How would i explain the readiness of allyl alcohol to react with the Lucas test????
Thanks!!
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I still don't understand-- I know Lucas Test is best for tert carbocations but 2-propen-1-ol gives secondary carbocation at best even with rearrangement? Is that the only answer?
I think the answer is in this part of what I gave you earlier. I have bolded the term allylic.
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.
To explain the readiness of allyl alcohol to react with the Lucas test, we need to consider the reaction mechanism and the properties of allyl alcohol.
The Lucas test is used to determine the reactivity of alcohols towards nucleophilic substitution reactions. It involves the reaction of an alcohol with Lucas reagent, which is a mixture of concentrated hydrochloric acid (HCl) and zinc chloride (ZnCl2). The reaction proceeds via an S N1 mechanism, where the alcohol is converted into an alkyl chloride.
Allyl alcohol, also known as 2-propen-1-ol, has a propenyl group attached to the hydroxyl group. The propenyl group consists of a double bond and an alkyl group. When allyl alcohol reacts with the Lucas reagent, the double bond of the propenyl group and the hydroxyl group play a significant role in the reaction.
Allyl alcohol undergoes a two-step reaction with the Lucas reagent. In the first step, the alcohol is protonated by the strong acid (HCl), forming a secondary carbocation. The double bond in the propenyl group contributes to the stability of this carbocation due to resonance delocalization of the positive charge.
In the second step, the chloride ion (Cl-) from ZnCl2 attacks the carbocation, resulting in the substitution of the hydroxyl group by a chlorine atom. The final product is 2-chloropropene.
Although allyl alcohol forms a secondary carbocation, the resonance stabilization provided by the double bond helps to stabilize the positive charge and makes it more reactive towards nucleophilic substitution. This resonance stabilization facilitates the reaction with the Lucas reagent.
Therefore, allyl alcohol reacts readily with the Lucas reagent because of its ability to form a resonance-stabilized secondary carbocation, despite it not being a primary alcohol or a strong tert-carbocation like a tertiary alcohol.