Regiochemistry

[Aperson444]

I've looked at the textbooks, I've looked online. I don't really get it. What is regioselectivity and how do you determine the major product of an alkene reaction (e.g 1-pentene with H2SO4). I'm talking about the Markonikov rule and whatnot. What is it, and how in hell is it determined?

Here is an example:

1-pentene is reacted with dilute sulfuric acid to form 2-pentanol

Looks like a rather useful reaction... but why 2-pentanol and not 1-pentanol?

If any of you are up to it, what is the mechanism for this reaction? God these books are awful at showing how something happens.

[The Jitterskull]

If you look at the structure here,

CH3-CH2-CH2-CH=CH2

There are two places the hydrogen can add... lets see what happens in both.

Say it adds to the end carbon of the double bond:
CH3-CH2-CH2-CH(+)-CH3
carbocation is formed, and it is a secondary carbocation

Now lets test it with the hydrogen adding to carbon number two
CH3-CH2-CH2-CH2-CH2(+)
primary carbocation formed


Primary carbocations are MUCH more unstable than secondary, and will be thermodynamically unfavorable. Thus, there will be protonation which will yield the most stable carbocation intermediate (assuming no hydrogen or methyl shifts!). The electron-dense group will then add to the carbocation.


Actually you can do anti-markovnikov addition of an alcohol with mercury I think...


Let me propose my shitty mechanism, one sec...

I DO NOT KNOW THE MECHANISM, THIS IS JUST A POSSIBLY BAD IDEA
And a shitty one at that, I'm too lazy too look it up and I don't have paper around me to fiddle around with a mechanism. Does anyone have a real link?

http://img34.imageshack.us/img34/4189/asdwoa.jpg

Honestly, do not take that mechanism as fact, I highly doubt it's right. I've never even seen a reaction like that before, I thought the sulfur nucleophile would just add and stay on?

[Aperson444]

THANKS!

But now I do not know exactly what Markinikov or Anti-Markinikov means. It has something to do with favoring either secondary or primary carbocations though, I know that.

I found an explanation of the mechnism. It involves forming H3O+, where the H goes out.... or is this the halogenation process.

Tell me if this is right:

Hexane + H2SO4 (and heat) --> 3-hexene

3-hexene + H2O (excess) + H2SO4 (dilute) + heat --> 3-hexanol

The reason you don't just end up with just 3 hexene in the second step is because there is so much H2O, that the equilibrium pushes towards 3-hexanol. Here H2SO4 acts as a catalyst instead.

Just tell me if I am correct.

[The Jitterskull]

Markovnikov means when a cation forms, it will take the more subsituted carbon, or the better resonance stabilized carbon.


Hexane + H2SO4 (and heat) --> 3-hexene

This I don't think would happen, you will learn something called elimination, but there is no leaving group. When you get to nucleophilic substitution and learn about elimination, you will be able to create elimination scenarios. I can show you all the tricks for that! It looks hard but I know the keys that will make it so easy for you if you're stuck.
Did you mean that in reverse?

3-hexene + H2O (excess) + H2SO4 (dilute) + heat --> 3-hexanol

I think 3-hexene + H2O would, technically alone, form alcohols.
If you look at the double bonds, when you protonate one side, the other will always be a secondary, so you could get 3-hexanol and 4-hexanol. Technically because they are the same, it works out alright here... you probably may have known this-- but look out in other molecules.

Have you covered hydride shifts and methyl shifts? They explain markovnikov much better.

Sorry these are mechanism deprived, but I do know the mechanism for both H3O+ / H2O and OH- / H2O addition to alkenes. Wait does OH/H2O add to alkenes? My brain is so screwed up today... I think so. Our two expert organic chemists will know this much better than I do.
I'd be happy to show you.

[Aperson444]

This I don't think would happen, you will learn something called elimination, but there is no leaving group. When you get to nucleophilic substitution and learn about elimination, you will be able to create elimination scenarios. I can show you all the tricks for that! It looks hard but I know the keys that will make it so easy for you if you're stuck.
Did you mean that in reverse?


Huh?

We learned this as dehydration or something along those lines. There was something else that used KOH and EtOH, but I am already forgetting what it means.

Have you covered hydride shifts and methyl shifts? They explain markovnikov much better.

Sorry these are mechanism deprived, but I do know the mechanism for both H3O+ / H2O and OH- / H2O addition to alkenes. I'd be happy to show you.

Yes we have.... But those only happen in the E1 reaction am I right?

Markovnikov means when a cation forms, it will take the more subsituted carbon, or the better resonance stabilized carbon.

So Anti means taking a less substituted C?

I think 3-hexene + H2O would, technically alone, form alcohols.
If you look at the double bonds, when you protonate one side, the other will always be a secondary, so you could get 3-hexanol and 4-hexanol. Technically because they are the same, it works out alright here... you probably may have known this-- but look out in other molecules.

H2SO4 and heat are catalysts. They are not necessary, but you have to tip the equilibrium, and be fast. That's how those work.

[The Jitterskull]

Huh?

We learned this as dehydration or something along those lines. There was something else that used KOH and EtOH, but I am already forgetting what it means.

Yep, what you can do is if you have an alcohol, you can protonate it to become H2O, which really wants to leave. The conjugate base of the molecule that protonated the water will then rip off a proton (hydrogen) from an 'alpha' carbon (aka, 1 carbon away from the carbon attached to the now H2O) and a double bond will form kicking it off. I think you know this since you mention E1 below this.

