6it is SN1 indeed................thats why u say that tertiary alcohols show turbidity instantly..........this hints at the fomation of a tertiary carbocation which is more stable than primary and secondary!!!
The reaction of alcohols with Lucas reagent undergoes
1) SN1
2)SN2
Arihant is saying SN1, but I was taught SN2 (I have a very logical mechanism by sn2.)
What u ppl think ?
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6 Answers
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21Keep laughing:(
hmm..okay... Then tell me what does zncl2 does in the reaction?
6HCl alone reacts poorly with 1° alcohols and 2° alcohols, but a combination of HCl with ZnCl2 , is eff. for the prep of alkyl chlorides.......
meanwhile , it is also said that this reacn. prob. proceeds via an SN2 mech with 1° alcohols but SN1 pathway with 2° alcohols.
1See Zncl2 actually forms a complex with the alcohol so that we get a better leaving group.
71Wikipedia :
"When Lucas' reagent (ZnCl2 in concentrated HCl solution) is added to the alcohol, H+ from HCl will protonate the -OH group of alcohol, so that the leaving group H2O, being a much weaker nucleophile than OH-, can be substituted by nucleophile Cl-. Lucas' reagent offers a polar medium in which SN1 mechanism is favored. In unimolecular nucleophilic substitution, the reaction rate is faster when the carbocation intermediate is more stabilized by greater number of electron donating alkyl group (R-) bonded to the positively charged carbon atom. Tertiary alcohols react immediately with Lucas reagent to produce turbidity while secondary alcohols do so in five minutes. Primary alcohols do not react appreciably with Lucas reagent at room temperature.
The reagent dissolves the alcohol, removing the OH group, forming a carbocation. The speed of this reaction is proportional to the energy required to form the carbocation, so tertiary, benzylic, and allylic carbocations react quickly, while smaller, less substituted, alcohols react more slowly. The cloudiness observed is caused by the carbocation immediately reacting with the chloride ion creating an insoluble chloroalkane.
Hence, the time taken for turbidity to appear is a measure of the reactivity of the class of alcohol with Lucas reagent, and this is used to differentiate between the three classes of alcohols:
no visible reaction: primary, such as normal amyl alcohol (1-Pentanol)
solution turns cloudy in 3-5 minutes: secondary, such as sec-amyl alcohol (2-Pentanol)
solution turns cloudy immediately, and/or phases separate: tertiary, such as tert-amyl alcohol (2-Methyl-2-butanol)
The test is usually conducted at room temperature."