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Transcript
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.