I'm finding organic chemistry difficult

[KnowledgeIsPower]

I find it extremely tough to remember the correct mechanisms and apply them, there seems to be so many different structures etc.
Can anyone offer any advice regarding learning organic chemistry? Are there any decent websites/material which lay out organic chemistry in a more accessible manner?
Thanks.

 

[GCT]

Yep, for most org chem simply consists of memorizing, while a few seem to have a natural ability to learn it. For people who were exposed to org chem when they were younger it becomes much easier in college. Being familiar with general chemistry principles and other sciences (such as physics) also helps to build a better infrastructure in learning org chem. Other than that, there are no simple principles to understand it.





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Online help with chemistry
http://groups.msn.com/GeneralChemistryHomework

 

[KnowledgeIsPower]

Thanks, guess I just need to devote more time to it.

 

[maverick280857]

Hi

I would like to add that you should not only devote more time to organic chemistry but also read it with interest. There aren't too many books for undergraduates which make learning very interesting at the outset, so I would suggest that you read a book called Organich Chemistry by Paula Yurkanis Bruice. Also, refer to Peter Sykes' book "A Guidebook to Mechanism in Organic Chemistry".

Do not memorize anything in Organic Chemistry. You will have to come to terms with the mechanisms but once you start solving problems without looking at your own notes and the books, you will have understood a major part of it. In fact, most of organic chemistry is about positive and negative charge interactions in a way and it is ironic to note that there are so many things to be read and understood despite this simple looking idea :-).

You might also want to read Morrison and Boyd, esp as a reference. The only problem you might have with Organic Chemistry is the lack of time if you're taking courses in other subjects as well. But remember--and I speak from experience (having been in a similar situation not too long ago)--that mechanistic organic chemistry is one of the most interesting you will study in your life :-).

And the best part of it is...that you keep learning and improving as you work on it.

Cheers
Vivek

 

[KnowledgeIsPower]

Thanks for the advice ^^

 

[Roarixer]

Can anyone suggest a good site to find information and learn more about organic chemistry?

 

[GCT]

Remember that your organic chemistry book is a result of centuries of research and step by step achievements. It is not about "understanding" the book for itself as you would understand a novel after you had finished reading it, it is more like a dictionary and I suppose if you are smart enough, great memory skills, the course is relatively easier.

Organic chemistry is merely a piece of the pie in the subject of chemistry, in reality the book covers so many fields of chemistry.

In short I would say that organic chemistry is about experiments and thus experimentational skills, it is not really about mastery of the subject.


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Online help with chemistry
http://groups.msn.com/GeneralChemistryHomework

 

[shrumeo]

I find it extremely tough to remember the correct mechanisms and apply them, there seems to be so many different structures etc.

Above is answered by this:

Do not memorize anything in Organic Chemistry. You will have to come to terms with the mechanisms but once you start solving problems without looking at your own notes and the books, you will have understood a major part of it. In fact, most of organic chemistry is about positive and negative charge interactions in a way and it is ironic to note that there are so many things to be read and understood despite this simple looking idea :-).

Learning the mechanisms is usually just realizing where the positive and negative charges will be and where the electrons are going to be pushed around. That's how you learn where to put all the half arrows and stuff. It's like solving a little puzzle. There is quite a deal to memorize, though, mainly trends in reactivity. Remember, though, if it's not obvious how to push the electrons from the structure you see on the page, think of any possible resonance structures that will make it more obvious.

 

[movies]

The best organic text I have found by Clayden, Greaves, Warren, and Wothers (through Oxford press). It has a great breadth of material from basic priciples on to very advanced ones. The explanations are all very clear with excellent illustrations. It's the kind of stuff that should be on a website somewhere, but it just isn't. Most of the websites I have found don't go into detail with respect to mechanisms, they just show the general transformations. This website seems decent though:

http://www.chemguide.co.uk/mechmenu.html

When I was learning OC in college my prof. told us to not learn reactions, but to learn mechanisms. If you know the general types of reactivity for various functional groups then you can predict what what reaction will occur without having a clue what that kind of reaction is called. That is the greatest power of learning mechanisms!

 

[KnowledgeIsPower]

Thanks a lot for the site. There's a great many mechanisms on there that we are studying this year so i'll use those to form my written up notes.

