Properties and reactions of alcohols

Alcohols are those compounds containing the –OH group.

Because alcohols can hydrogen bond with each other, alcohols have relatively high 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 they hydrogen bond with water).

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

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.

Each class of alcohols has characteristic chemical properties.

Oxidation of alcohols

Primary and secondary alcohols are oxidised by acidified potassium dichromate.

A beaker of hot water speeds up the reaction.

There is no reaction with tertiary alcohols.

Primary alcohols are oxidised to aldehydes, then further oxidised to carboxylic acids.

Propan-1-ol Propanal

Propanal

Propanoic acid

Secondary alcohols are oxidised to ketones:

propan-2-ol propanone (acetone)

Chlorination

Alcohols undergo substitution reactions with various chlorinating agents to form chloroalkanes.

Important chlorinating agents are:

• Phosphorus pentachloride, PCl5

• Phosphorus trichloride, PCl3

• Thionyl chloride, SOCl2

• Lucas reagent, HCl/ZnCl2

The first three also react with carboxylic acids to form acyl chlorides, while Lucas reagent only reacts with alcohols.

All these reactions are fastest with tertiary alcohols.

Be prepared to write one of these equations in the exam.

Click on this movie to see what happens when solid PCl5 is added to 2-methyl propan-2-ol (a tertiary alcohol).

Using Lucas reagent to distinguish alcohols

Lucas reagent be used to distinguish between low mass primary, secondary and tertiary alcohols.

Lucas reagent contains anhydrous zinc chloride dissolved in concentrated hydrochloric acid. It contains a very high concentration of chloride ions and the Zn2+ ion acts as a catalyst.

Take 1–2 mL of Lucas reagent in a dry test tube, add a few drops of the alcohol and shake. If there is no reaction, place the test tube in a beaker of boiling water for a few minutes.

Click on this movie to see 1°, then 2° and finally 3° alcohol added to Lucas reagent.

The primary and secondary alcohol test tubes you’ve just seen were placed in beaker of boiling water.

Once heated, the secondary alcohol quickly turned cloudy.

The primary alcohol tube is unchanged.

• Tertiary alcohol turns cloudy immediately

• Secondary alcohol turns cloudy after gentle heating for a few minutes

• Primary alcohol does not react.

Note:

1. Only add a few drops of alcohol.

2. Lucas reagent is very concentrated. Wash it off skin immediately. It’s also expensive, so use very small quantities.