alcohols

Alcohols

Alcohols all contain the hydroxyl functional group (R-OH), it is the presence of the -OH hydroxyl group which is responsible for the reactions and much of chemistry of alcohols. Alcohols are named from the corresponding alkane by replacing the -e at the end of the alkane name by -ol. Alcohols can be classified as primary, secondary or tertiary depending on the number of alkyl groups attached to the carbon atom bonded to the hydroxyl group.

primary alcohols secondary alcohols tertiary alcohols
contain 2 hydrogen atoms and one alkyl group(R) attached to the carbon bonded to the -OH functional group contain 1 hydrogen atom and two alkyl groups(R) attached to the carbon bonded to the -OH functional group contain three alkyl groups(R) attached to the carbon bonded to the -OH functional group

This difference between primary, secondary and tertiary alcohols is shown below:

primary, secondary and tertiary alcohols

Many of the reactions of alcohols are the same, since they they all contain the same hydroxyl functional group (R-OH). However one area where the reactions of primary, secondary and tertiary alcohols is different is in their reactions with oxidising agents such as acidified potassium dichromate..

Oxididation of alcohols

primary and secondary alcohols are 
 oxidised by acidified potassium dichromate

One thing that is often confusing in chemistry is the use of the words oxidation and reduction, this is simply because there are so many different definitions that chemists use for these two words, for example oxidation can be:

The defintion which is used simply depends on the actual chemistry taking place, bear in mind that no matter what definition is used they all amount to the same thing, we just use the definition that helps explain most clearly the chemistry taking place. Here it is simplest to think about oxidation as the loss of hydrogen. To oxidise an alcohol two atoms of hydrogen are removed by the oxidising agent. To help you grasp what is happening when an alcohol is oxidised I have colour coded all the hydrogen atoms in the different types of alcohols brown, as shown in the image opposite.

Looking at the image you can see that primary alcohols are oxidised to form aldehydes, while secondary alcohols are oxidised to form ketones. Tertiary alcohols are not able to be oxidised with acidified potassium dichromate.

The oxidising agent which is most commonly used to oxidise alcohols is potassium dichromate (K2Cr2O72-). Potassium dichromate is an orange coloured solid which is a powerful oxidising agent, the chromium atoms in the solid dichromate have an oxidation state of +6 and this means they make excellent electron acceptors or oxidising agents. However in order to act as an oxidising agent it needs to be dissolved in acid, 1M sulfuric acid is normally used for this purpose.

Mild oxidation of alcohols

As an example of the oxidation process consider the oxidation of the primary alcohol ethanol to the aldehyde ethanal, the apparatus set-up is shown below. The set-up is simple distillation, the alcohol ethanol has a boiling point of 780C while the ethanal has a boiling point of only 230C. The oxidising agent, the dichromate and sulfuric acid are added to the pear shaped flask first then the ethanol is dripped in slowly. Gently warming will cause the ethanal vapour to enter the Liebig condenser where it will liquify and collect in the flask. It is good practice to cool the flask in iced water since the boiling point of the ethanal is close to room temperature (250C) and from my experience ethanal is very good at giving you a thumping headache!

apparatus set-up for the oxidation of alcohols

The equation for this oxidation reaction is often written as shown below, here the symbol [O] is used to represent the oxidising agent.

equation for the oxidation of ethanol to ethanal

Further oxidation

further oxidation of ethanal to ethanoic acid

The aldehyde ethanal produced by the oxidation of the alcohol ethanol can be further oxidised to a carboxylic acid, in this case ethanoic acid. The same potassium or sodium dichromate can be used to oxidised the ethanal but this time the conditions are made more severe. Concentrated sulfuric acid is used along with more dichromate in order to carry out this second oxidation reaction.

This time we will define oxidation as the addition of oxygen, slightly different from the example above where we defined oxidation as a loss of hydrogen, but rememeber at the end of the day it all amounts to exactly to same thing! Ethanal can be oxided to ethanoic acid. However the apparatus set-up will have to be modified.

The ethanol as above is oxidised to the aldehyde ethanal, however ethanal is very volatile and evapoated easily, so we will have to set-up a reflux experiment. The set-up is shown opposite. Here the ethanol is oxidised to ethanal which will evaporate and enter the Liebig condenser where it will be liquified and simply drip back down into the oxidising mixture of sulfuric acid and dichromate. After around 20 minutes or so the oxidation reaction should be complete. Simply rearrange the apparatus back into a distillation set-up and distill off the ethanoic acid (boiling point 1180C) produced.

Secondary alcohols as mentioned above are oxidised to ketones and since ketones have no hydrogen atoms attached to the carbonyl carbon they cannot be oxidised further using acidified dichromate as the oxidising agent.

oxidation of secondary alcohols produces 
 ketones

Colour changes during oxidation

The dichromate ion (Cr2O72-) is a bright orange colour. It contains chromium atoms in the +6 oxidation state, it is the presence of these ions which are responsible for the orange colour of the dichromate ion. When acidified dichromate solution is mixed with a primary or secondary alcohol the Cr+6 ion is reduced to the green Cr3+ ion. This is shown in the image below. However as mentioned earlier tertiary alcohols cannot be oxidised with acidified dichromate and when a warm acidified dichromate solution has a tertiary alcohol added no colour change is observed. This is outlined in the image below:

orange dichromat ions are reduced to the green Cr3+ ion by primary and secondary alcohols

Testing for aldehydes and ketones

Oxidising alcohols with acidified potassium dichromate will enable you to distinguish between tertiary alcohols and primary/secondary alcohols. However it cannot distinguish between primary and secondary alcohols, as both of these react with the acidified dichromate. However it is possible to test the products of the oxidation of primary and secondary alcohols which will enable us to differeniate between them.

Aldehydes are readily oxidised to carboxylic acids but ketones, the oxidation product of secondary alcohols are not readily oxidised, unless powerful oxidising agents are used. So by using mild oxidising agents such as Tollens reagent or Fehlings solution it is possible to differientiate between aldehydes and ketones and hence primary alcohols from secondary alcohols.

Fehling's and Tollens solutions

Fehling's solution is a beauitiful dark blue coloured solution made by mixing copper(II) sulfate solution and a solution of sodium tartrate in potassium or sodium hydroxide. If you simply add an alkaline solution such as sodium or potassium hydroxide to a transition metal ions (M2+) such as Cu2+ you will simply produce a precipitate of the metal hydroxide, in this case copper hydroxide. However the tartrate ions form a complex ion with the Cu2+ ion which prevents this precipiate of copper hydroxide forming.
If about 2ml of an aldehyde is warmed with a few mls of Fehlings solution in a hot water bath the aldehyde will reduce the Cu2+ ions present in the Fehlings solution to form the Cu+ ion, which in the alkaline conditions forms a orange-brown precipiate of copper (I) oxide.

Tollens solution is prepared by adding about 5ml of silver nitrate solution to a boiling tube then add a drop of sodium hydroxide solution. This will produce a brown precipitate of silver(I)oxide, now add drop by drop aqueous ammonia solution until the precipiate of silver oxide dissolves. This new solution contains the silver diammine complex [Ag(NH3)]+OH-. If a few drops of aldehyde are then added to the Tollens solution and warmed on a water bath the aldehyde reduces the silver ions (Ag+) present in the Tollens reagent to metallic silver. This is usually seen as a silver coating on the walls of the test-tube, it is often called a silver mirror. This is shown below:

results of Tollens and Fehling's tests on aldehydes and ketones

Key Points

Practice questions

Check your understanding - Questions on alcohols.

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