Alcohols
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I am doing some AS level Chemistry coursework and I am trying to understand why the -OH functional group of alcohols makes such a bad leaving group in the Lucas test. Please can you explain it to me? Thank you.
Igloo writes ...
The –OH group is a relatively poor leaving group as the C-O bond is a relatively srong bond that needs to be broken. Furthermore, when the OH “leaves”, it becomes a hydroxide, OH-, ion. In the context of alcohols the following reaction would occur:
R-OH ---> R+ + OH-
The OH- ion, being an excellent nucleophile (largely since it has a negative charge), would simply attack the R+ (known as a carbocation) to form the original alcohol, i.e. no productive forwards reaction occurs.
However, in the Lucas Test, it is not the –OH group which leaves – it is the water molecule! The Lucas Test is carried out in the presence of a strong acid (concentrated HCl), and the alcohol molecules, R-OH, are rapidly protonated to form R-O+H2 ions.
When the R-O bond breaks, it forms a R+ ion and water (a much better leaving group!). If R is tertiary, the R+ carbocation is relatively well stabilised – therefore feasibly formed in the first place. It then readily reacts with Cl- ions to form RCl. This two-stage process occurs via what is called an SN1 mechanism. Secondary alcohols have R groups which produce R+ ions which are not so readily stabilised, so the Lucas Test only works rather slowly with these. As you know, primary alcohols do not give a positive result, since primary carbocations cannot be successfully stabilised – i.e. cannot be easily formed.
I haven’t the space to describe why tertiary carbocations are better stabilised than secondary, than primary, but that wasn’t your question anyway, so I’ll leave you to research further if necessary.
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updated: 24 April 2007
