PP's General Chem Question Thread

[b159510]

and then the nucleophile, which is actually neutral in charge, just polar, then adds to the carbocation and ejects a proton of it's own to sustain neutrality?
this would be the case for the acid catalyzed addition of
water to an alkene, but likely a water molecule removes the proton.

The nucleophile couldn't contain a negative ion, because the protons in solution would combine with it, wouldn't they?
a nucleophile can have a negative charge even in a strongly acidic solution, but not in this particular reaction.
But if the nucleophile is attracted to positive charges, why doesn't it react with the readily available protons in the system, forming NuH2+,
maybe it does. maybe it gets protonated and deprotonated
a thousand times a second. but when the deprotonated form
comes in contact with the carbocation and sticks, the next
step of the reaction can take place (this actually happens
very fast compared to the formation of the carbocation).
The rates of some acid catalyzed reactions are very pH
dependent, like an amine with a ketone.
As hypo says, there is an equilibrium factor. As foxy says,
reaction diagrams are usually not literal depictions.
They also almost never represent all the reactions that
are taking place. 
Back to the Primitive

[PrimoPyro]

Is this how acids of the species HX add to alkenes? The acid produces a carbocation and the halide anion attaches to it? Makes perfect sense.

Thank you for elaborating so much on this question, all of you.    I've started reading a book on Functional Group Chemistry, and it relies very heavily on electronegativities of functional groups, and how they play a crucial role in reaction mechanisms. It is highly interesting, I think I'll put the organometallics book aside for the moment and read this one first.

                                                   PrimoPyro

[Rhodium]

Is this how acids of the species HX add to alkenes? The acid produces a carbocation and the halide anion attaches to it?

Yes, definitely:

[PrimoPyro]

But the more stable carbocation is the secondary one, so wouldnt ISOpropyl halide be the chief product, chief?

[hermanroempp]

Correct, according to Markovnikov's rule, the 2-chloro-propane will form.
Quidquid agis, prudenter agas et respice finem!

[Rhodium]

You're right. I found it at

http://www.chemguide.co.uk/mechanisms/eladd/unsymprob.html

by googling for "HX addition alkene", and copied the first addition mechanism I saw, which was an example of what doesn't happen  

Here is the correct image:

[PrimoPyro]

Does precipitated nickel form good reducing amalgams/reducing catalyst with mercuric chloride? Is the power of the reducing metal in an amalgam determined by the metal's electronegativity? Nickel is slightly more electronegative than aluminum, so perhaps aluminum is a better reducing metal, but nickel has hydrogenation catalysis properties.

Im wondering if these properties could be enhanced by formation of a reducing amalgam with the precipitated nickel, and used in conjunction with a hydrogen donor. Would this be a better reducing system than precipitated nickel alone?

                                                   PrimoPyro

[Rhodium]

Does precipitated nickel form good reducing amalgams/reducing catalyst with mercuric chloride? Is the power of the reducing metal in an amalgam determined by the metal's electronegativity?

Yes, the power is determined (at least in part) by the metal's electronegativity. As Nickel is not especially electronegative, it would give an amalgam with very bad reducing properties. Very few organic compounds would oxidize Ni to Ni²⁺ (as in the Ni reducing the organic compound).
 
I'm wondering if these properties could be enhanced by formation of a reducing amalgam with the precipitated nickel, and used in conjunction with a hydrogen donor. Would this be a better reducing system than precipitated nickel alone?

I don't understand what you are trying to achieve here. Read up on Urushibara Nickel - the combination of NiCl₂ with either Al or Zn metal makes an excellent reducing system (again, the Ni is not reducing anything, it is the Al or Zn that does, the Ni is just acting as a catalyst for transferring electrons to the organic, just like the Hg does in Zn/Hg or Al/Hg amalgams.

[PrimoPyro]

I have read up on Urushibara Nickel, that is why I am asking. I am trying to see if added HgCl2 would enhance the power of urushibara by acting as an "electron conduit."

Thanks.

[Rhodium]

The electronegativity of Ni is just slightly lower that that if Hg - I wonder if Hg would deposit properly on Ni just like that. Let's ask one of the inorganikers here.

[PrimoPyro]

Nickel is only slightly more electronegative (0.3 more) than aluminum.

Ny the way, in a completely unrelated question, do you think I would be successful in an attempt to reduce an organomercury acetate to an alkane with something other than sodium borohydride/hydroxide mix used on your website?

