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 = C_nH_2n+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 40^oC.

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