Yes we have.... But those only happen in the E1 reaction am I right?
I think thats true from what I can remember, but when you cover electrophilic aromatic substitution it will come back I think. Thats my favorite stuff too.


So Anti means taking a less substituted C?
Correct! :)
Though always look for resonance, resonance is really key. You said you wanted to do organic chemistry in the future? My advice is that resonance is key. It literally beats out everything else at times in terms of stability.
The name for that I think is "Hoffman addition"

H2SO4 and heat are catalysts. They are not necessary, but you have to tip the equilibrium, and be fast. That's how those work.

Where did you find this? I'm interested, and wouldn't be surprised if there was a reverse reaction.

[nshanin]

So let's take 1-pentene, right?

There are 2 carbons that share this pi bond, C1 and C2. C1 has 2 hydrogens on it, and C2 has 1 hydrogen. If something adds in a Markovnikov fashion, that means that the carbon with more hydrogens will receive an additional hydrogen. This is a very, very useful heuristic at the lower levels.

"Markovnikov said 'the rich get richer and the poor get poorer'"
--My 1st semester OChem professor

Think about it another way: you're adding H2O across a double bond. Effectively, one carbon gains OH and another gains H. Which goes where? Markovnikov tells you; the one with more hydrogens gets the H and the one with less gets the OH.

Now why is this? It's a byproduct of the fact that since the carbon with less hydrogens is more substituted, it is a more stable carbocation. Since a proton won't attack a cabocation (and you'll never add anything more electrophilic than a proton across a double bond), the nucleophile (in this case OH) will attack at the carbocation, which is more often on the less substituted carbon (the one with less hydrogens).

Nightside Eclipse's mechanism is close, but SO2 is not a product. Instead, H2O attacks the carbocation and the proton on the positively charged oxygen is picked up by another water molecule. It's just as if OH attacked the carbocation. Now think about what would happen if you added HCl or HCN or NH3 (the last one is impossible but would be really badass if it worked)... same situation; the alkene grabs the proton and the more stable (more substituted, with less hydrogens) carbocation is formed. The nucleophile (the thing attached to the proton) then attacks the carbocation and you form your product.

Adding in an anti-Markovnikov fashion means that a different mechanism or different local effects also factor into the reaction. I won't go into this, but it just means the reverse of adding "Markovnikovly".

EDIT: Here (http://www.mhhe.com/physsci/chemistry/carey/student/olc/graphics/carey04oc/ref/ch15hydrationofalkenes.html) is a decent mechanism. Ask yourself why the double bond leaves from one carbon rather than the other, why a certain carbon grabs the proton rather than the other.

[Aperson444]

Where did you find this? I'm interested, and wouldn't be surprised if there was a reverse reaction.

Term One -- The professor said that you can have H2O form an alcohol with sufficient energy so the equilibrium tips over one way. He said that you would need an acid as a catalyst.


I pretty much get it now. Thanks so much.

So to wrap this up, regioselectivity is basically relocating the H "contained" in the double bond using the Markinokov rule, am I right?

So with 1-pentene and the H2SO4 and excess water reaction you would want 3-pentanol correct (tertiary carbocation..... or not?)?

[nshanin]

Term One -- The professor said that you can have H2O form an alcohol with sufficient energy so the equilibrium tips over one way. He said that you would need an acid as a catalyst.


I pretty much get it now. Thanks so much.

So to wrap this up, regioselectivity is basically relocating the H "contained" in the double bond using the Markinokov rule, am I right?

So with 1-pentene and the H2SO4 and excess water reaction you would want 3-pentanol correct (tertiary carbocation..... or not?)?

No. All carbons on pentane are either primary (the terminal ones) or secondary. There is no difference in the stability of 1-pentene or 2-pentene to hydration, but they will form different products. Remember, the bond doesn't relocate, it's only the most stable carbocation of those 2 carbons that share the pi bond (exceptions come later).

[Aperson444]

No. All carbons on pentane are either primary (the terminal ones) or secondary. There is no difference in the stability of 1-pentene or 2-pentene to hydration, but they will form different products. Remember, the bond doesn't relocate, it's only the most stable carbocation of those 2 carbons that share the pi bond (exceptions come later).

Yeah, I was thinking of alcohols for some reason.

The bond is basically taken out (this is an addition, eliminating the alkene), and the hydrogen is given to either one of the carbons on either side of the do.

Does the same go for cations as does alcohols?

[nshanin]

Does the same go for cations as does alcohols?

What?

[Aperson444]

Tertiary, Secondary, Primary, etc.

[nshanin]

Tertiary, Secondary, Primary, etc.

Naming conventions? Yes. The highest state of both is tertiary and primary the lowest. They correspond.

[Aperson444]

Thanks guys.

The quiz was today, but unfortunately I probably failed it (2/10 is what I counted). I screwed up most of the reactions, and there was very little actual regiochemistry :mad::mad::mad::mad:


...... This has been the hardest class I have ever taken in my life.

[nshanin]

...... This has been the hardest class I have ever taken in my life.

This... is... COLLEGE!:mad::mad::mad:

[Aperson444]

This... is... COLLEGE!

FUCK COLLEGE....:mad::mad::mad::mad::mad::mad::mad::mad ::mad::mad::mad:

:)

I'll get better. Stupid lectures. God I hate them........ I had to skip the lab... that's the best fucking part. :mad::mad::rant: