Sciencemadness Discussion Board

Alkylhalides

Jackson - 30-9-2018 at 15:06

What is the benefit if there is one of using an iodine or bromine compound over a chlorine compound for a grignard reaction.

Assured Fish - 30-9-2018 at 20:30

Thats an easy one.
The first reason is that alkyl bromides and alkyl iodides are easier to prepare than alkyl chlorides. To prepare a simple alkyl bromide such as n-butyl bromide you could simply refluxing aqueous HBr with n-butanol, or even simpler, add stoichiometric amounts of NaBr and butanol to a flask and slowly add another stoichiometric amount of H2SO4 with cooling, then reflux this mixture and distill.

Alkyl iodides can be prepared by reaction HI with alcohols also. However it should be noted that H2SO4 cannot be used to protonate potassium iodide to produce HI due to H2SO4 being able to oxidize the HI to iodine. HI must be prepared by other means.

In contrast alkyl chlorides are a bit of a pain to prepare and cannot be made by simply reacting HCl with a primary or secondary alcohol (tertiary alcohols i think work fine).
The reason for HCl not working so well is due to acidity, in order for the reaction to take place, the hydroxyl group must be protonated to a hydronium cation that can leave as water allowing the halide anion to pick up the free electon on the alkyl group.
HCl is not quite a strong enough acid to do this with primary alcohols at any desireable rate. Instead a lewis acid is usually employed to act as a catalyst to help facilitate the protonation of the alcohol.

The second reason, and probably the major one.
Alkyl iodides and bromides are better leaving groups and also form more neucleophilic organo magnesium halide reagents.
This is due to them having more electrons and more electrons means that the electrons on the outer shell are more loosely held in place.
Making it alot easier for the electronegative carbon to pull those electrons towards it, this results in a much more polarized species which is in turn more neucleophilic.
A stronger neucleophile means greater yields typically.

Sigmatropic - 30-9-2018 at 22:22

The heavier halides are used since they react faster and give better t
Yields in the grignard reaction. The theoretical explanation used by assured fish is not entirely correct, read a book on organic chemistry, not internet comments. (his practical explanation is spot on).

It has everything to do with the mechanism and the rate limiting step, the formation of a radical and the size of the outer shell of electrons on the halides. The larger the orbital, the more stable the formed radical anion. This radical anion then dissociates into a carboncentered radical and a halide. Like I said, read a book or you will never get a good grasp of the concepts.

Jackson - 1-10-2018 at 07:01

This explains why I didnt see any reaction of my sulfuric acid, ethyl alcohol, and non-iodized salt mixture. I do have one question though. My ethyl alcohol was 70% and had the impurites Acetone, Denatonium Benzoate, and Methyl Isobutyl Ketone. During the reaction, the whole room started to smeel fruity similar to snowcone syrup. What could be causing this?

Tsjerk - 1-10-2018 at 07:50

Not knowing what your reaction conditions were, nothing very useful can be said. Maybe ether? Still very dependent on your reaction conditions whether ether makes any sense.

Jackson - 1-10-2018 at 08:02

Excess of sulfuric acid, no heating, in a beaker. Used 3% hydrogen peroxide in the middle of it because the sulfuric acid was really dirty.

Assured Fish - 1-10-2018 at 13:12

By mixing hydrogen peroxide with sulfuric acid you would have made a small amount of charos acid (H2SO5).
This is an exceptionally strong oxidizer. Under the dilute condition you described i would expect it to oxidize some of your ethanol to acetic acid and even perhaps a small amount of acetaldehyde which has a very fruity smell (id describe it like rotting apples on an orchard).

Tsjerk - 1-10-2018 at 13:52

Ah, now some hydrogen peroxide pops out of the head in an again incomplete description of a reaction.

You oxidized some unknown mixture of organics with piranha! Complements, you could possibly have made TATP in a highly acidic and uncooled mixture.

Jackson - 1-10-2018 at 17:23

I was adding Hydrogen peroxide specifically to oxidize they dyes in the sulfuric acid. I Know It makes a dilute Caro’s acid solution. It was cooled using an ice bath because the addition of the 70% ethanol to the sulfuric was quite exothermic.
The smell is quite similar to hot overripe apples or apple juice.

Melgar - 1-10-2018 at 17:58

If you used denatured alcohol, there's no telling what's in that stuff. They'll often just mix in whatever low-purity organic solvents that there happens to be a surplus of at the time. For many chemicals, there aren't too many problems with using their hardware store incarnations. Denatured alcohol is not one of those chemicals.

When you have that many unknown chemicals mixed together, you never know what side reactions you'll have.

Jackson - 1-10-2018 at 19:27

It is 70 percent ethyl rubbing alcohol. The impurities are acetone, denatonium benzoate, methyl isobutyl ketone, water.

Ubya - 1-10-2018 at 22:27

Quote: Originally posted by Jackson  
Excess of sulfuric acid, no heating, in a beaker. Used 3% hydrogen peroxide in the middle of it because the sulfuric acid was really dirty.


you should "clean" your reagents before the reaction, not in the middle of it...

Tsjerk - 1-10-2018 at 22:55

I would be a whole lot more comfortable with a bit of dye in this reaction than peroxide. The fact you can see a compound doesn't make it more "dirty".

Jackson - 2-10-2018 at 10:27

I understand that but i remember hearing somewhere that there were inhibitors out into drain cleaner acid that could be broken down by the addition of hydrogen peroxide to form a dilute pirhanna solution. This reaction really was just done to see if chloroethane could be made this way.

