in  the case of allybenzenes 14 hours yeilded 70% conversion and it maxed out to 93% at 24 hours so somewhere between those timeframes is the (mol/unit time) curve.
if all your trying to do is be antagonistic i see no purpose in discussing this further

good to know this is an exothermic reaction though, in the future i will know that cooling will accelerate things.

Honestly, I wasn't trying to be antagonistic. I thought you had a research paper with the hydrohalogenation reaction kinetics of safrole + HBr.

WizardX: you're confusing reaction kinetics (which is based on activation energy) with reaction thermodynamics (which is based on the change in gibbs free energy).

Sedit: that's a pretty interesting claim, can you provide any references?
dG = dH - TdS  Increasing the temperature of a reaction significantly can increase the T/\S term to the point that the forward reaction is no longer spontaneous.  Just because a reaction is exothermic does not always necessarily make it spontaneous.

And the rate of the reaction is determined by the magnitude of the activation energy, not the overall change in energy.  The overall change in energy determines the equilibrium.  The data you provided was the over all change in energy, not the AE

Certainly true, the reaction is dependent upon a number of factors.  But the arrhenius equation tells us that there is a relationship between the rate constant and the activation energy:

It also suggests a logarithmic relationship between the reaction rate constant and the temperature.

Activation energy can be thought of as the height of the potential barrier (sometimes called the energy barrier) separating two minima of potential energy (of the reactants and products of a reaction). For a chemical reaction to proceed at a reasonable rate, there should exist an appreciable number of molecules with energy equal to or greater than the activation energy.

http://en.wikipedia.org/wiki/Activation_energy

minima of potential energy (of the reactants and products of a reaction)

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The rate of a reaction depends on the temperature at which it is run. As the temperature increases, the molecules move faster and therefore collide more frequently. The molecules also carry more kinetic energy. Thus, the proportion of collisions that can overcome the activation energy for the reaction increases with temperature.

http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch22/activate.html

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The hydrohalogenation reaction of safrole + HBr reactants at 0-1 oC react slowly over a period of 24 hours. At this 0-1 oC temperature, the activation energy is enough to break the bonds of the reactants, BUT the rate of the reaction is slow, requiring approx 24 hours for the hydrohalogenation reaction to complete 90%+ conversion.

At 0-1 oC temperature, the activation energy required for the cleaverage of the methylenedioxy ring with HX (X = Br or I) is inadequate, or negligible, thus the reaction rate is zero, or ultra slow.

The only relationship I've seen in the lab or read about in my gen chem book was the one mentioned by jon.  As I pointed out, the Arrhenius equation dictates a logarithmic relationship between temp and reaction rate.  I distinctly remember gen chem two lab where we graphed k against time and the relationship definitely was not a doubling in the reaction rate for a 50% inc in temp.

dG = dH - TdS 


can you define the equation i remeber this vaugely from chem 1
delta gibb's free energy = delta enthalpy - (what is T and d?) i know S is entropy

T and S aren't terms in this equation btw

wizard you are dead on about the rate of hydrohalogenation being 24 hours.

That is one of the fundamental thermodynamic relations.  The d(variable) is the differential change in that property.  Use of this equation and related equations is really Thermo 2 information, which is based on highly idealized situations.  In real life the complexity of these organic reactions are based on a lot of other things.  Based on reactor geometry and type, reaction kinetics and such. 

that relationship, and the other related ones, are some of the most fundamental building blocks of understanding thermo.

Reasearch Thermodynamic Web if you're more curious.

Actual high level organic chemistry like this cannot be explained with such basic thermo
relations.

EDIT:mostly used for multi component vapor liquid equilibrium, and very interesting stuff.

Looking forward to the results!

Found these writeups laying around, seems to have a nice level of detail to them.  The stuff about lighting in the Finkelstein writeup probably applies to the Bromination too.

Bromination with HBr in GAA

RATIOS

2:1 NaBr/KBr to oil
0.5:1 H2SO4 (actually, just a hair less than equimolar)
?:1 GAA (see METHOD for determination)

METHOD

To determine how much GAA is needed, you must first compute the mass of HBr required based on the 2:1 molar ratio.  The mass of GAA is equal to the mass of HBr for a 50% concentration.  Remember to account for GAA’s density when determining volume.

