Practical investigations
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Hi, I am doing a practical investigation on the synthesis and analysis of paracetamol. One form of analysis that I am doing is analysing paracetamol acid/base behaviour.
I know that it has the properties of a phenol as well as an amide, so will be able to react with both acids and bases. My question is, what is paracetamol, i.e. acid or alkali, and how do I work this out?
Igloo writes ...
Paracetamol – by virtue of its phenol group - can be neutralised by dilute cold alkali to form phenoxide ions, -O-C6H4-NHCOCH3.
With hot alkali, not only is the phenoxide formed, but in addition the amide group is hydrolysed giving amine phenoxide ions, -O-C6H4-NH2, as well as ethanoate ions.
If paracetamol is boiled with a dilute acid, e.g. HCl, the phenol group remains in place, but the amide group once again is hydrolysed, giving the protonated amine ion, HO-C6H4NH3+ as well as ethanoic acid.
The reactions of the amide group with alkali and acid do not involve proton transfer, so they are not classified as acid-base reactions – they involve hydrolysis.
However the interaction of the phenol group with alkali does involve proton transfer, so can be classified as acid-base.
Since only the phenol group needs to be considered in terms of paracetamol’s acid-base characteristics, we can classify it as a very weak acid. Its saturated aqueous solution has a pH of about 6.
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.
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updated: 17 December 2006