A small query i do have however, is the use of a reducting agent on (RRC=O) (CR2O) (acyl?) with a reducing agent such as LiAlH4.
We don't need to know how the hydrogen ions are produced, only that the reducing agent helps create them.
I've been told that the lone pairs on the oxygen creates a delta minus (that's fine) but I've also been told that this creates a delta positive on the carbon. My argument is that surely a lone pair on the oxygen would repel the electrons in the double bond and also make the carbon delta minus? Thus, wouldn't the molecule first be attacked by H+ (not H- as I've been told).
I understand how the double bond is then broken by the hydrogen ion.
It is merely the creation of the charges which i am unsure about.
Thanks for any input.

 

[movies]

First think is that in organic chemistry you should almost always consider any kind of double bond as "labile" or "easy to push around." Also remember that the polarization of the carbonyl group is based on the electronegativity of oxygen and carbon. If I recall correctly carbon's EN is 2.2 and oxygen's is 3.5. So that bond is really quite polarized (with oxygen having most of the negative charge). The lone pair repulsions don't really have much to do with it. Remember that oxygen anions are pretty stable (much more stable that carbon anions, in fact). So you shouldn't let that carbonyl polarization bother you too much. The short of it: the carbon of a carbonyl is electrophilic, the oxygen is nucleophilic. You can predict the reactivity based on whether the reagent is electrophilic or nucleophilic.

In this case it is a little tricky, and it is easy to get confused. The reagent, LiAlH4, is an aluminum atom with four hydrogens bound to it, thus having a net charge of -1. The lithium cation balances the charge so the reagent is stable. The first thing that happens is that the lithium cation dissociates from the AlH4- and then coordinates to the oxygen of the carbonyl group. Draw that out and then draw a resonance structure where the double bond of the carbonyl is pushed up on to the oxygen. You will find that in that case the oxygen has no charge and the carbonyl carbon has a positive charge. That should address your problem with the deltas. The AlH4- then delivers an H- (not an H+) to the carbon, thereby reducing it. When you quench the reaction, you end up with an alcohol as your product.

So in this case the lithium acts as a Lewis acid to further activate the carbonyl carbon to electrophiles such as the hydride from LiAlH4.

 

[Chemicalsuperfreak]

Actually, in the reduction of carbonyl compounds with LiAlH4, the oxygen coordinates with the aluminum. You end up with the aluminate. That's pretty well known.

And this is why I recommend against getting mechanism from anonymous second-hand sources. If you want to look up specific mechanisms, I recommend the Bibles of O-Chem: Carey & Sundberg, and March. You should be able to find both in your schools library.

If you just want to practice drawing mechanisms, practice makes perfect. Despite what some have said, memorization IS important in O-chem. You've had some mechanisms by now, just keep practicing them; the hyrdolysis of ketals and acetals makes for really good practice in electron pushing. If you want further help, I recommend "Writing Reaction Mechanisms in Organic Chemistry" by Miller and Solomon.

 

[movies]

It is true that you end up with the aluminate, but that can't happen with the initial reduction since the aluminum is coordinatively saturated. Once the first hydride has been delivered you end up with AlH3, aka alane, which is a reducing agent in itself, but far less powerful than LAH. Once you start removing the hydrides then the aluminum can act as the Lewis acid.

If you look in March, 5th ed., p. 1202, you will find that the mechanism is as I described (with lithium as the Lewis acid). It also states that evidence for this is that the reaction does not take place when the lithium cation is sequestered by a crown ether.

Furthermore, March states that it is most likely that the aluminate forms by disproportionation of the alane-alkoxide from the initial reduction. The products are AlH2(O-R)2 and LiAlH4, which goes on to do further reductions as described above. AlH4- is the only attacking species in this case.

 

[gravenewworld]

I hate to tell you this but 90% of what you learn in Organic Chem is useless. Sure it is nice to know theory, but in reality most of the reactions learned are not carried out because the conditions required to carry them out are rediculous. For example, rxns like the gringard rxn or oxidation with chromium containing compouds are almost never done in industry, there are plenty of other examples I just thought of those examples off the top of my head. I have been working in the lab at a pharmaceutical company for several years now, and I have to say that I agree with what someone said before, Organic in reality is about good laboratory practice. There exist data bases that contain thousands of reactions, all you have to do is draw the structure of your starting material and what you want as your desired product and the data base will find what reactions need to be done and the procedures required. Also most organic chemists don't even know how to name the compounds they make according to IUPAC nomenclature because you can just draw your compound in ISIS draw on the computer and it will name it for you.

 

[KnowledgeIsPower]

Hmm, very interesting. Thanks guys.
So it's safe to say that the greater electronegativity of oxygen has a greater effect at pulling the electrons from the double bond than the lone pair has in repullsion?

 

[movies]

Just because some reactions don't have a prominent place in pharmaceutical chemistry doesn't mean that the reactions are useless. Many of them are still used for academic applications. The utility of many reactions out weigh the concerns, e.g. most of the palladium cross coupling chemistry. That chemistry has revolutionized med. chem., but no one ever mentions that Pd is more toxic than lead!

KIP, I think that explanation is safe. Someone chime in if you have a better explanation though.

 

[KnowledgeIsPower]

Thanks for the help.
So you could say that organic chemistry forms an important basis while businesses are involved with manipulating knowledge to provide themselves with maximum product at minimum price.

 

[movies]

I suppose you could say that, but I think your definition is a little broad. Industry is much more based on cost/benefit analysis. They are willing to use very expensive reagents if it avoids several steps or avoids using some toxic reagents. All of that depends on disposal costs, etc. Sometimes no other reaction will give them the reactivity that they want so they have to use something like chromium that is very toxic (examples of this are Nozaki-Kishi or pyridinium chlorochromate reactions).

In academic total synthesis, it's more about getting to the final product. For example, if you had to do some oxidation early on in a synthesis and needed to do it on a fairly large scale (for academics) you probably don't want to use a very expensive reagent or a reagent that you have to make (e.g. Dess-Martin, which is also explosive, BTW) if the same thing can be accomplished in good yield with a chromium oxidation which is much cheaper and easier to make on big scale (although cleaning it up really sucks).

Another example from academia would be K. C. Nicolau's total synthesis of Vancomycin, where he performed an oxidation with potassium permanganate (a very powerful and often non-specific oxidizer) in the 4th to last step of a ~35 step synthesis. If a company were to make Vancomycin on large scale I would predict that they would do everything in there power to avoid using large amounts of KMnO4 because it is such a safety hazard. In an academic lab, however, you are using very small amounts of these chemicals, so the risks can be relatively controlled.

That's a long answer to a short question...

 

[KnowledgeIsPower]

A long answer but also an interesting answer.
Thanks for the help.

 

[Rodajc]

It's better to leave organic chemistry on the pages of a textbook, because that's where it works every time and you can predict what will happen. You can become good at organic chemistry by remembering a few basic principles (such as the electronegativity thing already mentioned) and then you can work most mechanisms out for yourself, even if you've never seen them before. Learning all of these mechanisms and reactions in isolation is impossible, unless you want to remain trapped under a copy of March in your bedroom forever.

Having said all this, organic chemistry and 'arrow pushing' is a poor approximation to how things really work and there are many mechanisms which shouldn't work, yet do anyway. These just need learning.

Hope this helps.

 

[maverick280857]

If you are new to organic chemistry, Morrison & Boyd + solutions manual are two things you will really benefit a lot from. As far as the mechanisms are concerned, there will be exceptions, but once you delve into problem-solving in organic chemistry, these mechanisms will stick to your brain and you won't need to mug them up.

Of course, during your course, you will come across some seemingly difficult/unreasonable questions. But remember, you only get better with experience and a lot of organic chemistry requires (a) interest + thought + insight and (b) experience (esp in problem-solving) as I said in my earlier post (below).

Enjoy organic chemistry!

Cheers
Vivek

 

[KnowledgeIsPower]

Thanks for all the help and suggestions. I'll follow up some of the references and think about the posts.
I think i may have to resit the chemistry module that i sat in May but with some hard work and thanks to your advice I'm sure i'll pass that one and the one i have to take next year.

 

[maverick280857]

Good luck mate!

have fun,

cheers
vivek