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Draw the structural formula of:
1.3,3-dimethylbutan-1-ol.
2.2-methyl-butan-2-ol.
Review of data exercise:
1.Explain the trend of BP of staight chain primary
alcohols.
2.Compare BP of alcohols with that of alkanes of
similar molar mass, what’s the difference, why?
3.What’s the BP of water, why does it have a
different BP.
Investigation:
Oxidation of alcohols.
Aim: To test different alcohols as reductants.
Method:
1.Place 5 drops of each alcohol in separate test
tubes, we have 4 alcohols.
2.Add 10 drops of dilute sulfuric acid to each
test tube followed by 2 drops of potassium
dichromate, shake.
3.If there is no change stand in warm water (60C)
for 1 minute, smell the product and record your
observation.
4.Repeat with potassium permanganate.
Reactions of alcohols
Oxidation of primary alcohols
Oxidation by permanganate
Methanol is a primary
alcohol.
Acidify the methanol
with dilute sulfuric
acid.
Put it in a beaker of hot
water.
Add a few drops of dilute potassium permanganate
solution.
The purple colour of the MnO4– ion
slowly fades.
The final solution is colourless.
Methanol has reduced the MnO4– to Mn2+:
MnO4– + 8H+ + 5e– → Mn2+ + 4H2O
Methanol is oxidised in this reaction.
It is a two-step reaction:
You only need to learn that methanol is oxidised to
methanoic acid (HCOOH).
Oxidation by dichromate
Pour a small amount of ethanol into a test tube and
acidify with dilute sulfuric acid.
Place in a beaker of hot
water and add a few
drops of orange
potassium dichromate
solution.
Leave in the beaker until no further change takes
place.
The orange dichromate solution turns blue/green.
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
In the days before electronic alcohol ‘sniffers’, New
Zealand police used ‘breathalysers’ to measure
breath alcohol. The subject blew through this tube
to fill a bag. The crystals of potassium dichromate
in the tube turned green if ethanol was present. If
there was sufficient ethanol in the breath to turn
crystals beyond the line green, then the person was
over the legal limit for driving.
Elimination of alcohols:
Elimination produces alkenes by the removal of
water, this is done by heating with
concentrated sulfuric acid, a dehydrating agent.
As with haloalkanes, this elimination will give more
than one product if the original alcohol is not
symmetric. The major product is the more highly
substituted alkene. (Saytzeff’s rule, the poorer
gets poorer.)
Substitution
Alcohols undergo substitution reactions in which the –
OH group is replaced with a halogen to form a
haloalkane. For example chorination can happen when
reacting alcohol with the following agents. The product
chloroalkane is insoluble in water and separates out.
Important chlorinating agents are:
• Phosphorus pentachloride, PCl5
• Phosphorus trichloride, PCl3
• Thionyl chloride, SOCl2
• Lucas reagent, HCl/ZnCl2
All these reactions are fastest with tertiary alcohols.
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 is 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.
Give the structural formula and the organic
products for each of the following reactions, if
there are two products, identify the major one.