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Topic: Lucas Test  (Read 10555 times)

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Offline Winga

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Lucas Test
« on: March 16, 2005, 04:17:53 AM »
What is the use of zinc chloride, ZnCl2?

Catalyst? How?

Increase Cl- concentration?
« Last Edit: March 16, 2005, 10:02:12 AM by Winga »
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Offline Winga

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Re:Lucas Test
« Reply #1 on: March 16, 2005, 10:07:20 AM »
I find that zinc chloride is acting as catalyst, but I find there are two mechanisms.

1. ZnCl2 binds to the hydroxyl oxygen that makes it becomes a good leaving group.
2. ZnCl2 abstracts Cl- from HCl that protonates the hydroxyl group.

Which one is correct?
« Last Edit: March 16, 2005, 10:12:14 AM by Winga »
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Offline Donaldson Tan

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Re:Lucas Test
« Reply #2 on: March 16, 2005, 10:18:25 AM »
isnt the lucas test carry out with zinc chlorode in concentrated HCl?
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Offline Winga

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Re:Lucas Test
« Reply #3 on: March 17, 2005, 11:16:37 AM »
Yes, but I don't know whether ZnCl2 abstracts the lone pair from O directly or it is used to ionize the conc. HCl for protonation.
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Offline Winga

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Re:Lucas Test
« Reply #4 on: March 17, 2005, 11:53:30 AM »
Does Zn(OH)2 present after the reaction?
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Offline Donaldson Tan

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Re:Lucas Test
« Reply #5 on: March 17, 2005, 02:30:37 PM »
duh.. effectively, this is a substitution reaction where hydroxide ion is displaced. naturally, you should expect the hydroxide ion to precipitate with the zinc(II) ions.
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Offline Winga

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Re:Lucas Test
« Reply #6 on: March 17, 2005, 11:18:36 PM »
Here is another mechanism. They make me confused.
(images are cut from files which are downloaded from websites)
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Offline Donaldson Tan

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Re:Lucas Test
« Reply #7 on: March 17, 2005, 11:47:27 PM »
what condition favours mechanism 1?
what condition favours mecbanism 2?
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"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline Winga

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Re:Lucas Test
« Reply #8 on: March 18, 2005, 12:01:44 AM »
Quote from: geodome on March 17, 2005, 11:47:27 PM
what condition favours mechanism 1?
what condition favours mecbanism 2?
I don't know. ???
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Offline movies

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Re:Lucas Test
« Reply #9 on: March 18, 2005, 12:54:38 PM »
Better yet, what reaction could you perform to differentiate these two mechanisms?
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Offline Mitch

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Re:Lucas Test
« Reply #10 on: March 18, 2005, 01:04:24 PM »
or even better, knowing Avogadro's number and mechanism 2 how can you prove the sky is blue. P.S. This is a joke.
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Offline movies

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Re:Lucas Test
« Reply #11 on: March 21, 2005, 12:21:41 AM »
Quote from: Mitch on March 18, 2005, 01:04:24 PM
or even better, knowing Avogadro's number and mechanism 2 how can you prove the sky is blue. P.S. This is a joke.

Heh, very funny Mitch  :P


What I was going for is that in mechanism 1 the alcohol is displaced like in an SN2 type mechanism.  In mechanism 2 the reaction is more SN1 type mechanism.  If mechanism 1 is operative then the configuration of that chiral center would be inverted, while as in mechanism 2 the product would be racemized because the carbocation would be achiral.  So if you were to perform the reaction with an enantiopure starting material you could determine which mechanism it was based on whether or not the product you got was chiral or not.
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