Alcohols
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I know that you can differentiate between primary and tertiary alcohols by using potassium dichromate(VI), but are there any other tests which can be used to differentiate between the two where there is a change in the tertiary alcohol.
Igloo writes
I am not sure what you mean by the phrase “where there is a change in the tertiary alcohol”, so I shall answer the question up to this point.
The majority of tests distinguishing between primary and tertiary alcohols rely on their differing behaviour in the presence of oxidising agents. The usual procedure is the one you mention using acidified potassium dichromate(VI).
One test which was once very much in vogue is known as the Lucas Test, and this is not concerned with oxidation, but with the conversion of alcohols to chloroalkanes, using what is known as Lucas’ Reagent. This is a mixture of concentrated hydrochloric acid and anhydrous zinc chloride (which acts as a catalyst):
ROH + HCl ----> RCl + H2O
The test relies on the difference in reactivity of primary and tertiary alcohols.
Tertiary alcohols react immediately with Lucas' Reagent, forming a cloudy mixture. This is because the chloroalkane formed is insoluble in water, so an emulsion is formed.
Primary alcohols do not react appreciably at room temperature.
So the observations are:
• no visible reaction : primary alcohol
• solution turns cloudy immediately: tertiary alcohol
Incidentally, as you might expect, secondary alcohols give an intermediate result, i.e. the mixture turns cloudy after about 3 minutes.
It is important that the reaction is carried out at room temperature. Why do you think that this is necessary?
You should research the reasons why tertiary alcohols tend to react the fastest.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 01 December 2006
