11.03.1 Alcohols
Syllabus
Alcohols contain the functional group –OH.
Methanol, Ethanol, Propanol and Butanol are the first four members of a homologous series of alcohols.
Alcohols can be represented in the following forms: CH3CH2OH or as shown right
Students should :
be able to describe what happens when any of the first four alcohols react with Sodium, burn in air, are added to water, react with an oxidising agent
be able to recall the main uses of these Alcohols. Aqueous solutions of Ethanol are produced when sugar solutions are fermented using yeast.
know the conditions used for fermentation of sugar using yeast.
be able to recognise Alcohols from their names or from given formulae.
Students do not need to know the names of individual alcohols other than Methanol, Ethanol, Propanol and Butanol.
Students are not expected to write balanced chemical equations for the reactions of alcohols other than for combustion reactions.
What does this mean?
Basic Organic
Previously we learned that:
molecules with one Carbon atom begin with Meth
molecules with two Carbon atoms and no C=C bond begin with Eth
molecules with three Carbon atoms and no C=C bond begin with Prop
molecules with four Carbon atoms and no C=C bond begin with But
We learned that a functional group is the reactive part of a molecule.
Alkanes don't have a functional group.
Alkenes have a C=C bond
Alcohols have a -O-H group
So the shortest alcohol will look a lot like Methane but with the -O-H group.
The next alcohol will look a lot like Ethane but with the -O-H group.
etc
So, the name Ethanol tells us what the molecule will look like:
Eth tells us 2 Carbon atoms, an tells us no double bonds, ol tells us that it has the OH functional group
The unlabeled molecule to the right has:
six carbons
no double bonds
the OH functional group.
So, we would call it Hex-an-ol - though we would just write Hexanol
Combustion
The syllabus says we should be able to write balanced equations for burning Methanol, Ethanol, Propanol and Butanol.
It would help to know that the general formula of alcohols is the same as the general formula of Alkanes with the addition of an Oxygen atom = CnH2n+2O
And also that they burn to form CO2 and Water.
Don't forget to count the O in the alcohol when balancing Oxygens at the end.
Methanol: CH4O + 1.5 O2 --> CO2 + 2 H2O
Ethanol: C2H6O + 3 O2 --> 2 CO2 + 3 H2O
Propanol: C3H8O + 4.5 O2 --> 3 CO2 + 4 H2O
Butanol: C4H10O + 6 O2 --> 4 CO2 + 5 H2O
Oxidation
If we leave alcohols in air long enough they react with Oxygen (are oxidised) without burning and become sour.
They turn into Carboxylic Acids, which is how vinegar is made.
Oxidising Agents are substances that cause this to happen quickly.
We don't need to know any other details
Early breathalysers oxidise alcohol in breath and change colour to show how much alcohol was there.
Reaction with Sodium.
Alcohols react with Sodium to form Hydrogen gas.
We could collect the gas and carry out a pop test to prove it is Hydrogen.
We don't need to be able to write symbol equations for these reactions.
The syllabus is unclear as to whether we should be able to write word equations, but it doesn't say we should.
So we can assume that knowing Hydrogen is made is enough detail.
Making Alcohol
Traditionally we make ethanol by fermentation.
Carbohydrates (Starches and sugars) and water are mixed with yeast at a temperature below 40oC.
The Carbohydrates break down into Glucose and enzymes in the yeast turn this into Ethanol and Carbon Dioxide.
We don't seem to need to know the symbol equation.
But we should probably know
that too high a temperature kills the yeast and stops fermentation
that the presence of Oxygen would cause the alcohol to sour
that all yeasts will die when the concentration of alcohol goes much above 14%, making it impossible to make pure alcohol this way.
that we can distill solutions of alcohol until they are about 98% pure
Uses of Alcohols.
All alcohols are poisonous - though that doesn't stop people drinking Ethanol.
They also make good solvents and can dissolve some substances that water cannot.
And they burn well so make fuels - which have the advantage of being renewable since we "grow" alcohol from sugar.
Though this 'wastes' agricultural land that could produce food
Videos
Past Paper Questions
2021
10.4 Fermenting / Fermentation
10.5 warm allow a value in the range 25 °C to 45 °C
Anaerobic (conditions) allow without oxygen / air
10.7 C4H9OH + 6 O2 → 4 CO2 + 5 H2O allow C4H10O for C4H9OH
allow multiples
allow 1 mark for C4H9OH + O2 → CO2 + H2O with incorrect / no multipliers ignore state symbols
2018