Charge a flask with a stir bar, the GAA, and with vigorous stirring add the bromide in doses.    This is to prevent overloading/stalling the stirbar, which is a real pain to restart.  A pan or other container needs to be placed on the stirrer, under the flask, so a water/ice mix may conveniently be added without disturbing the stirring.

When all the bromide has been added, fill the pan under the flask with a water/ice mixture, and give the contents some time to begin cooling.  Take care not to OVER-COOL or the GAA will freeze!

Now add the sulfuric, being mindful that unless the flask is stoppered in some way, HBr gas will fume off vigorously.  Do this addition SLOWLY (like drop-wise) so the generated HBr has time to dissolve in the GAA.  The cooling helps reduce gas loss, so add the sulfuric in a measured way to keep the temp (and fumes) down.

When all has been combined and the solution has cooled about as much as it’s going to, firmly stopper the flask (tape the stopper in to be certain!), wrap in Al foil or otherwise place in a light-tight container.  Place the entirety into the fridge for a day or so.

WORK-UP

While the flask contents are still cold (like RIGHT AFTER you get it out of the fridge!), carefully decant the acid layer from the solids (salts).  Add only enough water to the salts in the flask to fully dissolve them, then extract once using a chlorinated solvent.  Discard the dissolved salt solution. 

The trick is to add EXACTLY enough water to solvate the salts… this minimizes the amount of water that dissolves in the solvent and saves a wash step.

Add one to two volumes of water (the colder, the better) to the acid you decanted at the beginning.  This will crash the bromo out of solution.  Add the chlorinated solvent used to extract the salts (above) to the water layer you just created to extract any bromo that might have been left behind.  Drain off the non-polar and add it to the bromo that fell out.  Extract the aqueous solution once more, but use about half as much solvent.  Add separated extract to the bromo.

To the now isolated bromo, add an equal volume of water and neutralize using bicarb.  You’re done when the bicarb no longer fizzes when added.  Separate the bottom (non-polar) layer and discard the top (aqueous) one.  Wash the non-polar layer with water and then with brine. 

Finally, distill off the solvent AT THE LOWEST PRACTICAL TEMPERATURE.  Temps much above ambient can adversely affect the bromo, so a vacuum is mandatory.

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Finkelstein Conversion of Bromosafrole to Iodosafrole

RATIOS

1:1.1 Br to KI
n/a    Acetone (anh) – sufficient to ensure good mixing during stirring

METHOD

First, ELIMINATE all sources of UV or really bright lights (like flourescents, halogens, sunlight) before you expose the bromo. 

Weigh out the KI so it’s just a bit more than equimolar with the bromo compound you’re processing.  My suggestion is a 10% excess.  Since KI is hygroscopic and this reaction doesn’t like water, dry the KI as much as possible.  This will also skew the weight, so dehydrate before weighing. 

Place the bromo and a stir-bar in a suitably sized flask (RB recommended), add a bit of acetone and begin stirring.  Adjust stirring so the bromo is well agitated; add acetone if needed.

Now add the iodide, in small portions, being careful not to stall the stir bar.  Add acetone and/or adjust the stirring to maintain through agitation.  Leave it to stir vigorously for 30 minutes to an hour in a water/ice bath to keep it cool.

When the time is up, remove the acetone being ESPECIALLY careful to not heat the (now) iodo much above R.T.  Iodo is very easily decomposed by heat and energetic photons (like UV).

WORK-UP

At this point, you should have your iodo in the flask mixed with (what is now mostly) potassium bromide.  Isolate the iodo with an anhydrous solvent in which neither the bromide nor the iodide salts are soluble (but the iodo compound is).  BTW, the salts ARE soluble in acetone, which is why we had to get it out of the mix.  IPA would be a good choice, especially if you’re going to aminate your iodo next.

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Noticed the discussion of time for prep of bromo and this evening I came across this bit from Weygand and Hilgetag (1972 ed).

In laboratory practice addition of HBr is usually effected in glacial acetic
acid at 0-15°, or HBr is led into the undiluted olefin or a solution thereof in
CHCI3 at about —20°. Volatile olefins are left for several days in a bomb-tube,
higher-boiling ones for about 24 h in normal apparatus.

So it may be that you need to leave it in a pipe bomb if it is one of the lower boiling point oils, but 24 hours is plenty for safrole IMLE.