Lucas reagent

lucas reagent is a mixture of zinc chloride and conc. hydrochloric acid.
it's used to test for the presence of alcohol (alkanol), as well as the "degree" of that alcohol, i.e. primary, secondary or tertiary.

in conc. hydrochloric acid and salt (zinc chloride), polarity of solution is very high. if there's substitution reaction, uni-molecular (e.g. uni-molecular nucleophilic substitution, Sn1) is more favoured than Sn2 as the carbocation formed is stabilized by the highly polar solvent (by solvation of ions).

conc. acid can protonate the hydroxy group, making it a good leaving group for Sn-reactions (charge density of OH2 is lower than (-)OH ion).
Sn1 reaction occurs, substituting -OH by -Cl.

since chloroalkanes are less soluble than alcohols (for obvious reason: lack of H-bonding), presence of alcohol is indicated by formation of turbid (milky, non-transparent) mixture.

R-OH ----> R-OH2(+) <====> R(+) + H2O ---Cl(-)---> R-Cl


as you may have learnt, Sn1 is more favourable for tertiary alcohol for the alkyl-stabilized carbocation, the tertiary alcohol will become turbid in a very rapid manner.
primary alcohols, however, undergo Sn1 very slowly due to unstable carbocation. they give turbidity in slower manner.

some alcohols, which give rise to stabilized carbocation (like benzyl alcohol, C6H5-CH2-OH, which is stabilized by conjugation to benzene ring) also react very fast.


one reminder is, of course, the alcohol tested should be soluble in water in some extent -- such that it can turn the solution milky by formation of chloroalkane in water.


2011-04-29 23:16:17 補充：
Lucas reagent is to test the degree of alcohols.
upon mixing, turning turbid in faster manner indicates higher degree.

tertiary: R1R2R3-C-OH
secondary: R1R2-CH-OH
primary: R-CH2-OH
methanol: CH3-OH
rate: tertiary > secondary > primary >> methanol