Saturday, March 12, 2016
Alcohols
L.O: To understand the structure of alcohols and describe some
reactions involving alcohols
Do you remember making wine
in year 8?
 Alcoholic drinks contain the alcohol ethanol.
The drink is produced by fermenting plant
sugars
What is the combustion equation for ethanol?
Why does beer / wine smell
and taste of vinegar when it
is left out?
 Ethanol is oxidised into ethanoic acid by
microbes which live in the air
 Ethanol can also be oxidised by some
oxidising agents like potassium dichromate
Alcohols
 Alcohols (which are organic compounds as they contain
carbon) have an –OH functional group, with the general
formula
CnH2n+1OH
 Methanol – CH3OH
 Ethanol – C2H5OH
 Propanol – C3H7OH
Different alcohols are part of the same homologous series
as they have the same general formula
What do you observe when
sodium is reacted with water
http://www.youtube.com/watch?v=aGaWwnMy
KVE
Alcohol As Solvents
 Alcohols such as methanol and ethanol can dissolve most
compounds that water dissolves but they can also dissolve
substances that water cannot, such as oils, fats and hydrocarbons
 Ethanol is the solvent for perfumes and aftershaves (it can mix with
the oils (giving the smell) and the water)
 Methylated spirits (meths) is ethanol with other chemicals added to
it – meths are used as paint cleaners and fuels (though they are
poisonous to drink so are dyed)
Ethanol
 Ethanol is also useful as a fuel – for use in cars and other vehicles, it is
usually mixed with petrol
Producing Ethanol – Hydration
of Ethene
Ethanol can be produced in 3 major ways. You need to know the
advantages and disadvantages of each method
Ethene, which is obtained from crude oil, can be reacted with steam under
high temperature and pressure to form pure ethanol.
Ethene + Steam  Ethanol
Conditions = 300oc, 60-70atmospheres
C2H4
+ H2O
 C2H5OH
Advantages
• 100% atom economy (See
7.1)
• 95% yield
• Produces 96% pure ethanol
Disadvantages
• Feedstock is nonrenewable.
• Ethene must be produced
by cracking which requires
energy.
• Process requires a lot of
energy.
Producing Ethanol –
Fermentation of Sugar
For thousands of years humans have been producing ethanol by
fermentation of sugar. Common feed stocks are maize, corn and sugar cane.
Glucose  Ethanol + Carbon Dioxide
Conditions = 25-37oc, 1atmosphere
C6H12O6  2C2H5OH + 2CO2
However the product is impure as concentrations higher than 15%
denature the enzymes killing the yeast. To purify it distillation is used,
which uses energy.
Advantages
• Uses renewable feedstock
• Operates at low
temperatures and pressures
• Carbon neutral fuel source
Disadvantages
• Produces impure ethanol
• Atom economy is 51%
• Still requires fossil fuels for
purification
• Feed stock needs to be
grown which takes land
which could be used to
Exam Questions
Method 1 is the least sustainable as it uses
ethene which comes from crude oil and is
not renewable. Methods 2 and 3 use
renewable resources which can be
regrown.
Atom economy. A process which
produces less waste is more sustainable. If
waste products can be used elsewhere
the process is more sustainable.
Exam Questions
Sodium fizzes
slowly
Sodium fizzes
violently on
surface of
water
No reaction
Ethanol and water both contain O-H groups so
react similarly. However hexane only contains
C-H and C-C bonds so is unreactive.
Exam Questions
High concentrations of ethanol denature the
enzymes in yeast killing it.
Fractional Distillation
The mixture of ethanol and water is
heated. Ethanol has a lower boiling point
than water so evaporates and is
collected first.
Exam Questions
High concentrations of ethanol denature the
enzymes in yeast killing it and the reaction does
not complete. Also other reactions take place
using the glucose.
Exam Questions
At high temperatures the enzymes in the yeast
denature killing it. However at low
temperatures there is little energy for the
reaction to take place and the reaction is too
slow.
Exam Questions
Kerboodle alcohol ‘on your marks’
Ethanol
 Ethanol can be manufactured by reacting ethene (from cracking
crude oil) with steam –phosphoric acid is used as a catalyst: Ethene + Steam  Ethanol
C2H4+ H2O C2H5OH
 In the reaction ethanol is the only product – the process is
continuous as long as ethene and steam are fed into one end of the
reaction vessel, ethanol will be produced
 These features make it an efficient process, but as ethene is made
from crude oil, which is a non-renewable resource, it cannot be
replaced once it is used up and it will run out one day
Ethanol & Fermentation
 Ethanol can also be made via fermentation – sugar from plant
material is converted into ethanol and carbon dioxide…
 Enzymes found in single-celled fungi (yeast) are the natural catalysts
that can make this process happen (this is a renewable resource): C6H12O6  2C2H5OH + 2CO2
 More than 90% of the world’s ethanol is
made by fermentation