom
economy, in fact it is theoretically 100% since the reaction
involves the simple addition of two molecules.
Disadvantages of hydration of ethene
route to ethanol manufacture
- It uses a non–renewable
finite resource of crude
oil and more costly technology and may not be sustainable in the
- Most countries have to import the crude
oil to make ethene from cracking – supply may be subject to world market
prices or politically unstable situations eg in the Middle East.
- In the long term, as oil reserves
decrease, the production of ethene from cracking oil hydrocarbons
may become increasingly costly.
Is bioethanol a carbon neutral fuel?
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(6) More notes on
the homologous alcohols and reactions of alcohols including
- A homologous series is a family of
compounds which have the same general formula and have
a similar molecular structure and similar chemical
properties because they have the same functional group of atoms e.g.
C–OH for an alcohol.
- Members of the alcohol homologous series
have similar physical properties such as
appearance, melting/boiling points, solubility etc. BUT show trends in
them e.g. steady increase in melting/boiling point with increase in carbon
number or molecular mass or in the case of alcohols, they become
progressively less soluble in water.
- It is important to realise that members of a given
homologous series like alcohols have similar chemical reactions
because their molecules contain the same functional group (OH) and so
you can predict the chemical reactions and products of the other
members of the alcohol series
The hydroxy functional group, C–O–H, is
group atoms common to all members of the alcohol homologous series that confer a
particular set of characteristic chemical reactions on each alcohol molecule
of the series.
- The simplest homologous
series of alcohols have the general formula CnH2n+1OH
where n = 1, 2, 3 etc. i.e. the number of carbon atoms in the alcohol
molecule (see the diagram of the first five alcoholism the diagram near the
top of the page).
- You must always make sure the C–O–H group
(OH, hydroxy) is
clear in any molecular structure e.g. displayed formula, you draw of an alcohol
i.e. the C–O–H bonds are clearly shown.
- C2H5OH may not be good
ticks all the boxes!
- The last alcohol structure given below is
the full displayed formula which you should definitely know, but you
also need to know the various abbreviated ways of writing the molecular
structure of alcohols.
- The simplest
alcohol with the
lowest carbon number of one is methanol (the 1st in the homologous
series of alcohols is shown below, followed by the next four in the series.
- Ethanol, discussed in detail above,
is the 2nd in the series,
next three alcohols are propanol (strictly speaking propan–1–ol, 3rd in series),
butanol (strictly speaking butan–1–ol, 4th in series) and pentanol
(strictly speaking pentan–1–ol, 5th in series), note all the alcohol names end in ...ol, which means the
molecule is an alcohol. The –1–ol means the OH group is on the first
carbon atom of the molecule's chain –C–C– etc.
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(7) Some physical
properties of alcohols related to their use as solvents
- All the alcohols are colourless liquids with a characteristic
- The first three alcohols dissolve in water
(miscible) but as the carbon chain grows longer they become less and less
soluble in water. The fourth alcohol, butanol, is quite soluble in water.
- The boiling point steadily rises from one
alcohol to next with increase in molecule size (increase in carbon
number), just like the boiling points rise in alkanes.
- Comparison of alcohols with alkanes
at room temperature and volatility
||Alcohols are colourless
liquids, large alcohol molecules may be white waxy solids. Ethanol
is quite volatile and readily evaporates into the air.
||Colourless gases or
liquids, the first few alkanes methane to butane are gases, the rest
are colourless liquids or white waxy solids.
||Water is a colourless
liquid, but not as volatile as ethanol.
for the same size of molecule (molecular mass) alcohols have a much higher boiling
point than alkanes
||Alcohols have a wide
range from 65oC to over 500oC
ethanol boils at 78oC
|Alkanes have a wide range
from –164oC to over 500oC
Hexane boils at 69oC
is very high for a very small molecule, but since ethanol only boils
at 78oC, the two can be readily separated by fractional
Solubility in water
||The first few alcohols
like ethanol are completely soluble (miscible), after that they
become progressively less soluble in water
||All hydrocarbons like
alkanes are insoluble in water
What will they
||The first few alcohols
are very useful industrial solvents e.g. methanol and ethanol dissolves a wide variety of
compounds including hydrocarbons to some extent, other alcohols,
carboxylic acids. Ethanol is used to dissolve molecules used in the perfume and cosmetic
industries. The important point here is that ethanol will dissolve
many compounds water can't. Methanol cannot be used in cosmetic
products - its too toxic!
||Alkane liquids like
hexane have limited use as solvents, they will dissolve other
hydrocarbons from diesel to waxes, but not much else.
||Water is very useful solvent,
dissolves a wide variety of compounds e.g. lots of salts, some
organic compounds like sugars, smaller alcohols, smaller carboxylic acids
(like ethanoic acid). Water is used
widely in all sorts of domestic products from cosmetics to cleaning
fluids as the main media or solvent.
