why is this statement wrong?

[laya533]

Why cant we distinguish between a benzyl alcohol and a phenol by disolving them in HCl? I thought its the lucas test and we can do it to distingyish them.

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[aamartin81]

Why cant we distinguish between a benzyl alcohol and a phenol by disolving them in HCl? I thought its the lucas test and we can do it to distingyish them.

I'm pretty sure that they can be distinguished, as a benzlic carbocation is the most stable, while phenol will not create a carbocation (I think it would be an aryl, or something along those lines). Be careful in taking what you read in prep books as fact, there are many errors in them!

If I'm wrong, I'm sure someone will let me know...

Adam

 

[liverotcod]

The Lucas test requires a zinc chloride reagent, in addition to the HCl.

 

[aamartin81]

The Lucas test requires a zinc chloride reagent, in addition to the HCl.

This is true, but I assume that the question said Lucas Reagent and not just HCL? If not, the previous poster is correct, you need a catalyst.

 

[laya533]

No, it did not ask about the lucas. It just said that these 2 compounds cannot be seperated based on their activity with HCl. But I think we can. Why am I wrong?

 

[aamartin81]

No, it did not ask about the lucas. It just said that these 2 compounds cannot be seperated based on their activity with HCl. But I think we can. Why am I wrong?

An easy way to think about it is the ability of the two leaving groups involved. Which is a better leaving group: -Cl or -OH? Cl is a much better leaving group and will not displace -OH.

On the other hand, you could displace benzyl chloride to form benzyl alcohol with H20.

Hope this helps,

Adam

 

[adiddas125]

Why cant we distinguish between a benzyl alcohol and a phenol by disolving them in HCl? I thought its the lucas test and we can do it to distingyish them.

To my knowledge you could see which compound is more acidic. Phenol should be more acidic than benzyl alcohol.

You cannot displace OH in most cases, unless you protonate the OH to make H2O+ and it will leave as H2O: .

In benzyl alcohol it looks like it will be a primary carbocation when H2O leaves, and will become a secondary carbocation if, water leaves from this compd.

 

[Daichi Katase]

i like that last answer. i too think it has to do with acidity.

 

[KitEKatEli]

Couple things:

It just said that these 2 compounds cannot be seperated based on their activity with HCl.

1. What is the context of this statement?

I could see how benzyl alcohol might be converted to benzyl chloride through SN1. But I'm not sure that's correct. I would guess that no reaction occurs between phenol and HCl (see point 3).

In benzyl alcohol it looks like it will be a primary carbocation when H2O leaves, and will become a secondary carbocation if, water leaves from this compd.

2. Technically, the benzyl cation is a primary carbocation, but its stability is close to that of t-butyl carbocation b/c of resonance stabilization. So you can't go strictly by the primary < secondary < tertiary hierarchy of stability for aromatic carbocations. Which leads to my next point...

3. I'm assuming you meant to say "phenol will become a secondary carbocation if water leaves from this compound." But substitution and/or elimination of the phenolic hydroxyl group almost never occurs.

See the excellent website:
http://www.cem.msu.edu/~reusch/VirtualText/alcohol2.htm#alcrx5

Now here's a question on a related note:

I know that primary and secondary alcohols are "resistant to acid" (see the following website), so they must first be converted to sulfonate or phosphite esters (with tosyl chloride or PBr3):

http://www.uiowa.edu/~c004124/CHAP17.pdf
(Slide 15)

But they can be converted to alkyl chlorides with stoichiometric amounts of HCl or HBr. So why don't primary alcohols react with Lucas reagent?

 

[aamartin81]

I never would have imagined that this would be such a long answer. A tertiary carbocation is the strongest position, followed closely by benzylic, then allylic, then secondary carbocations. The strength of benzylic and allylic carbocations is due to the ability of the charge to be shared through resonance. A primary alcohol will react with a Lucas Reagant in an Sn2 reaction (a primary carbocation intermediate is too unstable to form for a Sn1 to occur). However, the rate of the reaction is extremely slow because Cl- is a poor nucelophile. The +ZnCl attaches to the O (similar to being protnated) and makes it a better leaving group. Remember though, phenol is not a primary alcohol.

The reason that Cl- cannot act as a nucleophile in phenol is that the -OH group is a strong activator which contributes to an increased electron density in the ring. The incoming Cl- nucleophile is repelled by this electron density.

I think I've confused myself as to what question I'm supposed to be answering... I hope this helps!

Adam