Baphomet
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Isomerisation of alcohols
This may be a dumb question..
How can one determine if an alcohol is prone to spontaneous isomerisation?
I have been experimenting with some terpenoids (e.g. farnesol, linalool), converting them to alcohols. But sometimes they switch, for example from
secondary to primary alcohols.
There doesn't seem to be any rhyme or reason to it. Just interested to know if there are any rules about what makes them isomerise. Thanks....
\"Who ARE you? You\'re like the drummer from REO Speedwagon - nobody knows who you are\" from \'Employee of the Month\'
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JohnWW
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Are you quite sure of your identification of the isomerized alcohols, from the IR and NMR spectra especially?
Farnesol and linalool, terpenoids or alcohols derived from terpenes, along with nerol and geraniol and citronellol etc., are alkenes as well as
alcohols. Linalool, a tertiary alcohol found in the oils of lavender, bergamot, and coriander, is (CH3)2C=CH-CH2-CH2-C(OH)(CH3)-CH=CH2 . Farnesol, a
primary from ambrette seed oil, with an odor of lily of the valley, and also a development hormone in insects, is
(CH3)2C=CH-CH2-CH2-C(CH3)=CH-CH2-CH2-C(CH3)=CH-CH2(OH). Neither of these are enols, which undergo keto-enol tautomerism, but in each of them the -OH
group is alpha to the double bond. Any mechanism causing the migration of the -OH groups would have to involve ionization due to added acid H+ in a
polar solvent, initially as ROH2+ and then as H2O and a carbonium cation. However, at the same time, H+ can also protonate the double bonds to form a
single bond and a carbonium cation. When this happens, migration of the charge can then occur to form the most stable carbonium cation, with tertiary
ones being favored over secondary ones, and these favored over primary ones. So, no charge migration is likely in the case of the cation derived from
linalool, but the charge in the cation from farnesol would certainly migrate at least one and probably two C atoms to form a secondary or tertiary
cation., with the double bond migrating in the other direction. Then, if the acid is neutralized, the newly introduced -OH- anions would react with
these cations to form the new alcohols.
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Baphomet
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Thanks for your response. So this means that the hydroxy group can only move around when it is next to a double bond or some other structure that
'destabilises' the molecule..?
I'm purplexed because some of the alcohols I'm dealing with appear to re-arrange at room temperature from secondary to become primary alcohols! No
double bond involved, as far as I know, the compound is saturated.
And as you say, secondary should be thermodynamically favoured over primary.. This doesn't occur in solution by the way. I'm talking about once it has
already been distilled-off and separated.
BTW I confirmed it is going to primary because it changes the colour of KMnO4 solution (aldehyde test).
[Edited on 5-11-2008 by Baphomet]
\"Who ARE you? You\'re like the drummer from REO Speedwagon - nobody knows who you are\" from \'Employee of the Month\'
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