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 21 
 on: January 13, 2019, 09:59:02 PM 
Started by owk9688 - Last post by phth
The rate depends on the pKa and the nature of the electrophile. The dimethyl sulfate can react in two positions making it faster even though the pka's are different, and solvent effects play a role in the reactions.

 22 
 on: January 13, 2019, 09:25:26 PM 
Started by ostudent - Last post by Irlanur
The argument is a bit hand-waving, but one can argue than in order to be fluoresecent, the light energy that was taken up also has to be released via a radiative process. If a molecule has a lot of vibrational degrees of freedom, i.e. if it can wiggle a lot, most energy will be released as heat, not as visible light.

 23 
 on: January 13, 2019, 07:41:33 PM 
Started by ostudent - Last post by ostudent
See attachment for problem.

In this question, I am confused about the effect of rigidity on effects of UV radiation.
I agree that C is the answer because it is the most highly conjugated. However, why does C being "the most rigid due to its fused aromatic ring system" make it more likely to have intense yellow fluorescence. Can this be explained?

 24 
 on: January 13, 2019, 05:22:00 PM 
Started by bootlegengineer - Last post by billnotgatez
topic locked
See forum rules

 25 
 on: January 13, 2019, 02:53:53 PM 
Started by Richard Jeong - Last post by Richard Jeong
TiCl4 is a strong Lewis acid but easily reducible to TiCl3.
Zn powder is an effective reducing agent.
Mg powder or chips can easily form organometallic compounds via an intermediate, free radical step.
Thus:
1). Study carefully the mechanisms of McMurry coupling, Pinacol coupling and Clemmensen reduction and see what is the role of the above metals, therein.
2). Search for alternative metals and/or their salts that have similar properties with Zn and TiCl4 and which could replace them (say Sn and ZrCl4, respectively?).
Hint: Firstly, check their neighbors in the periodic table, before searching for more exotic metals like lanthanides and actinides.
Good Luck!

That would help a lot! Thanks!
By the way, my professor and group are currently working with nickel and cobalt. It would take a lot of effort to tell this topic.

 26 
 on: January 13, 2019, 01:48:55 PM 
Started by Shannon Dove - Last post by Enthalpy
Same opinion: a nitro is highly improbable, a trinitro even worse. Extreme detection sensitivity won't suffice. Amino acids were a smaller step away from the reactants used.

But finding an interesting molecule in a big bunch is sensible as a first step before synthesizing it in a practical way. Hence my suggestion with atomic carbon, made more interesting because the synthesis was very little researched and may very well produce molecules still never obtained.

Similarly, you could prepare a wide mix of Grignard reactants, synthesize at once tens of thousands of branched alkanes, and sort out the ones with a wide liquid range. At least the separation on temperature criteria is decently easy. Once you have some drops that stay liquid from -100°C to +180°C, you can apply the more subtle separation and analysis methods. Low-freezing non-flammable alkanes would have commercial applications. Even for pure science, as we can't predict a freezing point, knowing more atypical compounds would help.

Or you could check if the selectivity rules for some synthesis methods are well understood. Make a mix of tens of type A and type B reactants, with similar reactivity, apply something like Diels-Alder, or 2+2 photoaddition, or any one you prefer, and check if the many product abundances are distributed about as expected. From the surprises, you might amend the guidelines.

 27 
 on: January 13, 2019, 01:41:24 PM 
Started by blokeybloke - Last post by blokeybloke
What is it meant by “elements and compounds can exist as discrete molecules”? The “exist as” part seems quite counterintuitive and I can’t seem to understand what is meant by it.

 28 
 on: January 13, 2019, 01:34:19 PM 
Started by blokeybloke - Last post by blokeybloke
What exactly do chemical formulas represent? For example a single sodium atom is represented by Na whereas sodium metal (the solid material) is also represented by Na. I am just confused as to how chemical formulas are used.

 29 
 on: January 13, 2019, 09:54:54 AM 
Started by Shannon Dove - Last post by wildfyr
It's all a matter of concentration. We can detect picomolar concentrations if an assay is performed correctly and we know what we are looking for.

And I think such a defense attorney would have to put a little more oomph behind the mechanism of formation for it to stand up against another expert witness like an organic chemist. Things are not truly "randomly made." They need atomic feedstocks, energy, and often catalysts. Part of the problem with the Miller-Urey experiment is that only quite simple molecules are really made. Single amino acids and sugars. Something like like an known active drug molecule is so enormously unlikely. It's like arguing that according to quantum mechanics there is a statistical chance I could teleport to Hawaii right today.

Your earthworm experiment is of the same vein, I think ppt would be orders of magnitude too high a concentration for nitroglycerin from such an experiment unless it was carefully designed to produce such molecules. Try to think of this in real numbers from a statistical mechanics point of view. Almost anything is possible, but we need to try to work with what is likely. Occam's razor and all that.

Its why the origin of complex biomolecules and life on Earth is still a problem we struggle mightily with.


 30 
 on: January 13, 2019, 09:04:53 AM 
Started by owk9688 - Last post by owk9688
Hey!

So I know that pKa is a thermodynamic property while electrophilicity would be a kinetic one, but I’m encountering a real life example right now that I can’t seem to wrap my head around:

in my lab I was charged with alkylating a bunch of phenols/phenoxides for others to use in their experiments. The reactions were clean but I noticed that dimethylsulfate finished the fastest (by TLC), faster even than methyl iodide. Also, butyl chloride took more than a full day to fully react (dimethylsulfate was done in 3 hours).

I guess I’m just really surprised bc in terms of pKa, sulfate should be far inferior of a leaving group compared to iodide and chloride, but with iodide the rates are essentially the same and even though chloride seems to be a far better leaving group, the butyl chloride was absolutely sluggish compared to the dimethylsulfate sulfate. Is the chloride example purely a steric issue? I know it’s the difference between methyl and primary but I really would not have thought that it would be so dramatic. And if it is, why then do methyl iodide and dimethylsulfate react at similar rates even with a pKa’ difference of nearly 6 orders of magnitude?

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