Assured Fish - 2-10-2018 at 15:09

Of all the experiments to conduct on your own. Trying to prepare a highly volatile very flammable central nervous system depresent that is a gas as room temperature is probably a bad idea, whats worse is that you had no way of knowing if you even had made the material unless you blacked out from inhalation, you put no research into the reaction prior to attempting it and tried to then add an oxidizing agent to this mixture of highly impure acidic hydrocarbons in the hopes of purifying the reagents while they were reacting (instead of before like a sensible person).
You did a dumb thing, a dumb thing that could have gotten yourself killed, a dumber thing than any dumb things ive ever done and i once threw a puppy out of a tree to see if it could fly.

It annoys me when i see this sorta shit, people jumping into practical chemistry without understanding the basics first, even worse still jumping into experimental chemistry without understanding the concequences. If you want to do some practical chemistry then why the fuck would you not start with something already tried and tested, like something simple that nurdrage or nilered have done or even something another member on the forum has done.

You notice that the members on this forum do not go around trowing reagents together all the time, they spend most of their time studying the field on paper, thats because this is what 99% of chemistry is about. You need to learn the theory before you start dabbling in this shit.
Chemistry and esspecially amateur chemistry is a really fucken dangerous persuit.
If you have an idea you want to test out that is not already established in the literature please for the love of science ask someone else about it first or do some serious research on the subject first, you will learn nothing by just adding things together and seeing what happens.

I hope you have learned something.

Tsjerk - 2-10-2018 at 16:43

I couldn't have said that any better

Jackson - 2-10-2018 at 17:22

I understand what i did was stupid. Could you point me to some literature to read up on for organic chemistry? Thanks

happyfooddance - 2-10-2018 at 18:06

Quote: Originally posted by Jackson  
I understand what i did was stupid. Could you point me to some literature to read up on for organic chemistry? Thanks


Check out the forum library, there is a link to it on this page: http://www.sciencemadness.org

Vogel is a good start, maybe you should read up on safety and stuff first. Someone mentioned it but some of the chemicals you haphazardly mixed make a high explosive that is very sensitive (shock, friction, static, heat) and has been responsible for very many deaths and even more lost fingers, limbs, eyes, etc.

Caro's acid + acetone is no joke

Jackson - 3-10-2018 at 08:50

Are you talking about practical organic chemistry by Arthur I. Vogel?

happyfooddance - 3-10-2018 at 09:24

Quote: Originally posted by Jackson  
Are you talking about practical organic chemistry by Arthur I. Vogel?


Yes. It has a lot of straightforward preps for useful reagents. Only problem is that the nomenclature (naming of substances) is a bit outdated. But it is a good idea to familiarize yourself with old terminology because you will come across it from time to time.

Chem Science - 4-10-2018 at 06:02

Hi :D !!
One thing that isn't mentioned is that Grignard reaction yield's vary with the halogen used. Chlorine is slower and harder to make grignards out of, but gives better yields. Bromine is easier but yields are lower than chlorine, and Iodine is even easeir but yiuelds are low ( relative to one onother, any grignard gives very good yields !! :) ) the reason is that there are some side reactions that grignards can have and deplete the magnesium and alkyl halide. some examples are, The alkyl halide can react in the surface of the magnesium lik in Wurtz reaction ( 2RX + Mg = R-R + MgX2)If the halogen is replaced fast ( Tertiary alkyl halides ) the exces halide can react with the grignard ( RMgX + RX = R-R + MgX2)formation of alkenes, and maeby others i don't know.
in general alkyl Bromides are the most often used. Because alkyl chlorides are hard to make, alkyl iodides are expensive to make.

Assured Fish - 4-10-2018 at 13:23

Quote: Originally posted by Chem Science  
Hi :D !!
One thing that isn't mentioned is that Grignard reaction yield's vary with the halogen used. Chlorine is slower and harder to make grignards out of, but gives better yields. Bromine is easier but yields are lower than chlorine, and Iodine is even easeir but yiuelds are low ( relative to one onother, any grignard gives very good yields !! :) ) the reason is that there are some side reactions that grignards can have and deplete the magnesium and alkyl halide. some examples are, The alkyl halide can react in the surface of the magnesium lik in Wurtz reaction ( 2RX + Mg = R-R + MgX2)If the halogen is replaced fast ( Tertiary alkyl halides ) the exces halide can react with the grignard ( RMgX + RX = R-R + MgX2)formation of alkenes, and maeby others i don't know.
in general alkyl Bromides are the most often used. Because alkyl chlorides are hard to make, alkyl iodides are expensive to make.

Speaking from experience?
While its not a reaction i have practical experience with (not yet anyway) it is a reaction ive read a fare bit on, including write ups other people have done, from everything ive read iodides give both best yeilds and are easiest to prepare. I suspect others will agree, perhaps you should do some reading of the drug literature, i realise its not the most reliable but its undoubtibly the most abundant.

Those side reaction you mention, specifically the one regarding the formation of alkenes seems unplausable, do you have references to back this up.
I wont deny the statement about teriary alkyl halides reacting too fast and then reacting with othe tertiary halides when introduced in the later stages of the addition of the halide to the flask but this should be mostly avoidable if you take care with the drip rate of you ether alkyl halide mixture.

Chem Science - 5-10-2018 at 09:24
Quote:
Speaking from experience?


Nop, i haven't ever done a Grignard ( It's actually one of my favorite reactions :D but i hope to do some time ) Ive reed a lot of it and that is actually from an Organic Chemistry book,I just review it and it has no specific reference for that. however what you say is things i've also read before regarding alkyl iodides. The book looks like is written by interpretation of experiences but it's old also .. it could be wrong. I really need to start to reed more papers rather than books, that's totally true !!

Sigmatropic - 5-10-2018 at 09:40

Gringards have are still common place but considering the availability of nBuLi, PhLi, and iPrMgCl or the turbo grignard reagent they should be replaced with transmetallation procedures (for increased efficiency, safety and ease). It a bummer for those who do not have access to afore mentioned reagents.