- As mentioned above, alcohols very
useful solvents – they
dissolve a wide range of compounds, some that water dissolves, but others
like oils, fats and hydrocarbons dissolve in alcohols, which are insoluble
- Ethanol is used as a solvent in cosmetic
products like perfumes and aftershave lotions because it mixes well with
natural oils (smell 'scent') and water which makes up the bulk of
many cosmetic preparations.
- In these sorts of cosmetic products the aromatic oils and
water base become compatible in the alcohol.
Esters are also used as solvents
- Why does a substance dissolve in
one liquid solvent but not another?
- There are three particle interactions going
on if you mix one substance with another e.g. a liquid solvent that
may or may not dissolve a solid.
- The three possible attractions are (i)
solid ... solid, (ii) solid ... liquid and (iii) liquid ...
- The relative strength of these attractive
intermolecular forces decides whether e.g. a solid will dissolve in a
- For example, nail varnish will not dissolve
in water, but will dissolve in organic solvents like an ester, alcohol or
- Nail varnish is insoluble in water
because the intermolecular forces between the nail varnish molecules
themselves, and between the water molecules themselves are much stronger
than the attraction between water and the nail varnish molecules, so the
nail varnish cannot possibly dissolve in water. Forces (i) and (iii)
override force (ii)
- However, nail varnish will dissolve in
organic solvents like butyl ethanoate or ethyl ethanoate (esters, old
names butyl acetate and ethyl acetate), ethanol ('alcohol')
and propanone (old name acetone) solvents. Here the organic solvent
intermolecular attraction to the nail varnish molecules can override the
nail varnish ... nail varnish and the solvent ... solvent intermolecular
forces and the nail varnish will dissolve. In this case attractive force
(ii) overrides both attractive forces (i) and (iii).
- Since different solvents are different
molecular affinities for different substances, the solubility of a solute in
a solvent can vary quite considerably from one solvent to another.
- The question of which solvent you choose
to use to dissolve a substance depends on two main factors ..
- (a) How soluble is the substance in
- (b) How safe is to use the solvent?
e.g. in terms of inhaling vapour or spillage on the skin (gloves!), is it
harmful?, irritating?, even toxic?, and is it highly flammable, so more
dangerous to use.
- Chlorinated organic solvents e.g.
trichloromethane ('chloroform') tend to be harmful, alcohols and esters are
safer but are more flammable.
is repeated in esters
TOP OF PAGE
(8) Chemical properties of alcohols – important reactions and uses
- The first two alcohols, methanol and
ethanol are important chemical feedstock ('starting materials') for the
manufacture of many other organic chemicals e.g. esters, carboxylic
- Both ethanol and methanol are important solvents
in the chemical industry because they both dissolve a wide range of
compounds e.g. hydrocarbons, oils, fats, carboxylic acids. To get two
solids to react it is convenient to dissolve them in a solvent to
provide a reaction medium. Normally on mixing two solids you only get
little or no reaction - think particle theory!
alcohols behave chemically in the
same way (same functional group C–OH) e.g. they all reaction with
sodium, they all react with carboxylic acids to form esters.
- All alcohols are flammable and readily burn
when ignited in air.
- 'Methylated Spirits' ('meths') is mostly
ethanol with other chemicals added to it like methanol to make it unpalatable to drink,
since pure ethanol is highly poisonous, but meths is more toxic!
- A purple dye is added so you don't drink it
- Methylated sprits is used as a fuel in
camping cooker burners (spirit burners – combustion use) and for cleaning
paint brushes (solvent use).
- Its use as fuel for cars is discussed
- biogas, biodiesel, gashol, alternative fuels - hydrogen - ethanol is
blended with petrol to produce a partial biofuel. Brazil, with no
oil deposits, grows lots of sugar cane cheaply and this is
fermented to make ethanol for fuels and other industrial uses.
- Details of the reactions of alcohols are
short note on
- Esters are another homologous
series of organic compounds (dealt with in detail on another page)
Ethyl ethanoate, an ester, is formed by the
of carboxylic acid and an alcohol e.g.
- ethanoic acid + ethanol
ethyl ethanoate + water
- sometimes more simply written as
- CH3COOH + CH3CH2OH
CH3COOCH2CH3 + H2O
- General word equation: carboxylic acid + alcohol ==>
ester + water
- This is a reversible reaction, so does not go
to completion. When the equilibrium point is reached, you have also
reached the point of maximum yield, and, you get about 2/3
conversion of the acid and alcohol reactants to the ester product.
