Introduction to alcohols

Alcohols

The functional group in an alcohol is an –OH (hydroxyl) group. For example, ethanol looks like:

Alcohols in everyday life

Ethanol is the alcohol in wine, beer and other alcoholic drinks.

Governments tax ethanol in drinks as a way to raise money.

Methanol is very poisonous. People accidentally drinking small amounts of methanol suffer blindness and brain damage. Others die.

Methylated spirits used to be made by mixing ethanol with methanol – to make it poisonous and avoid the tax.

Today’s ‘meths’ contains no methanol, but it still contains a very bitter compound to make it undrinkable. Purple dye is also added to make it easy to recognise.

Water has no effect on this permanent marker…

… but the writing is easily removed using methylated spirits.

Alcohols are good solvents.

water

meths

Alcohols are used as solvents in many food essences and perfumes

Alcohols are useful fuels.

Combustion products are carbon dioxide and water.

Making ethanol

Ethanol, the alcohol people drink, can be made by the fermentation of sugars found in fruit, grain or milk.

yeast 2CH3CH2OH + 2CO2

ethanol

C6H12O6

glucose

All alcoholic drinks, such as wine, beer, whiskey, vodka and rum, are made by fermentation.

New Zealand exports ethanol made by fermentation of lactose (milk sugar), which is left over after cheese making.

Making methanol

Methanol is made from methane in two stages:

First the atoms in methane and steam are rearranged or reformed into carbon monoxide and hydrogen:

CH4 + H2O → CO + 3H2

This reaction is done at 800 °C. Some of the hydrogen formed is burnt to supply the heat.

The rest of the hydrogen and the carbon monoxide is converted into methanol under high pressure:

CO + 2H2 → CH3OH

Methanex New Zealand has two methanol manufacturing facilities, both in Taranaki. Together, when at full production, they can produce 2.4 million tonnes of methanol per year, most of which is exported to countries in Asia Pacific.

Methanol production distillation tower at Methanex's Motunui facility.

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How to name alcohols:They are named as for alkanes but with the final –e replaced by –ol. If there are more than two carbon atoms then a number is needed to indicate the exact position of the –OH group on the alkyl chain.

e.g.

If the structure has got a side chain.

Used for making Polyester fibers

If there are two hydroxyl groups.

Count from the hydroxyl side, in this case it is called 2-methyl-2-butanol or 2-

methylbutan-2-ol.

Classifi cation of alcohols depending on the position of the hydroxyl group.

Classification of alcohols

Alcohols are classified as primary (1°), secondary (2°), or tertiary (3°), according to the number of carbon atoms bonded to the carbon atom attached to the –OH group.

Methanol is considered a 1° alcohol.

Primary

Secondary

Tertiary

Exercise:

1. Name and label each of the following alcohols as primary, secondary or tertiary.

2. Draw structural formulae for, and classify, each of the following alcohol:

a) butan-2-ol b) 3-methyl-pentan-1-ol c) 2-methylhexan-2-ol

A guide to naming organic compounds.

locator prefix stem locator suffix

Find the longest carbon skeletonIdentify functional groups and side chains.Find their locationChuck the things in the above categories.

Quiz:Name and classify the following alcohols:

Practicals

1.Observe the combustion of ethanol and write down the chemical equation of the equation.2.Find solubility of alcohols.

Questions:1.Write a balanced equation for the complete combustion of ethanol.2.If a molecular substance is soluble in water what does that indicate about the polarity of the molecule?3.Describe the nature of the intermolecular bonding present in alcohols.4.Cyclohexane is less soluble than any of the alcohols used above, explain the reason for this.

AlcoholsAlcohols are those compounds containing the –OH group.

Both the C–O and O–H bonds are polar, causing alcohol molecules to be polar and to have stronger intermolecular forces than alkanes or alkenes of similar size, with correspondingly higher melting and boiling points.Methanol and ethanol are liquids at room temperature, with boiling points of 64 °C and 78 °C respectively.

Low-mass alcohols are also soluble in water because the polar –OH group is attractive to water molecules.

As the hydrocarbon chain lengthens, the solubility decreases.

This photo shows ethanol, propan-1-ol and butan-1-ol in water. The first two are completely miscible in water, while butan-1-ol is not miscible in water.

Ethanol Propan-1-ol Butan-1-ol

Like alkanes, alcohols are covalently-bonded molecules, therefore they do not conduct electricity.

Because the –OH group in alcohols is formed by electron sharing it is not an OH– ion. This means that when alcohols dissolve in water they do not form basic solutions.

A bottle of ethanol and a beaker of water with universal indicator added.

Take a sample of the ethanol.

Add to the beaker The indicator does not change colour.

Ethanol is neutral.

Like alkanes, alcohols burn easily.

The flame is cleaner than with alkanes because of the oxygen molecule inside the alcohol.

The methanol flame is so clean it is almost invisible.

methanol

ethanol

The dichromate, Cr2O72–, has been reduced to Cr3+:

Cr2O72– + 14H+ + 6e– → 2Cr3+ + 7H2O

Ethanol is oxidised by this reaction. The final product of the oxidation reaction is ethanoic acid.

CH3CH2OH + H2O → CH3COOH + 4H+ + 4e–

3CH3CH2OH + 3H2O → 3CH3COOH + 12H+ + 12e–

2Cr2O72– + 28H+ + 12e– → 4Cr3+ + 14H2O

3CH3CH2OH + 2Cr2O72– + 16H+ →

3CH3COOH + 4Cr3+ + 11H2O