Im particularly interested in using sodium formate/hydroxide instead. Maybe sodium formate/Hydroxide/hydrazine if more power is needed.

                                                   PrimoPyro

[Rhodium]

Search at www.infotrieve.com and see if someone has done it before. NaBH4 it the perfect reagent for the reaction, so people has probably not bothered to look any further.

[Osmium]

> Nickel is only slightly more electronegative (0.3 more)
> than aluminum.

I don't think so!

Ni/Ni(2+) = -0.25V
Al/Al(3+) = -1.662V

Adding Hg to Ni will probably kill the catalytic acivity of the Ni.
I'm not fat just horizontally disproportionate.

[PrimoPyro]

Ni: 1.9
Al: 1.6

Ch. Elschenbroich, A. Salzer: Organometallics, A Concise Introduction, 2nd Ed. pg. 8, Table 2-1. VCH Publishing, New York, ISBN: 0-89573-983-6

Ref cited in book for table source:

L. Pauling, The Nature of the Chemical Bond, 3rd Ed., Ithaca (1960); A.L. Allred, J. Inorg. Nucl. Chem. 17 (1961) 215.

Would you like a picture of the page?

                                                 PrimoPyro

[hermanroempp]

PP, you're right with the electronegativities for Ni and Hg, I've just looked them up in the Elschenbroich/Salzer myself. But these are of little interest here, important are the standard potentials E₀ in volts and here is Os right with his statements. The Al/Hg reaction works by the principles described in this thread before, it's the reductive power of the aluminium that makes the reaction go.
But I think I understand what you mean, you want to use the reductive power of the hydrogen that is also produced in this reaction, your goal is to boost the yield. This could be done by addition of Raney-Nickel to the reaction mixture. Amalgamation of the nickel is unnecessary, it's even bad practice because the mercury will "clog up" the highly porous and active surface of the nickel catalyst, thus drastically reducing its performance.
Quidquid agis, prudenter agas et respice finem!

[PrimoPyro]

Has anyone ever tried to use Chavicol, a.k.a. 4-hydroxy-allylbenzene, as a safrole precursor?

If one used sodium hydroxide and chloroform to perform the Reimer-Tiemann formylation, the only available position for the formyl group is ortho to the hydroxy group.

Then if this reaction were followed up by reaction with sodium hydroxide and hydrogen persoxide in a Dakin Reaction, the ortho aldehyde is oxidized to a phenol in the same ring position, giving 3,4-dihydroxy-allylbenzene.

Methylenation of this compound gives safrole.

                                                   PrimoPyro

[Rhodium]

Hydrogen peroxide on a hydroxy-allylbenzene is asking for polymerization in my opinion.

[PrimoPyro]

Hm, and doing it at the acetone stage would form peroxides...

I see why it hasnt been done.  

[PrimoPyro]

Hi.  

Would a viable method of forming dicarboxylic aicd half esters of the formula HOOC-(CH₂)_nCOOR be the ring opening of the corresponding cyclic dicarboxylic anhydride with alkalai alkoxide, resulting in the half alkyl ester half alkalai salt of the dicarboxylic acid?

Illustratory example: Succinic anhydride would react with sodium ethoxide, forming NaOCO-CH2-CH2-COOEt.

The nature of the alkyl group is not important, it serves here only as a protecting group for the carboxyl. The half ester is desired for a Kolbe-like reaction where the product is to have a preserved terminal carboxyl group. This half ester faciliatates a protecting group for a single carboxyl while admitting the second carboxyl to facilitate the coupling reaction.

                                                   PrimoPyro

[lugh]

The electronegativity of Ni is just slightly lower that that if Hg - I wonder if Hg would deposit properly on Ni just like that.

Nickel amalgam cannot be formed by the direct union of nickel and mercury, according to J Nicklès in Comptes Rendus 104 154 (1853) While it's possible nickel amalgam could be formed under the described conditions, it's rather unlikely. Mercuric chloride is reduced by most metals, which could result in nickel chloride, and possibly form nickel amalgam at that point, though normally current or sodium are necessary to perform the task. Thus what's most likely to occur is what has been already stated, nickel's catalytic properties are dependent on being finely divided into small particles to adsorb hydrogen onto it's surface; which would be impaired by mercury on the surface    On the other hand, when nickel amalgam is formed, it does produce hydrogen, nascent hydrogen is a well known reducing agent, and thus may bee useful