- Esters are used in perfumes and
food flavourings. Lots of details
in section 10b. for the ..
Procedure for preparing
an ester, uses of esters, details of esters & carboxylic acids
Alcohols react with sodium to form
hydrogen and an alkoxide ionic salt
- alcohol + sodium ==> an alkoxide +
- Normal 'hydrogen' gas fizzing is observed
at a moderate rate, and the salt
product is soluble in the alcohol itself e.g.
- ethanol + sodium ====> sodium
ethoxide + hydrogen
- It is just the same for the other group 1 alkali metals i.e. just
change the Na to K for the reactions with potassium.
- (ii) 2C2H5OH + 2Na
- There is some similarity with the
reaction of sodium with water (Alkali
sodium + water ==> sodium
hydroxide + hydrogen
2Na + 2H2O
2Na+OH– + H2
Both reactions give hydrogen
gas, though the sodium reacts much faster with water. The ethoxide and
hydroxide are similar because on evaporation of the unreacted ethanol or
water, a solid white ionic compound is formed.
It can further be noted that
relatively 'unreactive' alkanes do not react with sodium.
- similarly with other alcohols ...
- (i) methanol + sodium ====> sodium
methoxide + hydrogen
2CH3OH + 2Na ====> 2CH3O–Na+
- (iii) propanol + sodium ==> sodium
propoxide + hydrogen
+ 2Na ====> 2CH3CH2CH2O–Na+
- (iv) butanol + sodium ==> sodium
butoxide + hydrogen
+ 2Na ====> 2CH3CH2CH2CH2O–Na+
Ethanol can be oxidised to form
- Which is a useful organic chemical. BUT it is this oxidation of ethanol that results in alcoholic drinks
turning sour (e.g. cider, wine) when exposed to air!
- The fruit material
already contains the enzymes that catalyse the oxidation of ethanol
('alcohol') in the presence of air, note that these aerobic conditions
produce a very different reaction than anaerobic fermentation of sugar.
- ethanol + oxygen ====> ethanoic acid +
CH3CH2OH + O2
===> CH3COOH + H2O
- Ethanoic acid (old name 'acetic acid') is the basis of
- This 'natural oxidation' is used to
manufacture vinegar in bulk for the food industry and domestic
consumption in cooking and eating.
- In the chemical industry ethanol is oxidised to
ethanoic acid to making esters (e.g. for flavourings like ester pear
drop essence as mentioned above).
- Oxidation here, is where a molecule gains one
or more oxygen atoms.
- Check out the formulae, ethanol has one oxygen
atom and ethanoic acid has two oxygen atoms.
- This oxidation can also be done in the
heating the ethanol with a mixture of sulfuric acid and potassium
- This is a complex reaction and the mixture turns from orange to green
as the ethanol is oxidised.
- In industry you can oxidise ethanol
directly with oxygen on a large scale.
- Other alcohols can also be oxidised to the
corresponding carboxylic acid e.g.
- propanol + oxygen ====> propanoic acid + water
+ O2 ===> CH3CH2COOH + H2O
- butanol + oxygen ====> butanoic acid + water
+ O2 ===> CH3CH2CH2COOH +
- These reactions require special chemical
reagents, you can't burn them in air to make the carboxylic acid, you
would just make carbon dioxide and water on combustion!
- The equations have been greatly simplified!
- Some textbooks quote potassium manganate(VII)
instead of potassium dichromate(VI) as the oxidising, but it doesn't
work very well.
When burned, ethanol, like any alcohol, on
forms carbon dioxide and water
- (i) ethanol + oxygen ==> carbon dioxide +
3O2(g) ===> 2CO2(g) + 3H2O(l)
- As mentioned in section
9b Fuels Survey, ethanol can be
blended with petrol to fuel road vehicles.
- Similarly, but the symbol equations are
more awkward to balance ...
- (ii) methanol + oxygen ====> carbon dioxide
3O2(g) ===> 2CO2(g) + 4H2O(l)
- (iii) propanol + oxygen ====> carbon
dioxide + water
9O2(g) ===> 6CO2(g) + 8H2O(l)
- (iv) butanol + oxygen ====> carbon dioxide +
6O2(g) ====> 4CO2(g) + 5H2O(l)
- As mentioned in section
9b Fuels Survey, ethanol can be
blended with petrol to fuel road vehicles.
- As already mentioned, ethanol is used in
sprit burners where it burns much more cleanly with a blue flame -
using a hydrocarbon in the same situation is more smelly and gives a
more yellow smokey flame - less efficient combustion.
the heat of combustion of alcohols
- This can be determined using the simple
copper calorimeter (diagram on the right).
- You can compare the heat energy released by
different alcohols e.g. methanol, ethanol, propanol and butanol.
- The alcohol is poured into a little spirit
burner which is then weighed.
- The burner is placed under the copper
calorimeter, which is filled with a known mass of water at a known
- After burning for e.g. 5 minutes, the flame
is blown out and the final temperature noted.
- The burner reweighed and the mass decrease is
equal to the mass of alcohol burned.
- From the mass of water, the heat capacity of
water and the temperature change you can work out the heat energy
- You can then work out the heat released per
gram or per mole of alcohol.
- Or more simply, you might just compare the
mass of fuel burned to give the same temperature rise.
- The results should show that the efficiency
of an alcohol fuel increases with increase in carbon chain length of
the molecule i.e. pentanol > butanol > propanol > ethanol >
For lots more details on the method and
measuring heat energy transfers in chemical reactions
Ethanol can be dehydrated to ethene
- This is done by passing the alcohol vapour over
heated aluminium oxide catalyst.
- This is actually the reverse of the
reaction by which ethanol is made from ethene from cracking oil.
- ethanol ====> ethene + water
- CH3CH2OH ====>
CH2=CH2 + H2O
- This reaction is potentially an
important source of organic chemicals e.g. plastics made by
polymerising ethene, and from a renewable
resource since the ethanol can be made by fermentation of
- It is being used in countries that do
not have oil reserves but have large areas of agricultural land
producing sugar cane or sugar beet that are the raw materials for the
fermentation process to manufacture ethanol.
- The ethanol, so produced, becomes an
important chemical feedstock for producing lots of other chemicals
including bioethanol fuel.
- Alcohols from propanol upwards,
carbon number 3 or greater, will form isomers.
- Isomers are molecules with the same
molecular formula but the atoms can be arranged in two or more different
ways e.g. there are two propanols with the molecular formula C3H8O,
they are very similar, but different molecules.
- they are similar physically and
chemically, but they are not identical.
- You will find plenty of examples on the
Advanced organic chemistry
page for alcohols
the uses of some individual alcohols
A solvent. All four of these alcohols are
colourless liquids and extensively used as solvents in the chemical and
pharmaceutical industries. They are particularly useful solvents because
they dissolve many substances that dissolve in water plus many organic
compounds. This makes them very useful in the chemical synthesis,
extraction and purification of many organic compounds. BUT, it should be
noted, that these four alcohols are all highly flammable!
Methanol: Methanol can be used as a fuel
e.g. in biofuels or blended with petrol, and methanol is cleaner burning
compared to hydrocarbons (alcohol combustion equations are described above
with equations). Methanol is used as an antifreeze.
Ethanol: As already mentioned - an
important solvent and use in the alcohol drinks industry e.g. beers, spirits and
wines. Its solvent uses include the manufacture of cosmetic formulations,
pharmaceutical preparations, detergents, ink solutions and coating
formulations. Also used as
biofuel and in blends
of petrol. Ethanol is used as an anti-freeze in windscreen de-icing
Propanol: Used as a solvent
in the pharmaceutical industry and the preparation of resin and cellulose
Is used in a wide range of consumer products. It is used to make butyl
esters like butyl ethanoate (old name butyl acetate) which is used as
solvent itself and as an artificial flavouring agent e.g. both butanol and
butyl ethanoate are used in butter, cream, fruit, rum, whiskey, ice cream
and ices, candy, baked goods and cordials (permitted in the US, not sure
about the UK?). It is an ingredient in perfume formulations. Butanol is also
used as an extractant (solvent extraction) in the manufacture of
antibiotics, hormones, and vitamins. Its also a solvent for paints,
coatings, natural resins, gums, synthetic resins, dyes, alkaloids, and
TOP OF PAGE
which contains the
alcohol group –OH is a sterol, a sub–group of organic molecules called
steroids (BUT not the body building type of steroid!, more to do with
the metabolism of fats!). Cholesterol is an essential steroid–sterol to humans but if
too much is produced it can cause heart disease.
image on the right gives the skeletal formula structure of cholesterol
structure representation is usually only dealt with at advanced level). All the lines in the structure
represent bonds between carbon atoms except the 'wedge dash' to the –OH alcohol
group in the bottom left of the molecule. Also note the 'alkene' double
bond functional group to the right of the –OH group.
So, even at advanced
level, the same organic functional groups crop up and can be recognised! [Cholesterol
image from NIST]